HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on December 01, 2021, 2141 UTC
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With and without a common mode choke made of 40 turns of RG-174 coax on an FT240-31 toroid core.
(https://i.imgur.com/XlaW3j9.png)
(https://i.imgur.com/fEsrDGl.png)
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Chris, thank you for the great screen shots. I have always been a firm believer of a CM noise choke at the receiver input. I personally use a Palomar Engineers CMNF-500 (38 dB rejection from 1 to 60 MHz) but the next one(s) I plan on winding myself. Did you do a straight winding or a differential winding on the core? A differential winding is where you wind on half of the core then pass the coax across to the other side of the toroid and wind the other half.
Here are some of my references for the readers:
Building Manual Common mode choke (nice instructions on how to build):
https://www.hfkits.com/manual-common-mode-choke/ (https://www.hfkits.com/manual-common-mode-choke/)
W1JR type chokes (nice pictures):
https://www.pa1m.nl/w1jr-type-chokes/ (https://www.pa1m.nl/w1jr-type-chokes/)
Paul's, W1VLF, YouTube Channel (https://www.youtube.com/user/W1VLF (https://www.youtube.com/user/W1VLF)) has some nice videos:
Common mode Chokes for loops:
https://youtu.be/cB6HeL6Eykc (https://youtu.be/cB6HeL6Eykc)
Common Mode choke Noise reduction On VLF signals:
https://youtu.be/b0Jx-oN3YlA (https://youtu.be/b0Jx-oN3YlA)
Building the noise reducing common mode choke with parts readily available:
https://www.youtube.com/watch?v=dcXq6u64HXg (https://www.youtube.com/watch?v=dcXq6u64HXg)
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Just a straight winding for this case. I'll have to try a differential and see if it offers any additional improvement.
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Nice illustration of the improvement Chris.
In tinkering around with reception of LowFER beacons and 630 m activity, I have found similar measures very helpful. The outdoor long-wire is transformer coupled at the feed-point, which also has a ground rod. Only the antenna side winding of the transformer is connected to the rod. The winding connected to the coax is "floating"
At the receiver end, I have another isolating transformer that can be inserted into the feeder. Not exactly a CM choke as such, but the intention is the same, add a high impedance for common modes. It seems to help a lot.
Touching the shield of the incoming coax to the centre conductor of the radio provides a very convincing demonstration that these measures are effective.
Ed
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Hey Chris? Have you tried that on the 1750M Experimenter's Band and even 60kHz from WWVB, to see if that improves RX'ing them? Passing thoughts....
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W1JR cross‑wound Guanella toroid choke
https://hamwaves.com/chokes/en/index.html (https://hamwaves.com/chokes/en/index.html)
cross-wound coaxial Guanella toroid choke with reduced interwinding capacitance, limiting the risk of deleterious common mode resonances, whilst maintaining the resistance for the common mode
Whenever I get around to winding another CM choke I plan on using a MIX 75/J core which is optimal from 150 KHz - 10 MHz or perhaps even 2 cores, 1 x MIX 75 and 1 x MIX 31 (1 - 10 MHz) or MIX 43 (30 - 300 MHz).
There is also nothing wrong with "stacking" toroids of the same MIX in order to reduce the number of windings and increase the coax size (I.E. RG-223/U for instance).
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A timely post. I too have found relief from various rfi maladies via the ubiquitous choke. On the main hf ant there's two commercial chokes followed by a length of coax festooned made with ferrite cores of varying mix to ensure overall performance from below bcb to 30kilocycles. There's also a number of cores at the ant feedpoint to reduce rf getting on the coax from the near field in and around the home - chokes at each end of the line keeps noises from inside the home from getting to the ant, something not oft considered by the hobbyist. What chokes do is more or less emulate a rf "portal" directly from the back of the rig to the ant feed.
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Cool, thanks, Chris.
This can go in a project box with SO239s, right? And put it right before the receiver...?
I think I could really use this and would love to try it out.
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If you "box it up" with connectors just make sure you use a plastic (or similar non-conductive) enclosure. A metal enclosure will defeat the purpose of the choke as is will be conductive and introduce an additional grounding point or ground loop. The aim of the choke is to remove the interference which is on the braid of the coax cable.
You might find some interesting reading here:
Chelmsford Amateur Radio Society Advanced Course 7 EMC
https://slidetodoc.com/chelmsford-amateur-radio-society-advanced-course-7-emc/ (https://slidetodoc.com/chelmsford-amateur-radio-society-advanced-course-7-emc/)
Also have a look at:
A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing by Jim Brown K9YC
Jim is constantly updating this document and it is very comprehensive. It is available at the following URL:
http://www.audiosystemsgroup.com/K9YC/K9YC-old.htm (http://www.audiosystemsgroup.com/K9YC/K9YC-old.htm)
and here: http://k9yc.com/ (http://k9yc.com/)
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Nice. Thanks, SIGINT.
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I'm now a true believer.
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Enquiring minds would like to know what you decided / ended up with or results.
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I have been having noise problems with my end fed antenna and Malachite receiver when the receiver is being charged by a USB charger. I have tried several approaches without much improvement. I saw a YouTube video by Steve Ellington on end fed antennas and the feed line as part of the antenna. I decided to try a common mode choke (aka 1:1 current balun). I put five (5) clamp on ferrite cores (covered with plastic electrical tape to weather proof) on the coaxial cable close to the connector of the 9:1 antenna transformer. To my surprise, they work quite well. Before the cores, the noise floor would come up about 15 dB with the charger on. With the cores present, the noise would rise by only 3 dB. They are staying on.
If you are using an end feed antenna, a common mode choke is required for low noise operation. Thanks to the original poster for starting this topic.
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Greg, most excellent. Small misconception ... toroids and snap-on chokes etc ... are weather proof by nature to start with. No need to get fancy with sealing them up.
Now, as for your end-fed antenna, you need a choke on the coax somewhere down the line towards the receiver. The reason for this is that the end-fed antenna uses the braid of the coax as part of the antenna but as some length, you need to choke that so that the braid no longer acts as part of the antenna.
Have a look at the Palomar Engineers application notes on End Fed Antennas.
https://palomar-engineers.com/tech-support/tech-topics/best-hf-end-fed-antenna (https://palomar-engineers.com/tech-support/tech-topics/best-hf-end-fed-antenna)
Scroll down to the bottum of the page and get a copy of Bullet End Fed Antenna Notes (PDF).
I have a couple of the Palomar Engineers bullets and where to place the choke on the coax is not apparent unless you read the manual which comes with them. I currently do not have access to that information.
P.S. I have similar noise issues with my Belka DX but that is due to the switch mode USB charger that I am using.
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Enquiring minds would like to know what you decided / ended up with or results.
If you were asking me, SIGINT, well, once it dawned on me that making a little box would've been fun but completely unnecessary, I simple wound as many turns of RG58 as I could cram onto a FT-240-31 and put some PL259s on and that was that. It sits just before the input. Between that and putting little snap-on ferrite things on everything near the RSPdx, I think I'm good to go.
Would it be better using the RG174 as Chris did? With more windings? I think I have some around somewhere.
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Erik, you are on the right track. Keep it simple. You can gain the same effect and perhaps even more benefit by stacking 2 or more FT-240-31 on top of each other and winding the coax on multiple cores vs. going to a smaller coax with a greater number of windings.
There is a couple of nice images courtesy of G3TXQ (http://www.karinya.net/g3txq/chokes/ (http://www.karinya.net/g3txq/chokes/)) in the link below that illustrate the number of cores vs. the number of turns. You will also find there a link to the K9YC 2018 Cookbook PDF with some great construction ideas.
https://ham.stackexchange.com/questions/13005/coax-or-bifilar-windings-for-a-choke (https://ham.stackexchange.com/questions/13005/coax-or-bifilar-windings-for-a-choke)
Snap-on ferrites are the least effective types of chokes. Toroids are somewhat in the middle and convenient to work with and 2 x parallel ferrite rings are the best but more difficult to work with.
You will want to get a minimum of 12 turns on a single core. If you can squeeze in 17 turns then that will be even better at lower frequencies. Ref here: http://www.m0pzt.com/baluns/ (http://www.m0pzt.com/baluns/)
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Thank you for your reply and references, SIGINT (aka signal intelligence, excellent handle and subject). Yes, ferrite will not rust. The problem, that I have encounter in the past, is insects. They will lay their eggs any place that seems protected. After hatching, they will follow the cable and get in house. The little bugs can be amazing. The outer plastic can be degraded by ultraviolet radiation. It takes time, but does happen. Also, the tape helps to hold them in one place. This is why I do cover them with tape.
Yes, the choke should be at the receiver. I have some more clamp on ferrite cores on order. When they arrive then I will place them on the receiver end of the cable. We will see what happens.
In my case, I always make the RF ground connection as close as possible to the antenna. In my setup, the ground wire to the antenna is about 30 feet. So, the ground wire also acts as a "counterpoise". Placing the choke at antenna end near the ground seem to me a good approach.
I also use a battery to power the equipment. I have found in the past having the RF ground and Powerline ground connected together at receiving equipment can cause some strange effects. I will agree with you now that the ground and bonding can be a controversial subject. It all depends on particular environment and which regulatory agencies are involved.
SIGINT, please do not take any of the above as criticism. Your posts show you are very knowledgeable on communications systems. Thanks again for trying to help others.
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Congratulations on getting ~SIGINT~. Most people do not get the abbreviation - LOL I tried to use just plain SIGINT but someone else who has been dormant on the HFU already scooped it. I asked Chris that if he ever cleans up the use database that I would like to take it over.
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I hope you get it.
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I recall some tests qst or whoever made that showed chokes most effective at the rx port, but choke at each end of the line eliminated the line from participating in reception of signals and increased effect over just one end being choked. From then one I try to have chokes at each end. Also consider using different mix cores as that will give you wider range as far as freq goes, some cores are peaky.
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Yes, Josh, I have added some more cores at the receiver end. It only helped a little bit. I got the biggest improvement at the antenna end, but, I did that first ;). Sometimes the order of testing colors the results. The good news the common mode choke did help.
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Congratulations on getting ~SIGINT~. Most people do not get the abbreviation - LOL I tried to use just plain SIGINT but someone else who has been dormant on the HFU already scooped it. I asked Chris that if he ever cleans up the use database that I would like to take it over.
I just assumed everyone got it. :)
And thanks again.
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Allow me to demystify a few things ...
For receive purposes, chokes are generally most effective at the receiver input basically choking out interference picked up along the coax's braid.
Chokes at the antenna input are generally installed when this is a transmitting antenna to prevent RFI due to the high current that can occur at the antenna input from the transmitter's RF output. As you will see in the example below, there are also some benefits to a receiving system.
The best approach to identifying and eliminating interference is to look at it in a "source" and "victim" perspective. For my example, I will use the proverbial and nasty switch mode wall wart we all love --- because of the lack of EMI control in North America which allows for junk to be imported. So, "Nasty" (the source) is generating interference and somewhere along the line this is being picked-up. Who is the "victim"? The antenna or the receiver? Odds are in this case it will be the receiver and that Nasty is located somewhere inside / vicinity so we will install chokes at the receiver input. Now, let me throw a curve ball into the mix (no pun intended). Nasty is located outside. Who is now the victim? Odds are in this case that it will be the antenna therefor chokes located at the antenna are appropriate.
If you have more than one antenna, definitely, chokes all around. Here is why. Using our previous example, Nasty is generating interference into antenna A (which now becomes the source). Antenna B, C and D have now become victims as they are receiving Nasty's interference emanating from antenna A. Hopefully antenna A is a nice full size dipole with 3 dB of gain so that Nasty's interference gets well splattered across the entire neighbourhood. The same goes if you have an antenna switch. Nasty on one coax will travel through the antenna switch making all the other antennae victims.
different mix cores as that will give you wider range as far as freq goes
Absolutely, but again, look at where the interference is. For example, there would be no reason to install a MIX 43 if the your interference is bellow 25 MHz. A MIX 31 should have you pretty much covered. If you are going to play in the VLF and LF band, you might consider a MIX 75 or combination there of.
Palomar Engineers have a great table on their Ferrite Mix Selection page:
https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selection (https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selection)
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If there's no choke at the feedpoint the line IS participating in reception and reception pattern, in the case of a tx in transmission. Then again anything conductive attached to the feedpoint, often a mast or ground wire, is too. This reception includes household noise from inside the house and near field picked up by the line and fed to the antenna and thus back down to your rx. As for reception pattern, no choke at the feed of a horizontal diploe means the feed is adding vertical polarisation to the erstwhile horizontal polarised antenna. The problem is if it's a conductor it's an antenna, and antennas typically only send down the feedline about 1/3rd the rf they pick out of the aether and reradiate the rest back into the aether. Shame of it all. Yes 1/3rd is all you get.
edit; (haha I was wrong here, you get 2/3rds of the received signal, not 1/3rd you big dummy, but that 1/3rd is still getting away)
For example of feedline effect, the tiny pa0rdt active antenna types typically show much more "gain" when grounded to a metal pole above ground than just hanging off a choked feedline, this reveals the nature of what's going on - the pole and/or line's doing most of the receiving.