009 Does a simple Quarter-Wave Stub Reduce Interference?
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- Опубліковано 11 лют 2022
- I have a problem in my shack. I discovered it when I wanted to decode SSTV pictures from the ISS using my SDR receiver. It did not work because I had a horrible interference on pretty much the whole 2m band. Obviously, there is a strong transmitter around my home. Interested in the less than 5-dollar solution I found to reduce the problem?
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Over the years, my ears have developed an adaptive filter for that Swiss accent. Thank you for producing material that made that very much a worthwhile endeavor.
Thank you for your nice words! I appreciate it!
I made a stub for 144.3 MHz to stop my VHF TX from stopping TV working. Cut the stub, tuned it, and fitted in the loft! Perfect!
Then the TV stopped working. Turns out that 721.5MHz (5th harmonic) was the frequency of the local BBC DVB-S transmitter! 🙈 It attenuated that perfectly, too!
Moral of the story, don't forget about the odd harmonics of the stub! They'll be attenuated too!
George M1GEO
True. This is the advantage of modern VNAs. We see what happens! It is also much easier to tune a stub. I would not want to tune it without a VNA. The last cuts were very short….
@@HB9BLA Yes. I was using hard-line coax and a file at the end, with a Keysight VNA. And of course, zoomed completely in at 2m band with no care for 750 MHz! 🙈
Really enjoying this new RF-centric content!
Thank you for your feedback!
That fancy SDR unit is amazing!
MANY years ago when I was a kid I lived in a rural area with a local (5 miles) channel 8 but I wanted to watch channel 9 but that station was 65 miles away and channel 8 interfered and bled over. I made a 1/4y open stub and added it to the twin lead input on the TV. It worked so well we had to rotate the TV antenna to point at CH to watch it without snow. 73 WC0H
Also other commenters wrote about this use case! I think it was an easy and cheap solution (and it was even easy to find the right length because you had your analyzer connected to the antenna (the TV set)
I cut it twice and it was still too short!
Thanks for another interesting video.. I really enjoy the physical nature of RF, I write embedded C for power electronics and it's fun to see the code I write make things get really hot, bright or loud, but just cutting stuff with pliers to make a filter.. that's really hands on!
That is why I thought it might be interesting for my viewers…
When I was young I worked at a TV shop in a small town and we did something similar. The closest TV station was 35 miles away, but there was a college FM station was downtown. Back then TVs all used 300 ohm twin lead so our traps were made from this, but the idea was the same. We installed hundreds.
Cool. This for sure also was not an expensive solution!
No, the next airport is 30km away...
4:55 That's exactly what my hairdresser said. ;-) And if I look at your picture, I think we both could have the same hairdresser. Thank you for sharing all your knowledge to the world Andreas. 73 de PB1SAM ...-.-
You are welcome! 73
I know this is 2 years old but something I didn’t see mentioned. If you add a second 1/4 wave stub spaced 1/4 wave from the first you will get a deeper and sharper notch. I don’t know how far you can take that, might get unwieldy after a few. You can make it tuneable also by adding a variable capacitor across the far end.
Great presentation! A few years ago I did the same stub filter! It works flawlessly!
Cool. Indeed, they age good because it is only a bit of coax…
Nice video, thanks for the reference to my video.
You deserve it!
Thanks for this. I've been following you for years, but after discovering the world of microcontrollers myself, I only sporadically watched your videos. This new ham-related content is absolutely perfect. Your practical style of explaining things is very helpful. 73 DE OM2LT.
Welcome aboard the channel!
Andreas, thank you for another great video. I'm very impressed with your wall of cables. I'm a big fan of crimp connectors but have never attempted them on thin coax. 73 Lloyd - W1LBM
It is very easy (much easier than soldering), particularly for the small cables.
We used this trick to drop a commercial data transmitter level that was plastering a UHF receiver at a commercial radio site once. Learned a lot about coax transformers back then! Fun stuff! Cheers!
Indeed. Coax is an interesting topic! And these stubs are really easy to make...
Glad to see you are now adding new videos to this channel rather than just moving old ones over. If you in the future are adding ones from the archives, could you mark them somehow, at least in the description so followers from your other channel know that it could be a repeat? Cheers!
They are easy to distinguish. Just look at the hat on the thumbnail… Also the writing is different.
An excellent presentation on 1/4 wave stubs. I've only made a couple, but I was pleased with the results. They are simple, and they work.
I suspect larger diameter coax would have a higher Q and provide an even narrower filter bandwidth, and heliax would provide the best. I've ignored the small pieces of it I have had access to in the past simply because I thought it to be useless, but next time I see some I'll grab it for stubs.
You are right. Thicker coax has a higher Q
I live within 5km of ~10 FM Broadcast stations, and anything better than the stock rubber duck would render a UV5R deaf with front end overload. Made a 5/8 flowerpot antenna, feedline, and coax stub tuned to 98MHz using some cheap RG6 I had lying around. The stub worked well enough that I managed to score a contact with a station around 50km away on 2m.
I've abandoned direct conversion and moved on to superhet radios since then lol. 73
In my last video I showed a stub, too ;-) But against a pager.
Larger coax = higher Q filter. Velociy factor changes with frequency for the same coax cable, the published vf is no use away from the frequency they tested it at. The quarter wave stub is a very useful and interesting thing, 1/4y transforms open circuit to dead short, so shorted end behaves like open circuit at resonance and vice versa. Very useful with a PIN diode for switching. Often used in microwave circuits for obtaining a DC ground.
Good to know about the Q factor. I have to check if I can make the same stub with PL259s and a bigger cable...
The beauty of the VNAs is that you do not need to trust the VF calculations (other than cutting the coax long enough).
I find PIN diode switching very elegant, but never tried it myself.
"Strange Swiss distortion is the signal"🤣
Welcome aboard the channel!
Love this! RF is like black magic to me. I follow along and understand principals, but trying to wrap my head around the reality of whats going on, then boom magic. I use to run a CNC shop years ago and we machined a lot of different cavities and filters and I just saw screws going into empty cavities and was like what is this mystical magic!
Indeed, the combination of mechanical designs and waves is an interesting topic!
Nice video!
I have seen coax stubs used on HF when having a group contest station, and it works.
If you were to use air dielectric coax, or make your own coax cavity, it would work much better than the coax stub.
I have seen where the cavities were silver plated to increase the Q.
RF is fun!
I agree with the cavities. They are also used for repeaters!
Thanks for sharing, had no idea you could use coax cable to form a filter!
Indeed, I also find it a cool idea!
Excelent. I have the same agilent and this notch is helpful in many case. Bravo
And simple to build…
@@HB9BLA yes.. (,even if you loose 3dB / for the splitter). 👍
Awesome thank you 😊
You’re welcome 😊
What a brilliant yet simple idea! Thanks for sharing! 73! Linas LY2H
You are welcome, Linas.
Six meters used to be a problem band to transmit on because of interference with VHF over the air TV signals. Similar filters sometimes helped if the TV wasn't too close to the six meter transmitter.
I can imagine that this was a problem. TV was below 100 MHz in the first years… so far I never tried 6m. It was not allowed when I was young and now I have no antenna.
Stubs are good, as are bandpass, however for this application you might want to look at a notch.
As always another excellent video, thanks.
Actually, this stub behaves like a notch filter. Just not steep enough for this application :-(
What if you use an open quarter wave at 147mhz. Or possibly 2 open quarter waves at 147 spaced a half wave apart and a short at 145.
@@joecarty8579 That's something I used to avoid receiver overload on some Motorola equipment caused by the nearby airport transmitter. Pretty effective solution.
You can make the notch deeper by wrapping some Al foil around the stub. You have to experiment with the length of the foil and its location on the stub. I've seen an extra 20 dB of rejection if you get the size and location just right. :-)
Thanks for the tip!
Interesting that you mention the aluminum foil. When I was a teenager the cable company would scramble Showtime unless you paid for it. If you paid for it you got a box to filter out the frequency they mixed in to scramble it. We would take some twin lead hooked to the antenna input and slide aluminum foil up and down on it until we filtered out the scrambled signal to get free Showtime so we could watch. It probably wasn't legal but it worked!
@@gibbywankenobi66 We did the same thing. It wasn't illegal if you called it a school science experiment!
Your videos are pushing me from my Arduino Nano to my NoolElec NESDR Nano . It is still in the shipping bag.
That is another complexity, I fear. But a lot of new possibilities.
@@HB9BLA I'm not sure what I'm doing. I have a box of cheap UHF modules mostly 433Mhz. One year into it and its taken me down many roads. I bought the SDR device to examine the variations in the transmitters. The 2.4Ghz stuff has me working with Arduino. It's busy fun.
GREETINGS from Va. W4GDM
4:41 something you forgot to explain very well about coax VF for a novice, different types of coax material have VERY different velocity factor percentages! But you can always Google that coax to get a close enough estimate! 😁As an example between the "same" coax... a favorite "fancy coax" here in the US, LMR-400 has an 'advertised' VF of 85%, but I've tested a cheaper version called KMR-400 and found it had a 90-93% VF and MUCH lower TX losses at a fraction the cost. Of course that means even using the "same" series of coax the lengths would be quite different.
I agree. This is why I use cut the cable too long and use the VNA to decide on the length ;-) (Then you can calculate the true VF)
Funny i have a similar problem at home. I live very close to the local tv tower which houses a strong DAB Transmitter at arround approx 180Mhz. Weak Receivers like my SDR Play or a the direct mixer of my Anytone Handheld are getting overloaded as soon as you connect broadband antenna. Adding a quarterwave stub to the feedline solves the issue.
Cool! 180 MHz fortunately is quite distant from 2m...
Many thanks. Also helpful is a shorted stub, which serves as a bandpass filter, passing the frequency it is cut for. Many years ago I worked at a USAF radar site that had various radars from high VHF to S-band, all blasting out multi-megawatts. Concerned that my 2-meter transceiver would get fried, I created a *shorted* quarter-wave stub at 146 MHz and never had a problem. Any signal outside that range and its odd multiples was seriously attenuated.
I have some old coax around and I'm wondering if the same idea might work on the HF ham bands to reduce out-of-band interference and offer some protection against lightning, a problem in my area. Each antenna would have a shorted stub tuned for that band.
I tried a shortened stub instead of an open one. Open it had a notch at 96MHz and its odd multiples. Closed it had a notch at 196MHz and all multiples. So I did not see a broadband rejection of the closed stub :-(
I measured mechanical and quarter-wave stub filters on today's lab. They were not on 2m though but on 900MHz
I do not understand :-(
I hope you make a video building band pass passive filters
I suggest watching Fesz' channel (link in another video on this channel)
Very instructive video. I am having a similar problem receiving 137MHz NOAA images because of a local pager signal near 138MHz. I may try a stub in addition to my homebrew LC filter although the interfering signal is extremely close. Have you any comments on SAW filters ? (Limited frequencies of course.)
The SAW filters I saw were for higher frequencies. But if the tho frequencies are so close you need an extremely narrow filter like the one I showed. They are also used for repeaters. Or you have to give up. :-(
Interesting !
Thank you!
yay
:-)
Nice video, and thank you for linking the useful things I need to immediately buy 😁. What crimp tool do you use for the SMA connectors? I have an IWISS HSC8 6-6, but I don't think it'll open wide enough.
These are made for ferrules. You need a different one. I left a link in the description to the one I use. Probably not the best quality. But ok for me (I solder the inner pin because I was not very successful with crimping it)
@@HB9BLA Thank you for replying. Silly me, I looked for the crimp tool in the description but didn't see it at the time. Of course, I can see it as plain as day now - ordered!
Nice, this is always a nice and simple way of filtering out a spur. Especially since the materials needed are easily available.
I guess you can do this with whatever media you have?
Like coax in all impedances, stripline, twisted pair, etc.?
Do you plan to make a video on the spectrum analyzer?
I came across them by coincidence and they seem to be very nice units.
The principle should work with other media, too.
There is a video about the NanoVNA (how to use it). And maybe I will once to a video about different tools...
@@HB9BLA
Thanks for the fast answer. Good to know, I since read other comments saying they used this technique for twin lead as well.
Thanks, I meant the Spectran V6 Spectrum analyzer. :-)
If you look at its price tag it is not really for private persons….
Cool!, usually losses are what define the BW, did you try using a better cable like RG-400 for example?. Also, do all notches have the same BW, I am expecting the higher order notches will have narrower BW, not sure though.
AFAIK you can change the bandwidth with different coax cables. I am not sure if the losses are the deciding factor, though. But I did not test it. And I do not remember if the bandwidth on the harmonics was different and how.
Hey Andreas, what software did you use on your TV to display the spectrum? I really like your videos! 👍
This software come with the receivers. The Spectran has a very good SW and the RSP uses SDRplay.
To bad the other way around isn't that easy as just cutting. I am talking about a bandpass filter. These VNA devices sure make it a lot easier to design stuff!
Maybe for one of your next video's a HB9CV antenna ?
I am not sure I will enter the antenna building soon. But you never know... I think that these nanoVNAs make antenna building much more attractive. Many antenna-building videos appear on UA-cam these days.
You may want to experiment with a shorted stub too, a bit more tricky to tune but gives a band pass version. BTW, another few passes of iss this morning with KG-STV on the 70cm
Others mentioned that stubs should behave like band passes, too. But I did not see such a behavior so far (if you do not count the wide space between two "notches" as the pass band).
@@HB9BLA if you short the end of should behave as a band pass element
I tried it and it just changed the frequencies of the notches. The wide part between the notches are of course a "pass band"
Superb video! The book by Cutsogeorge is worth reading…
Which book? I did not find one on amazon written by somebody called "CUTSOGEORGE"
Andreas, sent you an email with a link…
Frank
K4FMH
73's from downunder where we mount everything upside down
Exactly! That is what I always thought when I was younger. But I never checked myself. Maybe time to do so ;-)
Excellent video! It's a bit rude that they are using pagers on 147mhz. In VK, they are located above 148 and that's bad enough.
We have plenty of problems with them. Not only me...
@@HB9BLA Absolutely! I remember that my 2m handheld RX with lamba/2 antenna was blocked by one pager TX that was maybe more than 2km away.
I do have a similar problem here in Berlin. When using my Baofeng radio with my 4 element Yagi-Uda antenna I always use the 2m bandpass from YU1LM which works absolutely great (I do have a video on my channel if someone is interested). I recently set up a satNOGS station on a tower and unfortunately the biggest problem is a ham radio relay transmitter on 145.6MHz which leads to heavy intermodulation. My boyfriend already suggested using a big cavity filter to get the necessary steep curve for filtering. Maybe on day...
These cavity filters are great! But as said, not cheap…
Hi Andreas,
Does the SDR receive software provide a way to create a notch filter? My Flex 1500 PowerSDR can create a TNF tuned notch filter that can be made very narrow. 73 WJ3U
The SDR filters usually are made in software (one of the advantages of SDR). My problem most probably was created at the ADC or the Mixer stage. So before the signal was digitized. I assume your Flex still has lots of analog filters built-in to avoid this.
At 4:08 I'M SO JEALOUS OF YOUR CABLE RACK! I'm literally drowning in cables and boxes or drawers just doesn't work! Where can I find something similar?! (Besides Ikea... I've looked!😋) Is there a specific "name" for those or did you DIY it?
I printed them with a 3D-printer...
I think that I would do a 12 dbm antenna and lay it horizontal then make reflector on the earth side. The reflector should be able to cancel the earth bound transmitter . turn 90 degrees ( - reflector antenna
A directional antenna might help. But because the ISS passes are different, it probably would sometimes even amplify the pager station :-(
Hello Andreas, great explanation as usual ☺️👍
Would such a filter be possible on 13.56 MHz? Perhaps it will be very long? The piece of coax must be always straight right? Can a capacitor be used as a filter? Since cutting the third coax to the right length is in fact adding capacitance? 🤔 73! DE1CTL 🙂
You can calculate the length. It is quite immune to the position. You can wind it up. The make such filters also for low frequencies.
The coax is not only a capacitance. It has a clear resonance. So an L Must be involved. But everything distributed.
@@HB9BLA Thank you Andreas, I do learn more and more useful stuff from your Channel. 😉 👍
I would hope so ;-) otherwise it would not be worth your time.
@@HB9BLA Andreas perhaps you might find interesting the 2m Band small PCB Loop Antenna, I received one and it works. I would like to tune it´s variable capacitor on the FT8 Frequency and let it receive (perhaps add some filters now that I know how to build them). On Aliexpress you can search for: "70MHz - 200MHz VHF FM Schleife Antenne Schmale-band Antenne Für MALAHIT SDR Radio Empfänger" - 15€
Hi i live in South Africa and am interesred in setting up a Lora node to interface with the rest of tbe ground station network which yoj discussed a month avo, regards Tyrel Pepperell, Cape Town, Western cape south Africa
Go for it. It probably will be the first in your country!
Request expresslrs tx rx and more, thanks
I do not see expresslrs on this channel. And maybe you can help me where it would be useful outside of drone racing?
Do you know if there is any SPICE model to run a simulation ?
I do not know :-(
Good but why not include the filteras a directional antenna?
The ISS moves fast and directional antennas would add another level of complexity, I think.
@@HB9BLA Sounds like a 'fresh idea' and use of a micro-controller then? (rats! wrong channel :) R
@@HB9BLA but don't 'we want more?' :) (Ah sorry that's the other channel) You can see it plainly as it passes overhead
@@dicktonyboy Indeed this is a new project. And it might appear on this channel. Two ball bearings already occurred in the stream of mails here ;-)
Look, I am comment 74, so I missed 73 today. lol
:-))
What about making a BPF instead of a BSF?
I do not know how to do that with Coax only.
@@HB9BLA Terminate the end with a short. The inversion will make it an open on the desired freq (and multiples).
Also, there's a nice design by Zack Lau, titled "A No-Tune 2-Meter Bandpass Filter", that also mostly uses coax.
I tried it now (because others suggested the same thing. The result is the same curve, just with different notch frequencies. Now you can argue that between the notches there is a (very wide) passband.
I looked at Zack's article. It looks very similar to the one I showed with the two stubs. He even writes about "rejecting unwanted pager signals around 153MHz".
@@HB9BLA DL4XAV had a page on the topic, there it's also referred as "a band pass filter for a wide frequency band". It's a good read, and has nice pictures on the BPFs, and they are quite wideband. I'm not really sure how to make them narrower, but the no-tune 2m worked well for me, so I can recommend it.
Ussefull
Thank you!
Fantastic show very informative thank you for sharing de 2e0ree
Glad you enjoyed it
Have you considered a two meter repeater can/cavity? It would have a higher "Q" so would be sharper.
Cavity filters are very different. In performance, size, and price. They are much better than this simple stubs.