Hi, This warms my heart. My father was one of the founders of Potomac Instruments and one of the engineers who designed this instrument. This brings back such fond memories. Radio was his passion.
Thank you Geerlings! I have certianly enjoyed my career in RF communications. You are doing a greatjob exposing the next generation to RF and broadcast technologies that they may not have otherwise see. I'm sure you are influencing some in expanding their career options and choices.
I much prefer AM stations than FM. In FM there’s only a pre-recorded soundtrack that play no stop all day long with non-stop adds and barely any valuable information. It will be a loss when AM is no longer a financially viable medium to keep the stations going.
Back in 1975/76 I was an engineer at an AM/FM station in Jeff City with a 4-tower directional night-time array. I don't remember the exact model number, but we had a similar field strength meter. That station had to submit a "Proof-of-Performance" report to the FCC every 3 years. I was the newest, so I got to traipse around central Missouri walking out into farmer's fields "12 yards South of the corner fence-post and 5 yards East of the Oak tree" to take a measurement at least 2 hours after official sunset. I don't remember the exact number, but I think we had 5 stations we were protecting at night. So that would have been 5 radial bearings with at least 3 readings on each bearing. Lots of work in a "proof". I do remember getting a call at home from a Colorado station telling me that our jock had neglected to to the day/night power switch. He knew this because he was listening to our station on his over-the-air monitor. I called the control room, and reamed the guy for not switching from 5KW non-directional to 500W directional. I wanted to fire him because this wasn't the first (nor the last) time he was "too-busy". Alas, one of the sponsor's loved his show.
Man, what a job. Out in the field, not behind a desk. I think a lot of young guys today would want that instead of a desk job. Question, tho, this is the first time I've ever heard of day/night power switch. Why would there be a need to switch to lower power at night?
re: day/night switch - That's a great question! We didn't cover it exactly yet, but check out the video 'this tech splits my voice in 5 parts' - also the KMOX tower tour (If I touch this tower, I die), both talk a little bit more about day/night power and AM radio. It boils down to signal propagation. At night, AM radio frequencies bounce off the ionosphere and go waaaay further than they do during the day. Because of that, unless you're one of the few 'clear channel' stations in the US, you have to turn down your power and often use a directional antenna array to point your signal in the right places. Otherwise you blast through and other radio stations are overridden by your signal.
@@93LT1RamAir Although Jeff covered this briefly, I'll try to expand on this question. The short answer is that medium and short waves all propagate in two distinct modes: ground wave (day and night) and sky wave (effective only at night). Ground waves generally follow the surface of the earth and are attenuated by both distance (1/r^2) and ground conductivity and permittivity (irregular but high). Sky waves are highly attenuated by the "D-layer ionization" during the day when sunlight excites the ions to absorb medium and short wave signals. At night the D-layer becomes transparent to medium wave (MF) and short wave (HF) signals which then travel on to be reflected back to the earth by the E- or F-ionization layers attenuated only by the (1/r^2) factor. This is the so-called "skip zone" where signals can be heard several hundred or thousands of miles from the originating station who might be transmitting modest or low power levels. Amateur stations transmitting 100 watts or less are regularly heard at trans-oceanic distances at night. As to the need to switch power, the FCC in cooperation with similar regulatory agencies in other countries attempted to level the playing field by channelizing radio spectrum and licensing entities to "use" a portion of that spectrum thereby entities have a more-or-less equal chance at serving their intended audience. This basically means that in the mid-1930's the MF broadcast spectrum (525kHz to 1705kHz) was channelized every 10kHz from 530kHz to 1700kHz resulting in 117 channels. Since there are many more than 117 communities needing broadcast services, the bands were further divided into large, medium and small markets. 60 "large" market stations got "clear-channel" frequencies which meant that there were no other stations on that frequency: St. Louis (KMOX), Chicago (WLS), KFI (Los Angeles), San Antonio (WOAI), New York (WOR), etc. These were intended to reach large segments of the population at a state-wide or region-wide level with large antenna installations at high powers (>10kW). Other stations got medium power stations which they shared the frequency with other medium market stations service smaller regions but they had to accommodate sharing the frequency with other stations by either reducing power, operating a directional antenna, or both. And finally the lowest power (
At least this old 'tech' can always be fixed one way or other. It will not be, "Oh, it doesn't work because we don't have the latest version of software". Keep up the good work!
The SDR is open source, so software isn't much of an issue. If you can't repair the radio, well, you likely cannot repair any radio. This old tech is good for quick determination of signal strength and direction (with a directional loop like he showed), the SDR being able to double as a spectrum analyzer if say, a transmitter is detuned and splattering parasitic signals all over the spectrum. The tunable loop, saw models with even more filtering, not only to tune a resonant frequency, but could filter adjacent frequencies in the band, to help narrow down a single source and more. Of course, with the additional filtering, there is a cost in signal, because in engineering everything is a balance of trade-offs.
My first paying job in radio, the summer after I got out of high school, was doing field strength measurements for an AM daytimer. I think I measured about 20 points on each of 11 radials (twice!). A lot of driving and traipsing through farm fields. On the weekend I did a board shift on Saturday and Sunday, as well as took the base current readings.
I have some similar instruments (made by Narda and SImpson) that measure 2.450 Ghz in mw/cm2 and is used to check if microwave ovens are leaking. They come with probes and a 5cm styrofoam spacer to get a consistent accurate reading. The crazy thing is, while the instruments I have are decades old, very few places want to calibrate them saying "they are out of support and old" despite them working properly and just needing the calibration. Companies want you to spend thousands on new meters instead, rather than appreciate the quality of these old and very well built instruments.
I used a similar survey meter. The engineer who trained me said that the spare styrofoam cones that came with it were to be used when the old ones became too contaminated with metal particles. Every day I had to run it along the seam lines where the units were screwed into the resonance chambers. Best job I ever had.
The worst outcome would be driving up the price on the secondhand market... it's already hard enough to find one of these older units for sale for non-orbital prices!
Ha! Who would have thought for sure. I always wonder what next level development might happen from someone who watches and adds some new tech to improve/replace.
I was just a tinkerer at 12-13, but had a Walkman with AM & FM radio. I of course listened to Radio Luxembourg AM 1439 kHz to discover the latest hits, and had to rotate the small unit to get a good (less bad…) signal. Of course, I could only receive this at night (in south Norway), and the reflections shifted from the complex interaction with the ionosphere. So I kept turning the darn thing every minute… I’m not _that_ ‘ancient’ at 58, but this was “media Stone Age”… 😅
I found a VHF meter at a yard sale years ago. The meter came off it's jewel pivot, and it's battery compartment had corrosion, but it all cleaned up well on the bench. Cost me $200. Also, found several European FSMs from KSL Farnsworth Peak, which we gave to The Old Cable TV Equipment Museum.
I did HF radio for the US GOVT for a number of years. I had a couple of…..🧐…..either one of these sig-strength meters, or one similar to this one. But anyway, worldwide I had dozens of: rhombics, nested rhombics, RLP’s, HLP’s, VLP’s, curtains, high freq CONMONS, low freq CONMONS, quarter wave towers, shunt fed towers, serial fed towers, and so on. It was fun! But, you are correct, as long as you have calibrated records of initial antenna patterns, you can use a $20 pocket signal strength receiver to keep an eagle-eye on tower corrosion and as you say….to “look” at the condition of RF ground systems and radials. This is a wonderful video! Thanks! PS, And oh BTW, I don’t cook my hot dogs with a tower, I plug em in to a wall socket. When the breaker trips, it’s done! A couple of us (like many other goof-offs) back in college days did that for real. I think we found that aluminum nails work best! 🤪
Your method of cooking hot dogs reminds me of an actual commercial product available in the pre-microwave oven era--the Presto "Hot Dogger". It was a hot dog cooker that actually used 120 VAC service to cook hot dogs by way of two bus bar like poles, one connected to each AC line, with protruding spikes that you'd spear the hot dogs on. Here's a video of one in action: ua-cam.com/video/029jcmv4tIU/v-deo.htmlsi=aJ_qoTIECCfEb4Sh
@@channelview8854- I appreciate this sentiment. But I can assure you it wasn’t all fun and games. I have had very stressful moments when Jimmy Hendrix could have played the “Star Spangled Banner” using my intestines instead of his guitar because sometimes lives were at risk. But on the other hand I did enjoy myself at other times. Like when I was repairing a 50KW TMC BALUN (those puppies were about 5 feet tall, and about 6-7 foot circumference), on a short tower about 10 feet up. The antenna itself was a massively huge double HLP (probably took up about 4-5 acres of land plus another 12 or so acres all around to maintain integrity of the antenna pattern). Sooooo I just so happened to have with me in the man-lift bucket a ham radio transceiver and battery plus a coax adapter (reducing the 5 inch nitrogen filled coax down to a SO-239). I had a total blast doing things like that. A bad moment was when I was in the land-of-sand doing emergency repairs to the feed point of a Hy-Gain RLP (large log periodic, looks like a huge yagi…the antenna is up about 110 feet, and the feed point is all the way out (26 feet from the center mast)….so I am 110 feet in the air, out on the end of the antenna boom, changing the feed-point for 1.5 inch hardline nitrogen filled coax…when I started hearing 1 or 2 bullets whizzing past. Lives were on the line, so whaddia gonna do? Cry and walk home? I tried moving around as much as possible, and my sphincter was just kinda-sorta pinched shut (shoot, I cudda chopped a 10 foot length of 1 inch rebar into 1 inch chunks). Did I mention I hate heights, but I’ve climbed many hundreds of towers thru time. I’m not trying to impress, just saying there are downsides to every job. My spouse and my sister say I need to write a book, but I’d just get into trouble. 😂 Cheers es 73! And please enjoy what you do…life is too short!
Back in the 80's when I transitioned from landlines to wireless I often thought of how useful RF glasses would be to look at antenna patterns and such!
I remember doing engineering proofs with that very same model back in the early '90s. Had the cops stop us a couple of times atoask WTF we were doing, when we were checking the night pattern ...
The more I learn about AM radio the more it reminds me of how EM surveys work for exploration geophysics. Even everything having a specific orientation and going "null" if its 180 degrees out. Unlike radio though we have 3 "antennas" now for X Y and Z measuring the strength of the signal in 3 orientations allows a 3D "image" so it can be a pain in the ass to get lined up correctly. I cant help but think a random geo was screwing around with an old AM radio in the bush back in the day and figured out that theyd sometimes get a signal with the antenna "pointed" at the ground. These AM field strength testers remind me a lot of the very original looking 2D EM recievers from way before I was born.
7:42 that's because when you hold an antenna YOU become part of the antenna circuit, your body becomes part of the grounding system! That's a dead giveaway the antenna lacks a good ground leg or you're standing on poor soil/rock for good ground. All antenna in theory are some form of a dipole (like classic rabbit ears) with a positive and negative/ground element that rapidly flip polarity as the radio waves hit them, some of them just "hide" one pole/ear to save space by using different tricks. You can often solve that by adding the right length of wire trimmed to roughly match the frequency used to a ground point somewhere, using say an eye-crimp fitting onto a chassis grounded screw somewhere on the radio. Those are called a "Tiger Tail" in amateur radio and often work well for handhelds or portable setups once you tune the length.
Wow! Blast from the past. Back in the mid-80s I was an assistant Chief Engineer at a AM/FM station in Baltimore. I remember driving around in Pennsylvania in the station remote van with a similar unit testing signal strength when we moved the AM transmitter. Thanks for resurrecting a memory from 40 years ago!
PI also does lots of specialized government contracting. First time we invaded Iraq, I had a 4 month back order on some custom accessories for one of their phase monitors. They were 100% flat out trying to build elements of the directional shortwave transmitter sites we set up across the borders to send propaganda and info towards Bhagdad, before resuming normal operations.
The FIM-41 is the big brother that goes up to 5 MHz. That works on some shortwave systems, but was mostly used to certify 2nd and 3rd harmonic compliance, before NRSC cutoff rules required a spectrum analyzer. A major use not mentioned is ground conductivity studies of co- and adjacent channel signals to a potential new AM DA, in regions of the country where rock often makes soil conductivity demonstrably lower than 47 CFR presumptive charts. The nature of FCC designated DA monitor points as license conditions, usually 1-4 miles from a TX site and based on larger studies after new construction or major changes, could be described far better. Also missing from this video is how use of the null perpendicular to the received signal is used to evaluate quality of a potential measurement point. A null less than 10:1 is a horribly disrupted spot, or one with power or phone cables or metal structures distorting the signal levels. Get over 40:1 null at right angles to signal, and that's a really clean point. There's not just one antenna in that meter lid, but two. One is the normal receive antenna, and the other a transmit loop for the calibrate oscillator. The BNC input could also allow use of the FIM as a sensitive detector, such as with a Delta OIB-3 Operating Impedance Bridge. That could be done at increments above and below carrier frequency, trying to adjust for a flat resistive and balanced around zero at carrier reactive impedance curve. Try parsing that calibration certificate again. 1 or 2 percent tolerances.... with so many stacked details as to the basis of the reference fields and their use calibrating meters, that the lumped sum of worst case errors could stack into the 20-40 percent potential inaccuracy range, even if far better precision. Not that I've evern learned to lean slightly, to anchor an elbow in one side, while holding an FIM stable in one hand while fine tuning adjustments with the other.... And then rotate for the null, evaluate point quality, calibrate, rotate back, and measure, and record notes. Lather, rinse, repeat, sometimes out 100-250 miles from another station's site, if doing ground conductivity field studies Rarely past 20 miles in normal proofs.
I like to take one of my AM radios I have fixed up with me when I go for a walk to see what stations it picks up. If I had one of these field strength meters to take along, I'll bet every passing farm truck would probably have some concern as to what I'm doing 😅
Years ago I knew the Chief Engineer Don Holmes at 10.000 Watt 580KHz CJFX Radio in Antigonish Nova Scotia Canada. I remember seeing him using one of these, I take it, for reports to Industry Canada, DOC in those days. This was many years ago and Mr. Holmes now rests in peace. This Instrument will bring back many more memories. May all the old Chief Engineers rest in peace, and may perpetual light shine upon them. The Reverend Peter Rafuse.
Neat stuff. Back in 1986 I was the engineer for a two-tower AM in Connecticut, and I would take these readings, then tune the phaser to ensure we were in compliance with our license terms. Great blast from my personal past!
@@markmalasics3413 WCTF Vernon-Hartford (1170 AM) in Connecticut. Ran 1KW daytime, 500W “critical hours.” That was in 1986. Don’t know whether it’s still on the air.
@@eclexian looks like they're still on the air, they even have a Wikipedia article. They apparently moved transmitter locations in '88. en.wikipedia.org/wiki/WCTF Tuning phasers, boy that brings back memories... WSRN, back in the late '70's. They're obviously still around, as Swarthmore College isn't about to go anywhere, though they did go dark for a bit during COVID.
@@spvillano I had no idea there was a Wikipedia page for it. That's neat! Thanks. Just to clarify, the transmitter site didn't move; the studio moved to colocation with the transmitter. I remember building the original studio from WRTT in an office building in Vernon. Moving it to the xmitter site was a smart move. One of the common points of failure (not to mention ongoing cost) was the low-tech unweighted leased phone line STL. It also meant someone was there a lot of the time, keeping vandals away. These were both ongoing problems there.
Also note, manufacturers of sub station and power distribution equipment use a signal strength meter to check for corona. This was at 1020 Khz from what I recall.
I went and found the circuit disgram of this unit. This is no ordinary radio…the bloke who designed this was an absolute analog Wizard! I thought…(from “fallout era”) that it wiuld be full of valves, but far from it, almost 80% of the circuit is designed around CA3045 transistor arrays. The only bits where descrete devives sre used is in the audio amp and a j-FET at the RF front end with its fate protected by zeners should someone take a reading right next to a tower with no attenuation swotched in! Another odd feature is it has a Positive Earth, and its rail is -6v that looks to be furnished by four D-cells in series….but the odd thing is, its internal rail is regulated to -6v and the battery can’t be much more than that…so he has used a very low drop out regulator designed around three descrete silicon transistors, two PNP’s in a diffirential configuration and an NPN pass transistor. It is common to see old semicinductor radios built around PNP germanium transistors and using positive earth with these is fairly common practice because then all the transistors’ emitters (the lowest impedance connection on a BJT) can then go to ground by a fairly short path, but this thing has six PNP transistors in it, viz, two in the reg already mentioned, one for the meter and dial lamps, one in the headphone amp and two in the AF amp. all the rest, four descretes and 30 others, five each in six CA3045’s, are all NPN. So the circuit has a strange “upside down” look to it. Looking closer, most of the stages, RF amp, two IF amps, cal oscillator, local oscillator and null circuit are all designed around a diffirential pair with a current sink underneath and a few common emitter gain stages or emitter followers. The RF amp is a very wierd capacitively coupled cascode amp with the drain of the front end j-FET capacitively coupled to the emitter of a subsequent grounded base stage, but has a current sink underneath…..and all this of just 6 volts! So there is barely any headroom to move about. Three things that make this a very different thing from a commercial AM radio are….lack of any AGC, (and this is why you failed to get a good null with the commercialradio…its AGC was working against you), presence of the cal oscillator and null circuit, which a commercial radio lacks…and lastly…designed by an absolute Wizard. The loop antenna of this has TWO active ends that are out of phase, one end goes to the RF amp like in an ordinary radio, but the other end feeds into a common base stage in the null circuit. It is a VERY interesting circuit which looks deceptivey simple st first glance, but the minute you look a little closer you see interesting and mysterious things, and the closer you look, the more you see. One could learn a hell of a lot from this circuit and its clever designer! And that is where the cost is…along with the calibration and lack if drift over time. Ordinary AM radios can have a multitude of evils that a pasted over by the AGC, but here, all is lain bare and has to be spot on, so thanks for making me aware of this marvellous circuit. P.S. If you do go snd look up the circuit, it is drwan like a wiring diagram around the CA 3045 packages which makes in even more confusing to read….as if the positive Earth wasn’t enough….but if you carefully redraw it as diff amps with current sinks underneath and follow the dignal path, that will largely “unscrumple the paper it was drawn on” and allow you to see what a triely clever design it is.
Long post, but a lot of detail. The advantage of using a CA3045 transistor array is the inherent matching, since all devices were fabricated at the same time, so the doping of the semiconductor material is uniform for all devices on the wafer. Matching for gain but also for temperature characteristics.
This is definitely a 1980s or earlier design in that it only tunes to 1600 kHz, and CA3045 transistor arrays are used. The CA3045, I suspect, was originally an RCA part (due to the CA prefix), but Intersil, National Semiconductor, and others later second-sourced it.
I have come across the CA3046 and CA3086 transistor arrays in many pieces of test gear where relatively cheap matched transistors are required…had never heard of the CA3045 ‘till I saw it here. Suspet it is probably a lower noise version of the 3046. Still a bloody clever design though!
A level of understanding I will never achieve, but sure appreciate. I am much more suited to digital, where it either works or it doesn't. I can't screw that up too badly. :-)
I wish I had a buck for every time I jumped over a farmers fence, into a muddy field with a Potomac FIM-21 to get a measurement for a directional antenna proof of performance.🤓
My father w89job spent many of his retirement years going out and doing field strength readings with various equipment. I'm pretty sure I saw a lot of these in his stash.
That's the unit the FCC guy had when checking my Part 15 station. Also our local KELO-AM is 1320. KMOX being 1120 I wonder what 1110 KFAB looks like from your area as KFAB is also 50,000 watts.
I'm now calling it official. Your father is now Purple Shirt Geerling. Blue and Red make Purple. I also notice that your dad through challenge down, and you blanked it. Pi based RF detecting glasses.
We have a guy that drives around once a year or so and gives us reports on our signals. I am not sure who it is or what equipment he uses but the report is quite detailed. As for the "glasses" Ubiquiti makes a spectrum analyzer wifiman wizard that lets you "see" the RF as it maps the room/building you are in. You need an iphone with the lidar on it to take full advantage of it but it's a neat tool. Also laughs in 80 year old grounds sitting on rock basically for my AM tower. Chasing RF was fun till I went nuts with new grounds.
@@GeerlingEngineering Probably the modern equivalent. When I first saw what you have here I thought it was like a meter my silent key mentor gave me. I have only seen it used once big meter with dials goes across the tuning coil of an AM transmitter and us it to set the matching. If you want I can take pics of it. Been meaning to show it to some radio groups.
@@JamesHalfHorse Ah yes when you tune an AM tower, you usually have one of those massive boxes with a couple tuning knobs-my Dad has footage from a few sites where guys would set up towers with it.
@@JeffGeerling I need to pull it out and take a few pictures for the radio show and tell groups and maybe shoot you one in email if you like. I am not sure exactly how it works only saw it used once when we replaced the AM transmitter. Mark P designed a new coil for it to match the tower, I built it and then the more seasoned engineer helping us put this his own version of this thing that I think has a preamp in it mine don't across it to set the taps and it's held a 1:1 on the swr since. Okay 1:12 or so after I had them take a 2 way antenna off that is now my gmrs repeater antenna but not enough to tune out. It's the old way of doing things, I don't need it any more even if I did know how to use it. Last times transmitter had problems my rigexpert read the 80 year old shunt fed tower with bad grounds just fine matching the reading the Nautel gives me just to check for any major problems before trying to send RF up it. It like my other mentors old Simpson meters will likely go on the shelf of my shop their tools are kinda my memorial to them even if outdated and consider it an honor to have them. The bird meters however do get used but came from someone that holds some of my deepest respect... I think that is why the new Ipad commercial was so painful for a lot of us yeah there is better tools but the old ones have meanings and stories to tell. I hope to talk my remaining mentor into letting me stick a camera in his face for a few hours and say tell me your 70 year career in radio. A lot of that information is dying with these guys and it saddens me.
When I was a kid back in the 1960's, I made a medium wave loop antenna, with a variable tuning capacitor, so I could listen to US East Coast AM radio stations in the UK. The setup allowed me to null out interference from European transmitters and listen to US content. One station which I listened to regularly during the winter months was WKBW Buffalo NY, which is still going strong.
The SONY radio did hit a null on the 1320 station, but the AGC hid the signal strength dip pretty well. Listen closely as the radio is moved around; the interference level goes up in the null.
I worked for the Civil Aviation Authority here in Australia, we used to use the Potomac to quantify the output from our Non Directional Beacons (NDBs).
Certain older pieces of test gear are still considered a valuable must for calibrating particular types of equipment. This is why for instance a particular HP VTVM model is considered more desirable over another.
Being able to accurately compare antenna's signal strength is nice to have. I've gone down the rabbit hole and now own way too many meters. With TV signal strength meters, ones that can measure ATSC and QAM, they will show you the bit error rates and constellation. You can get a really good idea of he health of your antenna and amplifier setup. Gone are the days of being able to see ghosting and intermod on analog TV.
A.M. died here long time ago in Europe , now they want to get FM out off the air. DAB + remaining as "radio" .To me this sounds like a bit like censorship.
Hmm , a feild stregth meter didn’t used to cost that much years ago . I used mine to match antenna to radio RX/ RT two way system . $10 or $20 or so . Seems a bit excessive to me . Because the operator in any case has to calibrate the device himself any way . But ok , what ever .
I was lucky enough to be able to use one of these back in my radio engineering days in small market radio in the 70's and early 80's. Thank you so much for bringing back those memories!
I got a Simpson rectangular style panel meter, on a AC meter I found out near an airport on a farm beyond the runway... measures volts... or amp draw through it, with two switches to switch between 110/220, or the scale on the amps.
This Potomac FSM is built for the MF AM broadcast band. Other versions work on the LW Aviation Navigation band or the SW bands. All on first appearances look identical and all are still in high demand by the radio industry worldwide today as it is one of the few devices around that can give a accurate calibrated repeatable numerical value to the received RF signal strength of a selected radio station, thus the high price.
I'm surprised that there are so few field strength meters in the U.S. given that theres almost 5K AM broadcsat stations and ~6K FM stations. Do the stations share them amongst each other?
Often stations hire an engineer with the good and up to date models of test gear. A lot of stations don’t have a full time engineer and hire contractors who have the gear.
Once the gas in the tube fires, it becomes a low-Ohms conductor ... the reason a fluorescent bulb uses a ballast in series. Actually touching the tube could be an issue. Normally they just light in the proximity of strong RF signals. No need to touch ...
@@GeerlingEngineering looking forward to those! I've just started to get (back) into SDRs, and I've got the V3 of yours that doesn't have the built-in upconverter for HF/MF, it just uses the direct sampling method... Been thinking about if I should upgrade to the V4 or if the V3 performs about the same!
This brought back memories. Around 50 years ago I went out with the engineer with a receiver, but it used tubes and it was much larger and heavier. We put it on our shoulders. We spent the whole morning doing just one lobe. Both of us had our First Phone licenses but I had never worked at a station. It was just as you described. The station was a 3 tower array in the RF ghetto of 1410 KHz. :) I guess we were testing the daytime pattern, but I don't remember for sure.
@@Iridium43 Yeah, images have always been a problem in those low IF radios. But all the car radios were 262.5 KHz. Was that from someone driving by the tower? I used to get the second and third harmonic from a station on 1350 about 5 miles from the tower.
Awesome as always but what really struck me was the joke about magic glasses that allow you to see RF. I remember day dreaming about that early in my RF nerd hobby 30 years ago. I don't think I've ever heard someone else mention the same idea. Made me smile.
I have many different boxes for field strength readings, but most are for aligning the receiver antenna for best reception. They work on VHF, and UHF TV and also FM radio bands, as they are fun to use to do antenna installs, kind of like the units for Satellite TV dishes.
Hey Jeff, have you come across Meshtastic / LoRaWAN yet? Thought it might be a nice crossover between radio and raspberry pi for you and your dad to cover and experiment with!
In the early 60's I read in an electronics magazine that if you held an AM radio with a ferrite antenna upside down you would lose the signal. I swear at that age it did work as they said. This may have been the April Fools edition. Like the Black Noise Generator made out of a black bean soup can. Unfortunately, I haven't seen an AM portable radio in so long to try it. Yes, I do have a ghost detector.
Great video! Maybe next one you could ask you Dad to show us how you check the AM mask with a spec analyzer. RF is just electromagnetic radiation, a couple of orthogonal fields in motion. Still, it fascinates anyone with a sense of curiosity. Also, maybe you've already explained this, but lots of folks have no idea how to properly hook an antenna to a television receiver. Carry on! de NX8J
Any of the models with valid calibration certificates (that aren't expired) will go for considerably more... unless you're lucky! If you find a calibrated model for under $1k please let us know, it'd be nice to have a 2nd!
The successor to this is the Potomac Instruments FIM-4100 which is a modern software controlled digital readout instrument which is really great to use. Problem is the high price. Over 14 grand. It makes the analog FIM 21 and 41 look pretty good. Although no longer manufactured used ones can be found on used equipment sites for 2 grand or less. Calibrating the older analog ones is costly and Potomac Instruments is the only company that will certify the calibration.
Guys, this is right up my alley! But I´m a noob, here in Germany AM is dead, exept for some Turkish and Chinese stations. Could You do an antenna giude for dummies who don´t have any sphisticated gear like this. Btw it´s very cool that the company is still around to calibrate it. I´d love to just scroll thru the scale for some Johnny Cash... Yeah, no, I´d love to get by with the least financial expense- so to say. I collect old tube radios, still use my FM receiver and cannot imagine a future without analogue radio. Kind regards
When I lived in Chicago, I was able to watch TV stations over 80 miles away with rabbit ears and bow tie by picking up the bounce off the Sears Tower. I was on the side of the building away from South Bend and toward the building.
I bought a small digital-ready television in December of 2007 in anticipation of broadcast tv going digital in 2008. I have yet to take it out of the box. AM and FM radio are all I have been into all this time, plus some UA-cam, obviously, on my android. Some one of these years I will get around to unpacking that television. I sure hope it works, because forget about the warranty. I often eat out by myself, so I sit at the bar. I look at all those television screens and realize I'm not missing anything.
Is the Dad's knowledge being taught in a tech school or college anywhere? This is science but I fear that it's becoming a lost art. Are there engineers working for the big radio conglomerates with these skills and knowledge sets? I find it all fascinating. In my market, I've almost always preferred the programming on AM radio. Sadly, I know the audience is dying off.
On receiving am. There is a long wire as the antenna and a ground wire. They are not connected. Loop antenna are connected. With experience, when young a loop antenna don't really work on a radio that is designed for the other. I have also noticed that antenna very with thickness of wires, many are tubes and different materials. What's the deal there. Can I just use a loop copper wire for UHF. Shorter wire of course. Or a VHF with a single wire and ground. If there fundamentally differences what is being used besides then just directional or not. Maybe something is more suitable how the signal is being carried.
Cool collectible radio, however I can do the same thing with my cheap AM pocket radio by turning it. I know where the null is, so I know where the transmitter is and my signal strength meter is my ears. I do really like that tunable external loop antenna, more than the radio. I might build one of my own. Have been meaning to do it some day. You can also enhance reception of a radio by putting it near the loop even without a connection.
Unfortunately all I see happening as a result of this video is negative. People will think these are toys. Now that the masses are “ aware” the ones that are available for professional use will disappear. They will be bought for the cool factor only.
Have you considered using the AM tower as a Malehood compensator hhehehe, just kidding. Foolish things aside, would it be able to roast a chicken properly? And would a chicken roasted using a music channel taste different than one using a politics channel? 🤣
I have no idea or knowledge of any of what is talked about in this video. I'm here because I enjoy to see a son doing stuff with his father, that is so rare today. And who knows I might learn a thing or two :D
a spectrum analyzer used for the same purpose today is much more expensive, maybe 10-20 (but ofc does many more things) calibrated antenna alone will run you more than the entire device AM demod option will be a few thousand, too, if you don't just want to look at the signal, but also listen and do more specialized analysis stuff
That is one of the most expensive radios I have ever seen, and I used to be a meteorologist many years ago. We had calibrated RF "radio" equipment used in an RF chamber to measure both conducted and radiated emissions. You have the most expensive hotdog cooker ever! As a youngster I used to cook hotdogs with a board, a couple of bent nails, and a killer cord! Those were the days!
We use FIMs in the wireless industry to determine if the detuning on wireless communications sites is working, not messing with the AM station pattern.
This reminds me of my Grandfather. While he was living in northern Ontario Canada around the mid 30's. He would turn his AM radio on after dinner and sit in the corner so he could see up the road for oncoming headlights. He decided not to pay the radio license from the hardware store, so if he saw headlights from a vehicle, he would shut the radio off. The bylaw was enforced by a vehicle that had a directional antenna that could detect the intermediate frequency from the radios when they were turned on. Imagine a knock on your door and someone asking if you have a license for that radio!.☺
re: "hat could detect the intermediate frequency from the radios when they were turned on" Heh. I'll bet they had better results looking for the LO (Local Oscillator) in the radio rather than the IF. ALSO, the LO 'leakage' from an FM radio is MUCH more pronounced (a lot stronger) than the LO leakage from an AM radio.
@@uploadJ That's a good point. He had a directional style antenna and pointed it out the window while driving. I think it was just an AM radio he had at that time. I still listen to a few of the old radio programs on the internet archive.
Hi, This warms my heart. My father was one of the founders of Potomac Instruments and one of the engineers who designed this instrument. This brings back such fond memories. Radio was his passion.
That's so cool to hear! Potomac has always had a high standard of quality, that usually comes from the top :)
Respect to ur dad
Thank you Geerlings!
I have certianly enjoyed my career in RF communications.
You are doing a greatjob exposing the next generation to RF and broadcast technologies that they may not have otherwise see. I'm sure you are influencing some in expanding their career options and choices.
AM radio stations love these guys because when they tune in, their audience triples!
🤣
I much prefer AM stations than FM. In FM there’s only a pre-recorded soundtrack that play no stop all day long with non-stop adds and barely any valuable information. It will be a loss when AM is no longer a financially viable medium to keep the stations going.
Back in 1975/76 I was an engineer at an AM/FM station in Jeff City with a 4-tower directional night-time array. I don't remember the exact model number, but we had a similar field strength meter. That station had to submit a "Proof-of-Performance" report to the FCC every 3 years. I was the newest, so I got to traipse around central Missouri walking out into farmer's fields "12 yards South of the corner fence-post and 5 yards East of the Oak tree" to take a measurement at least 2 hours after official sunset. I don't remember the exact number, but I think we had 5 stations we were protecting at night. So that would have been 5 radial bearings with at least 3 readings on each bearing. Lots of work in a "proof".
I do remember getting a call at home from a Colorado station telling me that our jock had neglected to to the day/night power switch. He knew this because he was listening to our station on his over-the-air monitor. I called the control room, and reamed the guy for not switching from 5KW non-directional to 500W directional. I wanted to fire him because this wasn't the first (nor the last) time he was "too-busy". Alas, one of the sponsor's loved his show.
Man, what a job. Out in the field, not behind a desk. I think a lot of young guys today would want that instead of a desk job. Question, tho, this is the first time I've ever heard of day/night power switch. Why would there be a need to switch to lower power at night?
re: day/night switch - That's a great question! We didn't cover it exactly yet, but check out the video 'this tech splits my voice in 5 parts' - also the KMOX tower tour (If I touch this tower, I die), both talk a little bit more about day/night power and AM radio.
It boils down to signal propagation. At night, AM radio frequencies bounce off the ionosphere and go waaaay further than they do during the day. Because of that, unless you're one of the few 'clear channel' stations in the US, you have to turn down your power and often use a directional antenna array to point your signal in the right places. Otherwise you blast through and other radio stations are overridden by your signal.
@@GeerlingEngineering That makes a ton of sense, thank you for the answer! Love your channel :)
@@93LT1RamAir Although Jeff covered this briefly, I'll try to expand on this question. The short answer is that medium and short waves all propagate in two distinct modes: ground wave (day and night) and sky wave (effective only at night). Ground waves generally follow the surface of the earth and are attenuated by both distance (1/r^2) and ground conductivity and permittivity (irregular but high). Sky waves are highly attenuated by the "D-layer ionization" during the day when sunlight excites the ions to absorb medium and short wave signals. At night the D-layer becomes transparent to medium wave (MF) and short wave (HF) signals which then travel on to be reflected back to the earth by the E- or F-ionization layers attenuated only by the (1/r^2) factor. This is the so-called "skip zone" where signals can be heard several hundred or thousands of miles from the originating station who might be transmitting modest or low power levels. Amateur stations transmitting 100 watts or less are regularly heard at trans-oceanic distances at night. As to the need to switch power, the FCC in cooperation with similar regulatory agencies in other countries attempted to level the playing field by channelizing radio spectrum and licensing entities to "use" a portion of that spectrum thereby entities have a more-or-less equal chance at serving their intended audience. This basically means that in the mid-1930's the MF broadcast spectrum (525kHz to 1705kHz) was channelized every 10kHz from 530kHz to 1700kHz resulting in 117 channels. Since there are many more than 117 communities needing broadcast services, the bands were further divided into large, medium and small markets. 60 "large" market stations got "clear-channel" frequencies which meant that there were no other stations on that frequency: St. Louis (KMOX), Chicago (WLS), KFI (Los Angeles), San Antonio (WOAI), New York (WOR), etc. These were intended to reach large segments of the population at a state-wide or region-wide level with large antenna installations at high powers (>10kW). Other stations got medium power stations which they shared the frequency with other medium market stations service smaller regions but they had to accommodate sharing the frequency with other stations by either reducing power, operating a directional antenna, or both. And finally the lowest power (
@@93LT1RamAir AM radio signals travel farther at night. Some stations are required to completely shut down at night because of this.
At least this old 'tech' can always be fixed one way or other. It will not be, "Oh, it doesn't work because we don't have the latest version of software". Keep up the good work!
The SDR is open source, so software isn't much of an issue. If you can't repair the radio, well, you likely cannot repair any radio.
This old tech is good for quick determination of signal strength and direction (with a directional loop like he showed), the SDR being able to double as a spectrum analyzer if say, a transmitter is detuned and splattering parasitic signals all over the spectrum.
The tunable loop, saw models with even more filtering, not only to tune a resonant frequency, but could filter adjacent frequencies in the band, to help narrow down a single source and more. Of course, with the additional filtering, there is a cost in signal, because in engineering everything is a balance of trade-offs.
My first paying job in radio, the summer after I got out of high school, was doing field strength measurements for an AM daytimer. I think I measured about 20 points on each of 11 radials (twice!). A lot of driving and traipsing through farm fields. On the weekend I did a board shift on Saturday and Sunday, as well as took the base current readings.
I did the same thing. And it was all atlas's and paper maps
I have some similar instruments (made by Narda and SImpson) that measure 2.450 Ghz in mw/cm2 and is used to check if microwave ovens are leaking. They come with probes and a 5cm styrofoam spacer to get a consistent accurate reading. The crazy thing is, while the instruments I have are decades old, very few places want to calibrate them saying "they are out of support and old" despite them working properly and just needing the calibration. Companies want you to spend thousands on new meters instead, rather than appreciate the quality of these old and very well built instruments.
I used a similar survey meter. The engineer who trained me said that the spare styrofoam cones that came with it were to be used when the old ones became too contaminated with metal particles. Every day I had to run it along the seam lines where the units were screwed into the resonance chambers. Best job I ever had.
Never in my life did I ever expect a UA-cam channel to create internet 'sizzle' around a Potomac FIM. I'll admit to being slightly annoyed by this.
The worst outcome would be driving up the price on the secondhand market... it's already hard enough to find one of these older units for sale for non-orbital prices!
I know nothing about radio but where do I get one?
Ha! Who would have thought for sure. I always wonder what next level development might happen from someone who watches and adds some new tech to improve/replace.
Move on
I was just a tinkerer at 12-13, but had a Walkman with AM & FM radio. I of course listened to Radio Luxembourg AM 1439 kHz to discover the latest hits, and had to rotate the small unit to get a good (less bad…) signal.
Of course, I could only receive this at night (in south Norway), and the reflections shifted from the complex interaction with the ionosphere. So I kept turning the darn thing every minute…
I’m not _that_ ‘ancient’ at 58, but this was “media Stone Age”… 😅
Eh, I'm 38 and watched most of Earth: Final Conflict by holding on to the antenna. Very sci-fi.
I found a VHF meter at a yard sale years ago. The meter came off it's jewel pivot, and it's battery compartment had corrosion, but it all cleaned up well on the bench. Cost me $200.
Also, found several European FSMs from KSL Farnsworth Peak, which we gave to The Old Cable TV Equipment Museum.
I did HF radio for the US GOVT for a number of years. I had a couple of…..🧐…..either one of these sig-strength meters, or one similar to this one. But anyway, worldwide I had dozens of: rhombics, nested rhombics, RLP’s, HLP’s, VLP’s, curtains, high freq CONMONS, low freq CONMONS, quarter wave towers, shunt fed towers, serial fed towers, and so on. It was fun! But, you are correct, as long as you have calibrated records of initial antenna patterns, you can use a $20 pocket signal strength receiver to keep an eagle-eye on tower corrosion and as you say….to “look” at the condition of RF ground systems and radials. This is a wonderful video! Thanks! PS, And oh BTW, I don’t cook my hot dogs with a tower, I plug em in to a wall socket. When the breaker trips, it’s done! A couple of us (like many other goof-offs) back in college days did that for real. I think we found that aluminum nails work best! 🤪
Your method of cooking hot dogs reminds me of an actual commercial product available in the pre-microwave oven era--the Presto "Hot Dogger". It was a hot dog cooker that actually used 120 VAC service to cook hot dogs by way of two bus bar like poles, one connected to each AC line, with protruding spikes that you'd spear the hot dogs on.
Here's a video of one in action: ua-cam.com/video/029jcmv4tIU/v-deo.htmlsi=aJ_qoTIECCfEb4Sh
From where I sit you are a lucky guy. I'm a bit jealous.
@@channelview8854- I appreciate this sentiment. But I can assure you it wasn’t all fun and games. I have had very stressful moments when Jimmy Hendrix could have played the “Star Spangled Banner” using my intestines instead of his guitar because sometimes lives were at risk. But on the other hand I did enjoy myself at other times. Like when I was repairing a 50KW TMC BALUN (those puppies were about 5 feet tall, and about 6-7 foot circumference), on a short tower about 10 feet up. The antenna itself was a massively huge double HLP (probably took up about 4-5 acres of land plus another 12 or so acres all around to maintain integrity of the antenna pattern). Sooooo I just so happened to have with me in the man-lift bucket a ham radio transceiver and battery plus a coax adapter (reducing the 5 inch nitrogen filled coax down to a SO-239). I had a total blast doing things like that. A bad moment was when I was in the land-of-sand doing emergency repairs to the feed point of a Hy-Gain RLP (large log periodic, looks like a huge yagi…the antenna is up about 110 feet, and the feed point is all the way out (26 feet from the center mast)….so I am 110 feet in the air, out on the end of the antenna boom, changing the feed-point for 1.5 inch hardline nitrogen filled coax…when I started hearing 1 or 2 bullets whizzing past. Lives were on the line, so whaddia gonna do? Cry and walk home? I tried moving around as much as possible, and my sphincter was just kinda-sorta pinched shut (shoot, I cudda chopped a 10 foot length of 1 inch rebar into 1 inch chunks). Did I mention I hate heights, but I’ve climbed many hundreds of towers thru time. I’m not trying to impress, just saying there are downsides to every job. My spouse and my sister say I need to write a book, but I’d just get into trouble. 😂 Cheers es 73! And please enjoy what you do…life is too short!
@@thomthumbe If you do write a book I would really like to have a copy. I hope you have some photos. 73 OM.
The perfect receiver for the engineer who's out standing in a field.
All this and no poking it with a hotdog? Please make that a running gag for these videos with your dad scolding you for it.
Back in the 80's when I transitioned from landlines to wireless I often thought of how useful RF glasses would be to look at antenna patterns and such!
Geordi LaForge visor...
I used to think the exact same thing! Haha!
I remember doing engineering proofs with that very same model back in the early '90s.
Had the cops stop us a couple of times atoask WTF we were doing, when we were checking the night pattern ...
Haha I can imagine... even their new model with an LCD looks a bit 'suspect'
@@GeerlingEngineering Yes, it looks like a prop from a 50s SF movie.
I just used to tell people I was measuring radiation, and that they should stand far away otherwise they might absorb some.
The more I learn about AM radio the more it reminds me of how EM surveys work for exploration geophysics. Even everything having a specific orientation and going "null" if its 180 degrees out. Unlike radio though we have 3 "antennas" now for X Y and Z measuring the strength of the signal in 3 orientations allows a 3D "image" so it can be a pain in the ass to get lined up correctly.
I cant help but think a random geo was screwing around with an old AM radio in the bush back in the day and figured out that theyd sometimes get a signal with the antenna "pointed" at the ground. These AM field strength testers remind me a lot of the very original looking 2D EM recievers from way before I was born.
7:42 that's because when you hold an antenna YOU become part of the antenna circuit, your body becomes part of the grounding system! That's a dead giveaway the antenna lacks a good ground leg or you're standing on poor soil/rock for good ground. All antenna in theory are some form of a dipole (like classic rabbit ears) with a positive and negative/ground element that rapidly flip polarity as the radio waves hit them, some of them just "hide" one pole/ear to save space by using different tricks. You can often solve that by adding the right length of wire trimmed to roughly match the frequency used to a ground point somewhere, using say an eye-crimp fitting onto a chassis grounded screw somewhere on the radio. Those are called a "Tiger Tail" in amateur radio and often work well for handhelds or portable setups once you tune the length.
Wow! Blast from the past. Back in the mid-80s I was an assistant Chief Engineer at a AM/FM station in Baltimore. I remember driving around in Pennsylvania in the station remote van with a similar unit testing signal strength when we moved the AM transmitter. Thanks for resurrecting a memory from 40 years ago!
Great explainer! And I will admit to that curiosity, along the lines of a 1240 khz "grilled cheese." 😉
Oh my! I had never heard of Potomac Instruments, and it turns out I've lived just down the street from their shop for years!
Same!! I bet it's no coincidence that they are a short distance away from NIST, whose reference it seems like their calibrations derive from.
PI also does lots of specialized government contracting. First time we invaded Iraq, I had a 4 month back order on some custom accessories for one of their phase monitors. They were 100% flat out trying to build elements of the directional shortwave transmitter sites we set up across the borders to send propaganda and info towards Bhagdad, before resuming normal operations.
The FIM-41 is the big brother that goes up to 5 MHz. That works on some shortwave systems, but was mostly used to certify 2nd and 3rd harmonic compliance, before NRSC cutoff rules required a spectrum analyzer.
A major use not mentioned is ground conductivity studies of co- and adjacent channel signals to a potential new AM DA, in regions of the country where rock often makes soil conductivity demonstrably lower than 47 CFR presumptive charts.
The nature of FCC designated DA monitor points as license conditions, usually 1-4 miles from a TX site and based on larger studies after new construction or major changes, could be described far better.
Also missing from this video is how use of the null perpendicular to the received signal is used to evaluate quality of a potential measurement point. A null less than 10:1 is a horribly disrupted spot, or one with power or phone cables or metal structures distorting the signal levels. Get over 40:1 null at right angles to signal, and that's a really clean point.
There's not just one antenna in that meter lid, but two. One is the normal receive antenna, and the other a transmit loop for the calibrate oscillator.
The BNC input could also allow use of the FIM as a sensitive detector, such as with a Delta OIB-3 Operating Impedance Bridge. That could be done at increments above and below carrier frequency, trying to adjust for a flat resistive and balanced around zero at carrier reactive impedance curve.
Try parsing that calibration certificate again. 1 or 2 percent tolerances.... with so many stacked details as to the basis of the reference fields and their use calibrating meters, that the lumped sum of worst case errors could stack into the 20-40 percent potential inaccuracy range, even if far better precision.
Not that I've evern learned to lean slightly, to anchor an elbow in one side, while holding an FIM stable in one hand while fine tuning adjustments with the other.... And then rotate for the null, evaluate point quality, calibrate, rotate back, and measure, and record notes.
Lather, rinse, repeat, sometimes out 100-250 miles from another station's site, if doing ground conductivity field studies Rarely past 20 miles in normal proofs.
It's just a video, not a tutorial
note that Fallout took place in the year 2161, but the title is still technically correct because this device will still be working in 2161
That's right, even after everyone is dead, you'll still be able to tune into AM radio to hear about that jesus fellow
Isn't the big bang in 2077? That's definitely reachable for this device!
Also, inflation was running rampant in the Fallout universe - Thousands of dollars are more like hundreds for us...
I have the FIM-41. It has two bands letting you tune up to 5 MHz. This is handy for checking harmonics of the BC station you are working on.
Ooh, also for hitting WWV if you want...
I like to take one of my AM radios I have fixed up with me when I go for a walk to see what stations it picks up. If I had one of these field strength meters to take along, I'll bet every passing farm truck would probably have some concern as to what I'm doing 😅
Ha, but this radio is pretty good for AM DX! I've heard a few people pick them up for that purpose.
Years ago I knew the Chief Engineer Don Holmes at 10.000 Watt 580KHz CJFX Radio in Antigonish Nova Scotia Canada. I remember seeing him using one of these, I take it, for reports to Industry Canada, DOC in those days. This was many years ago and Mr. Holmes now rests in peace. This Instrument will bring back many more memories. May all the old Chief Engineers rest in peace, and may perpetual light shine upon them. The Reverend Peter Rafuse.
Neat stuff. Back in 1986 I was the engineer for a two-tower AM in Connecticut, and I would take these readings, then tune the phaser to ensure we were in compliance with our license terms. Great blast from my personal past!
Which station?
@@markmalasics3413 WCTF Vernon-Hartford (1170 AM) in Connecticut. Ran 1KW daytime, 500W “critical hours.” That was in 1986. Don’t know whether it’s still on the air.
@@eclexian looks like they're still on the air, they even have a Wikipedia article. They apparently moved transmitter locations in '88.
en.wikipedia.org/wiki/WCTF
Tuning phasers, boy that brings back memories... WSRN, back in the late '70's. They're obviously still around, as Swarthmore College isn't about to go anywhere, though they did go dark for a bit during COVID.
@@spvillano I had no idea there was a Wikipedia page for it. That's neat! Thanks. Just to clarify, the transmitter site didn't move; the studio moved to colocation with the transmitter. I remember building the original studio from WRTT in an office building in Vernon. Moving it to the xmitter site was a smart move. One of the common points of failure (not to mention ongoing cost) was the low-tech unweighted leased phone line STL. It also meant someone was there a lot of the time, keeping vandals away. These were both ongoing problems there.
Also note, manufacturers of sub station and power distribution equipment use a signal strength meter to check for corona. This was at 1020 Khz from what I recall.
I went and found the circuit disgram of this unit. This is no ordinary radio…the bloke who designed this was an absolute analog Wizard!
I thought…(from “fallout era”) that it wiuld be full of valves, but far from it, almost 80% of the circuit is designed around CA3045 transistor arrays. The only bits where descrete devives sre used is in the audio amp and a j-FET at the RF front end with its fate protected by zeners should someone take a reading right next to a tower with no attenuation swotched in!
Another odd feature is it has a Positive Earth, and its rail is -6v that looks to be furnished by four D-cells in series….but the odd thing is, its internal rail is regulated to -6v and the battery can’t be much more than that…so he has used a very low drop out regulator designed around three descrete silicon transistors, two PNP’s in a diffirential configuration and an NPN pass transistor.
It is common to see old semicinductor radios built around PNP germanium transistors and using positive earth with these is fairly common practice because then all the transistors’ emitters (the lowest impedance connection on a BJT) can then go to ground by a fairly short path, but this thing has six PNP transistors in it, viz, two in the reg already mentioned, one for the meter and dial lamps, one in the headphone amp and two in the AF amp. all the rest, four descretes and 30 others, five each in six CA3045’s, are all NPN.
So the circuit has a strange “upside down” look to it.
Looking closer, most of the stages, RF amp, two IF amps, cal oscillator, local oscillator and null circuit are all designed around a diffirential pair with a current sink underneath and a few common emitter gain stages or emitter followers. The RF amp is a very wierd capacitively coupled cascode amp with the drain of the front end j-FET capacitively coupled to the emitter of a subsequent grounded base stage, but has a current sink underneath…..and all this of just 6 volts! So there is barely any headroom to move about.
Three things that make this a very different thing from a commercial AM radio are….lack of any AGC, (and this is why you failed to get a good null with the commercialradio…its AGC was working against you), presence of the cal oscillator and null circuit, which a commercial radio lacks…and lastly…designed by an absolute Wizard.
The loop antenna of this has TWO active ends that are out of phase, one end goes to the RF amp like in an ordinary radio, but the other end feeds into a common base stage in the null circuit. It is a VERY interesting circuit which looks deceptivey simple st first glance, but the minute you look a little closer you see interesting and mysterious things, and the closer you look, the more you see.
One could learn a hell of a lot from this circuit and its clever designer!
And that is where the cost is…along with the calibration and lack if drift over time. Ordinary AM radios can have a multitude of evils that a pasted over by the AGC, but here, all is lain bare and has to be spot on, so thanks for making me aware of this marvellous circuit.
P.S. If you do go snd look up the circuit, it is drwan like a wiring diagram around the CA 3045 packages which makes in even more confusing to read….as if the positive Earth wasn’t enough….but if you carefully redraw it as diff amps with current sinks underneath and follow the dignal path, that will largely “unscrumple the paper it was drawn on” and allow you to see what a triely clever design it is.
Long post, but a lot of detail. The advantage of using a CA3045 transistor array is the inherent matching, since all devices were fabricated at the same time, so the doping of the semiconductor material is uniform for all devices on the wafer. Matching for gain but also for temperature characteristics.
This is definitely a 1980s or earlier design in that it only tunes to 1600 kHz, and CA3045 transistor arrays are used. The CA3045, I suspect, was originally an RCA part (due to the CA prefix), but Intersil, National Semiconductor, and others later second-sourced it.
I have come across the CA3046 and CA3086 transistor arrays in many pieces of test gear where relatively cheap matched transistors are required…had never heard of the CA3045 ‘till I saw it here. Suspet it is probably a lower noise version of the 3046.
Still a bloody clever design though!
A level of understanding I will never achieve, but sure appreciate. I am much more suited to digital, where it either works or it doesn't. I can't screw that up too badly. :-)
You should try an active loop for HF RX; broad bandwidth with no tuning.
I wish I had a buck for every time I jumped over a farmers fence, into a muddy field with a Potomac FIM-21 to get a measurement for a directional antenna proof of performance.🤓
You did get a buck every time you did that.
My father w89job spent many of his retirement years going out and doing field strength readings with various equipment. I'm pretty sure I saw a lot of these in his stash.
That's the unit the FCC guy had when checking my Part 15 station. Also our local KELO-AM is 1320.
KMOX being 1120 I wonder what 1110 KFAB looks like from your area as KFAB is also 50,000 watts.
I'm now calling it official. Your father is now Purple Shirt Geerling. Blue and Red make Purple.
I also notice that your dad through challenge down, and you blanked it. Pi based RF detecting glasses.
We have a guy that drives around once a year or so and gives us reports on our signals. I am not sure who it is or what equipment he uses but the report is quite detailed. As for the "glasses" Ubiquiti makes a spectrum analyzer wifiman wizard that lets you "see" the RF as it maps the room/building you are in. You need an iphone with the lidar on it to take full advantage of it but it's a neat tool. Also laughs in 80 year old grounds sitting on rock basically for my AM tower. Chasing RF was fun till I went nuts with new grounds.
Probably uses something like this! (Or maybe the newer model with built-in GPS).
@@GeerlingEngineering Probably the modern equivalent. When I first saw what you have here I thought it was like a meter my silent key mentor gave me. I have only seen it used once big meter with dials goes across the tuning coil of an AM transmitter and us it to set the matching. If you want I can take pics of it. Been meaning to show it to some radio groups.
@@JamesHalfHorse Ah yes when you tune an AM tower, you usually have one of those massive boxes with a couple tuning knobs-my Dad has footage from a few sites where guys would set up towers with it.
@@JeffGeerling I need to pull it out and take a few pictures for the radio show and tell groups and maybe shoot you one in email if you like. I am not sure exactly how it works only saw it used once when we replaced the AM transmitter. Mark P designed a new coil for it to match the tower, I built it and then the more seasoned engineer helping us put this his own version of this thing that I think has a preamp in it mine don't across it to set the taps and it's held a 1:1 on the swr since. Okay 1:12 or so after I had them take a 2 way antenna off that is now my gmrs repeater antenna but not enough to tune out. It's the old way of doing things, I don't need it any more even if I did know how to use it. Last times transmitter had problems my rigexpert read the 80 year old shunt fed tower with bad grounds just fine matching the reading the Nautel gives me just to check for any major problems before trying to send RF up it. It like my other mentors old Simpson meters will likely go on the shelf of my shop their tools are kinda my memorial to them even if outdated and consider it an honor to have them. The bird meters however do get used but came from someone that holds some of my deepest respect... I think that is why the new Ipad commercial was so painful for a lot of us yeah there is better tools but the old ones have meanings and stories to tell. I hope to talk my remaining mentor into letting me stick a camera in his face for a few hours and say tell me your 70 year career in radio. A lot of that information is dying with these guys and it saddens me.
When I was a kid back in the 1960's, I made a medium wave loop antenna, with a variable tuning capacitor, so I could listen to US East Coast AM radio stations in the UK. The setup allowed me to null out interference from European transmitters and listen to US content. One station which I listened to regularly during the winter months was WKBW Buffalo NY, which is still going strong.
The SONY radio did hit a null on the 1320 station, but the AGC hid the signal strength dip pretty well. Listen closely as the radio is moved around; the interference level goes up in the null.
I worked for the Civil Aviation Authority here in Australia, we used to use the Potomac to quantify the output from our Non Directional Beacons (NDBs).
Certain older pieces of test gear are still considered a valuable must for calibrating particular types of equipment. This is why for instance a particular HP VTVM model is considered more desirable over another.
Being able to accurately compare antenna's signal strength is nice to have. I've gone down the rabbit hole and now own way too many meters. With TV signal strength meters, ones that can measure ATSC and QAM, they will show you the bit error rates and constellation. You can get a really good idea of he health of your antenna and amplifier setup. Gone are the days of being able to see ghosting and intermod on analog TV.
Stop being so interesting, Jeff and dad! I need to get back to work! Thank you so much for the awesome videos.
Prefect ending to the video. Both father and son, at the same time: "... see what happens".
A.M. died here long time ago in Europe , now they want to get FM out off the air. DAB + remaining as "radio" .To me this sounds like a bit like censorship.
Hmm , a feild stregth meter didn’t used to cost that much years ago . I used mine to match antenna to radio RX/ RT two way system . $10 or $20 or so . Seems a bit excessive to me . Because the operator in any case has to calibrate the device himself any way . But ok , what ever .
Brain surgery can be performed with an Exacto knife too. No need for suction, sedatives, restraints needed either.
When my AM went dark I still have mine, AM is the greatest modulation method for long distance. Long live AM
I was lucky enough to be able to use one of these back in my radio engineering days in small market radio in the 70's and early 80's. Thank you so much for bringing back those memories!
All terrestrial radio costs too much money! But you have to love those still involved in this technology.
Ooh, an illuminated 4.5-inch Simpson rectangular style panel meter, with a mirrored scale! I'm in love!
I got a Simpson rectangular style panel meter, on a AC meter I found out near an airport on a farm beyond the runway... measures volts... or amp draw through it, with two switches to switch between 110/220, or the scale on the amps.
This Potomac FSM is built for the MF AM broadcast band. Other versions work on the LW Aviation Navigation band or the SW bands. All on first appearances look identical and all are still in high demand by the radio industry worldwide today as it is one of the few devices around that can give a accurate calibrated repeatable numerical value to the received RF signal strength of a selected radio station, thus the high price.
Because someone put thousands of dollars inside?
I'm surprised that there are so few field strength meters in the U.S. given that theres almost 5K AM broadcsat stations and ~6K FM stations. Do the stations share them amongst each other?
Often stations hire an engineer with the good and up to date models of test gear. A lot of stations don’t have a full time engineer and hire contractors who have the gear.
How about touching a four foot fluorescent bulb to the tower?
Once the gas in the tube fires, it becomes a low-Ohms conductor ... the reason a fluorescent bulb uses a ballast in series. Actually touching the tube could be an issue. Normally they just light in the proximity of strong RF signals. No need to touch ...
Another VERY VERY interesting video!
Loooove this content! :)
Excited to hear about you getting into SDR, looking forward to some videos potentially? (:
Definitely!
@@GeerlingEngineering looking forward to those! I've just started to get (back) into SDRs, and I've got the V3 of yours that doesn't have the built-in upconverter for HF/MF, it just uses the direct sampling method... Been thinking about if I should upgrade to the V4 or if the V3 performs about the same!
@@KenPiper save up and just get a SDRPlay RSP1A, you'll be far happier
This brought back memories. Around 50 years ago I went out with the engineer with a receiver, but it used tubes and it was much larger and heavier. We put it on our shoulders. We spent the whole morning doing just one lobe. Both of us had our First Phone licenses but I had never worked at a station. It was just as you described. The station was a 3 tower array in the RF ghetto of 1410 KHz. :) I guess we were testing the daytime pattern, but I don't remember for sure.
I was checking at 1470. Our station jammed KJR 950 on car radios. A car radio (262.5kc IF) tuned to 950 was also tuned to 1475.
@@Iridium43 Yeah, images have always been a problem in those low IF radios. But all the car radios were 262.5 KHz.
Was that from someone driving by the tower?
I used to get the second and third harmonic from a station on 1350 about 5 miles from the tower.
Awesome as always but what really struck me was the joke about magic glasses that allow you to see RF. I remember day dreaming about that early in my RF nerd hobby 30 years ago. I don't think I've ever heard someone else mention the same idea. Made me smile.
Maybe someday we'll have some awesome augmented reality tech 🤞👍
I have many different boxes for field strength readings, but most are for aligning the receiver antenna for best reception. They work on VHF, and UHF TV and also FM radio bands, as they are fun to use to do antenna installs, kind of like the units for Satellite TV dishes.
Hey Jeff, have you come across Meshtastic / LoRaWAN yet? Thought it might be a nice crossover between radio and raspberry pi for you and your dad to cover and experiment with!
We're actually talking to some folks from the Meshtastic project at Open Sauce!
@@JeffGeerling Ah cool, look forward to the videos then!
I used to work on these (and the FIM-41) when they were still located in Silver Spring, MD
By the way, the loop that you are using had a measured Q of about 800, when we measured it. Works well, and cheap.
Used one in the mid 70's Pattern checking AM broadcast station. That's a precision instrument.
In the early 60's I read in an electronics magazine that if you held an AM radio with a ferrite antenna upside down you would lose the signal. I swear at that age it did work as they said. This may have been the April Fools edition. Like the Black Noise Generator made out of a black bean soup can. Unfortunately, I haven't seen an AM portable radio in so long to try it. Yes, I do have a ghost detector.
Used many times. I do not miss the days of NRSC proofing. Retirement is such a lovely thing.
I get it! No more ticks and chiggers either!😂
Great video! Maybe next one you could ask you Dad to show us how you check the AM mask with a spec analyzer. RF is just electromagnetic radiation, a couple of orthogonal fields in motion. Still, it fascinates anyone with a sense of curiosity. Also, maybe you've already explained this, but lots of folks have no idea how to properly hook an antenna to a television receiver. Carry on! de NX8J
Bullshit ... a lot of listings ~ $500 on ebay
Any of the models with valid calibration certificates (that aren't expired) will go for considerably more... unless you're lucky! If you find a calibrated model for under $1k please let us know, it'd be nice to have a 2nd!
Oh the classic Potomac field strength meters. I still see them on eBay for an arm and leg.
NIST Calibration
The successor to this is the Potomac Instruments FIM-4100 which is a modern software controlled digital readout instrument which is really great to use. Problem is the high price. Over 14 grand. It makes the analog FIM 21 and 41 look pretty good. Although no longer manufactured used ones can be found on used equipment sites for 2 grand or less. Calibrating the older analog ones is costly and Potomac Instruments is the only company that will certify the calibration.
Guys, this is right up my alley! But I´m a noob, here in Germany AM is dead, exept for some Turkish and Chinese stations. Could You do an antenna giude for dummies who don´t have any sphisticated gear like this. Btw it´s very cool that the company is still around to calibrate it.
I´d love to just scroll thru the scale for some Johnny Cash... Yeah, no, I´d love to get by with the least financial expense- so to say.
I collect old tube radios, still use my FM receiver and cannot imagine a future without analogue radio. Kind regards
When I lived in Chicago, I was able to watch TV stations over 80 miles away with rabbit ears and bow tie by picking up the bounce off the Sears Tower. I was on the side of the building away from South Bend and toward the building.
I bought a small digital-ready television in December of 2007 in anticipation of broadcast tv going digital in 2008. I have yet to take it out of the box. AM and FM radio are all I have been into all this time, plus some UA-cam, obviously, on my android. Some one of these years I will get around to unpacking that television. I sure hope it works, because forget about the warranty. I often eat out by myself, so I sit at the bar. I look at all those television screens and realize I'm not missing anything.
Put your camera on a tripod!!! You're making me sick with your camera jumping around...................
Is the Dad's knowledge being taught in a tech school or college anywhere? This is science but I fear that it's becoming a lost art. Are there engineers working for the big radio conglomerates with these skills and knowledge sets? I find it all fascinating. In my market, I've almost always preferred the programming on AM radio. Sadly, I know the audience is dying off.
On receiving am. There is a long wire as the antenna and a ground wire. They are not connected. Loop antenna are connected.
With experience, when young a loop antenna don't really work on a radio that is designed for the other.
I have also noticed that antenna very with thickness of wires, many are tubes and different materials.
What's the deal there.
Can I just use a loop copper wire for UHF. Shorter wire of course. Or a VHF with a single wire and ground.
If there fundamentally differences what is being used besides then just directional or not.
Maybe something is more suitable how the signal is being carried.
Potomac is pronounced puhTOWmik. It’s the river that runs through Washington DC.
Cool collectible radio, however I can do the same thing with my cheap AM pocket radio by turning it. I know where the null is, so I know where the transmitter is and my signal strength meter is my ears. I do really like that tunable external loop antenna, more than the radio. I might build one of my own. Have been meaning to do it some day. You can also enhance reception of a radio by putting it near the loop even without a connection.
Similar a fricking 50ct screw costs $500, because it's FAA aircraft certified or something.
Unfortunately all I see happening as a result of this video is negative. People will think these are toys. Now that the masses are “ aware” the ones that are available for professional use will disappear.
They will be bought for the cool factor only.
Have you considered using the AM tower as a Malehood compensator hhehehe, just kidding.
Foolish things aside, would it be able to roast a chicken properly? And would a chicken roasted using a music channel taste different than one using a politics channel? 🤣
I have no idea or knowledge of any of what is talked about in this video.
I'm here because I enjoy to see a son doing stuff with his father, that is so rare today.
And who knows I might learn a thing or two :D
Thanks for linking to the loop antenna in the video description! 👍
id be really surprised if your meter would get kmox 1120 up here in my town, canton illinois, which is on your map at the top.
Pretty cool, but I can't watch the whole video because I can't get past you constantly talking over your guest.
Plenty of time spent cradling this thing doing monthly monitor point readings, partial, and full proofs. So glad I don't do that any longer!
Very popular 1945 August , in army green colour , powerful , durable
a spectrum analyzer used for the same purpose today is much more expensive, maybe 10-20 (but ofc does many more things)
calibrated antenna alone will run you more than the entire device
AM demod option will be a few thousand, too, if you don't just want to look at the signal, but also listen and do more specialized analysis stuff
This instrument would be good in broadcast engineering. It can do a screening measurement before bringing in the big guns($$$).
That is one of the most expensive radios I have ever seen, and I used to be a meteorologist many years ago. We had calibrated RF "radio" equipment used in an RF chamber to measure both conducted and radiated emissions. You have the most expensive hotdog cooker ever! As a youngster I used to cook hotdogs with a board, a couple of bent nails, and a killer cord! Those were the days!
Metrologist perhaps?
Metrology - Metrology is the scientific study of measurement.
We use FIMs in the wireless industry to determine if the detuning on wireless communications sites is working, not messing with the AM station pattern.
What about FM?
I don't mess with that newfangled sorcery.
There was no time in history known as "the fallout era." You're thinking of fiction.
Boring video at least you could have opened it did some explanation of the schematic.
I want one.
IT'S NOT AN AM RADIO, IT IS A FIELD INTENSITY METER, MADE BY POTOMAC INSTRUMENTS. NO JUNK CHINA PARTS
AM, Ancient Modulation. Except for Air Traffic Control.
Potomac Instruments are still in business and moved to Frederick
AM broadcastings in US, 2024 ? In Brasil is rare.
THE FIM41 HAS SHORTWAVE FREQUENCIES AND IS MORE EXPENSIVE
Because there are still some people that will pay for such things.
Have you ever seen a signal that you were not able to identify
Seems like you two have got your calibration set as well.
This reminds me of my Grandfather. While he was living in northern Ontario Canada around the mid 30's. He would turn his AM radio on after dinner and sit in the corner so he could see up the road for oncoming headlights. He decided not to pay the radio license from the hardware store, so if he saw headlights from a vehicle, he would shut the radio off. The bylaw was enforced by a vehicle that had a directional antenna that could detect the intermediate frequency from the radios when they were turned on. Imagine a knock on your door and someone asking if you have a license for that radio!.☺
re: "hat could detect the intermediate frequency from the radios when they were turned on"
Heh. I'll bet they had better results looking for the LO (Local Oscillator) in the radio rather than the IF. ALSO, the LO 'leakage' from an FM radio is MUCH more pronounced (a lot stronger) than the LO leakage from an AM radio.
@@uploadJ That's a good point. He had a directional style antenna and pointed it out the window while driving. I think it was just an AM radio he had at that time. I still listen to a few of the old radio programs on the internet archive.