Ham Radio - Magnetic loops and height. Do they change near to the ground?
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- Опубліковано 8 лип 2024
- I have been curious, to see if magnetic loops change tuning or performance if they're less than 1 diameter from the ground. So I attempt to compare with mine at 5 feet off the ground vs 1 foot.
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I think the change with close proximity to ground is that the loop loses directionality. I put a receive loop for WWVB down in a basement to get away from local noise, and it worked much better.
Yet another GREAT video! If I might add a comment for one reason to run a loop high on a pole.
With a loop even at QRP levels, one can have RF safety hazards, just because you're at low power does not mean that the RF levels are safe. Magnetic loops can have massively high currents and voltages creating incredibly strong nearfield B and H and thousands of volts. according to the math, when using a compact loop for CW or processed SSB (i.e. 30-40% duty factor) and QRP power, a distance of 2 meters from the closest part of the antenna system provides a reasonable margin of safety for controlled exposure. Casual observers and bystanders (e.g. other people at a park when operating portable) should be kept 10-15 feet away. At 100 Watts, or if a high duty factor mode is in use (FM, RTTY, PSK31), the distances should be doubled at a minimum.
H=(IR^2)/(2(R^2+d^2)^(3/2))
Where
H is the magnetic field in A/m
R is the radius of the loop
I is the current in the loop
d is the distance from the axis of symmetry
FCC limits 0.163 A/m maximum permissible exposure (MPE) for controlled environments in OET bulletin 65b
73
AD0WQ
Agree had minor RF burns on a home made, uninsulated loop at less than 10W - when I touched one of the feed points
Great test Kevin! And a bit of surprise on the result.
I moved from an ideal location with multiple rotating towers at 4000 feet land elevation on a steep mountain that dropped off 2000 feet to both east and west and on a clear day could see the Pacific Ocean 200 miles to the west, to a city center of a large city in Eastern Europe where no outside antenna was possible. So my only viable option was an interior mag loop and after lots of experiments found despite the 3 ft thick brick and stone walls, it worked surprisingly well with 10 or 100 watts. I am on the second floor of a 5 story building. So the Mag loop is a very practical antenna for unfavorable conditions.
Love your videos :) very well done!
Interesting. Chameleon states lower SWR if their loop is raised - though do not tell how much it should be raised. One loop diameter is commonly agreed upon.
Had the first test of my F-loop 3.0 today. Placed. on a tripod about 1.5 meters above ground, i made a clear contact on 10W SSB from Dennark to Canada - about 4.000 miles/6.400. km+
It did DX nicely even though often described as a NVIS antenna.
Good video Kevin, Thanks for posting it.....
Thank you for another great video!
Very good once again Kevin with the bonus of seeing the local geography as you travel around. Good luck, John G7RWF
Thanks for sharing, really to learn from the experience of others, rather than starting from scratch
I worked a station on Chagos Archipelago who was operating with a loop like this. I received his QSL card with a shot of it resting on a park bench, with an ocean view. Of course this was during the sunspot maximum a few years ago.
I'm using MFJ 1886 with SDRPlay RSP 1A. I bought the tri-pod along with it extended to about 5 feet out in the back yard. I do a lot of SWL, no ham license, and I determined the loop to be quasi-directional! I get North and South, etc. when I move it around; I'll add elevation this Spring or Summer to around 30 feet. I'm in North-East Kansas and on a good night I can hear RNZ using the amplifier. Going to turn it East and will get West at the same time. Mag loops are pretty good to work with; summer time will probably build a more directional antenna or put up an inverted V just to see what happens. I get a bunch of foreign language Asian and Spanish like languages will see what happens with broad side of loop if it really cuts down North and South. Have a great trip around.
When modeling these antennas, it takes far more of a change in height at 20 and 40 meters to see the changes that will happen. A change of only four or five feet really is insignificant at these frequency wavelengths. I wouldn't expect this change to show any difference on an s-meter.
Hi Kevin.
Interesting experiment with the loop.And yes those sdr remote receivers are very useful for monitoring your own signals.I always check using that method when I'm operating on HF from portable locations here in the UK . 73.G7HFS
Another good video and it looks like your not the only ham out there looking at the other antennas there.
73
WD4DDA
VERY informative -- Thank you!
Thanks Kevin. I never heard of WebSDR before this. Have no idea what Hak 5 was talking about but I'm having fun listening to English stations in my part of the world.
Those SDR websites are great! I love it. Anyone online can tune a radio and listen!
Heeey Kevin great video I fell in love with that FT817 base I want one ;)
Greeat and interesting video, Kevin. To me- this is real ham radio. Thanks! I wish I had a loop.. I'd love to try the same test (on Rx anyways) while tuned to WWV, using a step attenuator, and my ftdx3k's S meter in analog mode. Great place for antenna fun you're at. Have fun! 72 OM
Hi Kevin, like your video's I really like your stand for the 817 take care and enjoy yourself
I found it on Thing-i-verse and 3D printed it.
Yo Kevin:
Great video again! I'd love to see that elevated on the pole off the back of the RV. Maybe even rotatable! See what directionaliy it may have. Maybe even put it at the end of your end fed wire? Great experiments to try eh? Great info to know about loops now eh?
Motor on! 73's!
de seeker/Jeff WA7LFP
Looks like a good application for an arduino. Have it find the best direction to point the antenna and use some NEMA 17 to get it pointed right. Also if you bring a crank up pole, it might test for the sweet spot as far as height.
Hi Kevin, I certainly enjoy your U-Tube videos. Living in an HOA I am antenna limited and have an MFJ 40-15 tunable loop hidden on my flat garage roof with a loop center height of about 15 ft. This antenna has performed surprisingly well. I was curious about its good performance so I modeled it in EZNEC. (I make no claims as to my modeling expertise but the model seems to compute fine) What I found, for what it is worth, is that Mag Loops do seem to be height sensitive in performance in models at least. With increasing height resulting in better performance. At 15 ft my model vertical oriented loop shows modeled performance comparable to a quarter wave length vertical with four raised radias. This seems comparable to my real world performance. I Know real world antenna results and models can be quite different but I thought i would share this as an avenue for further research on the topic. Thanks for the fine videos. Cheers & 73 Jim
The key word is "magnetic". A dipole uses the electric field, so is affected by ground proximity. I would not expect a magnetic loop to suffer from the same problem.
Hi Kevin,
Nice experiment. Most recommend having the loop at least one loop diameter above ground. I have about 40 ft of 3/4" refrigeration tubing and a vacuum cap of 20 - 1000 pf. May make a 12 ft diameter loop for 160 - 40m. Not sure if the inverted terminated U antenna was working well during the CQ 160m SSB contest. I made 8 contacts to NH, NJ, MD, and DE from Philly. Heard many others, but no contacts. 73 WB3BJU
Good test! I need to do some more loop experimenting. I have some materials I want to try. I made a loop using half inch wide copper tape with conductive adhesive, on a substrate of polyurethane plumber's pipe strap, but it's too soft and needs too much physical support. It makes a dandy loop if properly supported, though. So I'm going to experiment with layered wood edge veneer, 3/4" wide, with preapplied hot glue. Two layers was pretty stiff, and could be shaped to a nice freestanding curve. Maybe four layers, with the copper foil sandwiched inside, will work well.
I also want to try 3/4" aluminum BX cable armor. No wires inside, just the flexible metal armor, for the main loop. I'll probably use the same coupling loop from the pipe strap loop for the feed point, though. I've got some ideas along that road as well.
I have to wait for finances to recover from the winter, though. I just had some significant expenses, and need to let things recover.
Hope to work you someday. You're doing something I've wanted to do for years -- travel around in an RV, working radio and meeting new people.
It sounded to me that when you were receiving WWV on the ground, it was much clearer than when it was up higher on the pole despite no change in sig strength. I’m currently going back and studying you mag loop videos since my wife and I are now full time RV’ers.
Maybe physical ground level is not electrical ground level in case of very dry desert ground? Can you dig a trench 6 ft deep and try again :) :) :) But maybe there is a narrow deep ditch around?
Looking forward to further experiments if possible to obtain that 40 ft difference. Would be very interested in seeing the results!
Hi Kevin - Thanks for making and sharing this and other excellent radio-related videos. I’m recently retired and decided it would be fun to look into radio technology. As a first project I built a small(13.5 inch diameter) single loop antenna using 1/4” copper tubing. I’ve got a variable capacitor (20 - 370 pF) connected as well. My intention is to use the loop in receive only mode. Given this, does it make sense to couple to the antenna with a small loop, such as the one shown in your antenna, or is that only necessary for transmitting, where impedance-matching is critical? Thanks very much! - Jim
I have found that having set the shape etc, of the coupling loop to optimum at approx. 2 diam. above ground, lowering
it to less than 1 diameter makes achieving a good VSWR difficult if not impossible.
The ground effect or lack thereof might be different in wet Ft Wayne soil vs the desert there.
Thank you!! Do you have a tutorial on magnetic loop antennae? This seems like such a mysterious thing to me.
I have many videos on them. Search youtube for my call and the word magloop.
As Kevin kindly pointed out the current in a mag loop is highest at the feed point and voltage is highest at the capacitor. Sorry all for the error in my comment
Thanks for your contributions. The question that your experiment cannot solve is what happens over more conductive soil. It looks like the location used is at the extreme of being dry and arid. I suspect that the soil conductivity is very low. More typical soil of a type that might support vegetation likely to be much more moist and conductive.
Ground interactions and height effects on propagation might be much greater.
I must have missed the video on your radio mount tilt base for your radio . Is that a 3d printed tilt ... Nice video.
Didn't actually do a video, I didn't make it. I found it on Thing-i-verse.
Just noticed this video. Too late, but I would love to see your mag loop near that Airstream, to see if there is any impact with all that aluminum. I’m considering buying or building another Mag loop for camping and was worried about all the aluminum interfering with it. I had an AlexLoop before and my truck interfered with it, I couldn’t use it elevated above the bed, I had to move it fairly far from the truck to be effective.
What about mounting the loop horizontally? I.e. the plane of the loop tilted 90 degree, to make it horizontally, in parallel to the ground. AFAIK the radiation pattern of a mag loop has the shape of a doughnut, like the loop itself. Mounting the loop horizontally would make it a good omnidirectional radiator in the horizontal plane. When mounted vertically, like you did, part of the radiated power gets send into the ground and part of it steep up into the sky. But good HF propagation is achieved with a shallow angle. With a horizontal mount you would get much more radiated power in that direction.
I have played with the loop in a horizontal plane. It worked, but was much noisier due to the omni-directional sensitivity in that configuration. It behaved a bit more like a vertical that way.
@@loughkb Thanks for the confirmation. That is exactly what I expected from my uneducated guess.
Great experiment. Also, is the Airstream in you video a ham?
There's several hams parked around here. They call themselves, The Rat Pack.
I would have been interestes at 15ft, 6ft, and ground and in transmit directivity
@6:00 'Whose Boat is This Boat?
Does putting the loop lower increase the NVIS ability or does that remain the same also?
Loops radiate in a toroid pattern, the same as their shape. If you want to use one NVIS, just turn the loop so it's perpendicular to the ground and it will radiate upwards.
Should have tried WSPR at 5 W.
good video Kevin, thankyou. Would be interesting to put an analyser on it it in the 2 positions to see if there is any difference in resonance. I also have a similar loop here (indoor)...check my vids. All the best and enjoy the "ham-desert". 73, Nick M1DDD, UK
After reading some of the other fine comments I thought I would add that the EZNEC Loop model also shows better performance with the capacitor at the top -- I think because the highest current is at the top and a little higher above ground. In both the model and my real world situation the loop shows , as expected a strong null perpendicular to the plan of the loop. I live near an rf noisy downtown area and the real world null of local city noise is about 8-10db on my Elecraft P3 panadapter. As a speculation, I wonder if part of the reason Mag loops get reported as having such good performance when experts often pan them, is that they act as very sharp preselectors on receive with a relatively small band width and pretty sharply drop off of signal strength outside the 2:1 band width. This should give improved signal to noise performance even if, as experts say, the overall receive performance antenna efficiency is lower. This certainly seems true of my particular situation compared with a 15ft high non-resonant stealth dipole. As the fine results of QRP operating show, transmit efficiency is often less of a real world effectiveness factor than other considerations. Cheers & 73 Jim
It's my understanding that the voltage is highest at the capacitor side of the loop and current is highest at the coupled side. Jeri Ellsworth recently demonstrated this in a video also.
I would expect radiation to be highest where the current is highest. Lot's of debate over this though.
I have built with the cap at the bottom, mostly for construction convenience. Always had pretty surprising performance too.
@@loughkb Hi Kevin, Right you are!---sorry to have contributed to any miss information. The voltage is highest at the capacitor and current highest at the coupled side! Getting old I guess! Thanks for the important correction.
I know that guy running around in the background..lol
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Kevin......Have you considered placing some aluminum foil on the ground below the loop?