Technical Topics > Equipment
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....
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