Can a common mode choke really help reduce RFI?

[~SIGINT~]

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/) 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

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/

[JustGreg]

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.

[~SIGINT~]

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.

[JustGreg]

I hope you get it.

[Josh]

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.

[JustGreg]

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.

[Erik Mattson]

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.

[~SIGINT~]

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

[Josh]

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.