Can a common mode choke really help reduce RFI?

[ChrisSmolinski]

With and without a common mode choke made of 40 turns of RG-174 coax on an FT240-31 toroid core.

[~SIGINT~]

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/

W1JR type chokes (nice pictures):
https://www.pa1m.nl/w1jr-type-chokes/

Paul's, W1VLF, YouTube Channel (https://www.youtube.com/user/W1VLF) has some nice videos:

Common mode Chokes for loops:
https://youtu.be/cB6HeL6Eykc

Common Mode choke Noise reduction On VLF signals:
https://youtu.be/b0Jx-oN3YlA

Building the noise reducing common mode choke with parts readily available:
https://www.youtube.com/watch?v=dcXq6u64HXg

[ChrisSmolinski]

Just a straight winding for this case. I'll have to try a differential and see if it offers any additional improvement.

[Ed H]

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

[ThaDood]

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....

[~SIGINT~]

W1JR cross‑wound Guanella toroid choke
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).

[Josh]

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.

[Erik Mattson]

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.

[~SIGINT~]

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/

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

and here: http://k9yc.com/

[Erik Mattson]

Nice. Thanks, SIGINT.

[Erik Mattson]

I'm now a true believer.

[~SIGINT~]

Enquiring minds would like to know what you decided / ended up with or results.

[JustGreg]

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.

[~SIGINT~]

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

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.

[Erik Mattson]

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.