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Author Topic: Common-Mode Chokes  (Read 3804 times)

Offline ChrisSmolinski

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Common-Mode Chokes
« on: July 06, 2013, 1552 UTC »
This is something I have been meaning to try...

Your ability to hear weak MF and HF signals is limited by noise, generated mostly by solid-state electronic switches within your own house, conducted via the 60-Hz power line to your shack, and from there to your antenna by common-mode current on the feed- line. Putting common-mode chokes on your feedline, power, and other cables will sub- stantially reduce your received noise level. A good choke has >> 1 kΩ impedance for all MF and HF bands and costs $12 (for a small cable) to $120 (for a large, QRO cable)...

http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf
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Offline ChrisSmolinski

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Re: Common-Mode Chokes
« Reply #1 on: July 06, 2013, 2110 UTC »
This inspired me to see what i could do here to reduce RFI. I had some luck, with a few ferrites: http://www.radiohobbyist.org/blog/?p=1374
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Offline skeezix

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Re: Common-Mode Chokes
« Reply #2 on: July 07, 2013, 0000 UTC »
Interesting.

Been experimenting with cores here as well recently.

Put a 16-turn FT-140-J in line with the input to the amp box on my ALA-1530S+, but no noticeable difference in @!)#*! RFI. Going to revisit this. The thing is, also know that its not just on the coax, as other receivers with other antennas also show similar noise, so picking it up via some !@#*!@ radiating .

Also at work, have some KRK VXT4 speakers that are rectifying a couple of FM broadcast stations (different speakers are rectifying different stations, but both are around 100 MHz). On Monday, going to put a FT-something-43 on their antenna, er, I mean, speaker cable, and see if that cures it. Put a couple of unknown chokes from the power cables on it and it did decrease it noticeably, but not a lot. The FT-something-43 shout.

Another good link:
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
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Offline refmo

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Re: Common-Mode Chokes
« Reply #3 on: July 07, 2013, 0121 UTC »
I have had similar positive results with common-mode chokes.

It should be noted that the actual antenna should be kept away from these local noise sources for the benefit to be realized.
Although the choke will prevent common-mode noise induced on the transmission line from entering the receiver, noise signals induced on the actual antenna will not be attenuated by the choke.
Users of indoor or attic antennas will not see as much benefit from the use of common-mode chokes.
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Offline Zoidberg

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Re: Common-Mode Chokes
« Reply #4 on: July 08, 2013, 2120 UTC »
I've had good luck with feedline chokes on receive antennas, both with ferrite chokes and just coiling up the feedline neatly around an air form (empty plastic containers or pipes work fine).

As refmo noted, it won't help if the antenna itself is picking up noise.  But even with my indoor antennas I can minimize local RFI just by reorienting the antenna a bit.  Chokes on the coax feedline really help minimize noise from nearby computers and other doodads.
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Offline RobRich

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Re: Common-Mode Chokes
« Reply #5 on: July 14, 2013, 2252 UTC »
A choke at the antenna feedpoint can help the antenna from receiving noise picked up by the outside of the coax shield further down the feedline. As already noted, it is recommended to at least wind an "ugly balun" (really an air-core UNUN) with coax, though note ugly baluns are often rather limited in bandwidth - YMMV depending upon length, diameter, coax type, etc.

Coax is built as a two conductor feedline, but it is more like three conductors from an RF standpoint: center, inside shield, and outside shield. The outside shield is the most exposed to external EMI/RFI.

I like to use active antennas like the DXE-ARAV3-1P as good examples of decoupling the feedline shield. Active antennas tend to work or fail based upon N, since S is already usually low due to the short antenna element and a typically even worse RF ground/counterpoise. Feedline shield isolation is explicitly called out as an important design consideration in the specs. Without effective isolation, the coax outer shield can become part of the counterpoise, right along with delivering unwanted locally picked up noise to the antenna feedpoint, thus further degrading S/N.

Of note, it works the other way around as well. If the outer shield becomes part of the antenna, then "RF in the shack" can become a problem when transmitting.

I use multiple active antenna designs - mostly verticals - due to lot size considerations. I can personally recommend the MFJ-915 RF isolator from personal usage, but admittedly, it is kind of overboard for just a receiving antenna. It is little more than a bunch of ferrites on a piece of coax, placed in a PVC pipe. I suppose it could be replicated for less money, especially with lower grade ferrites if not transmitting.

It is much the same reasoning for a 4:1, 9:1, or whatever baluns on non-resonant antennas. Not only are they doing impedance changing, but they are acting as feedline shield isolators as well.
« Last Edit: July 14, 2013, 2256 UTC by RobRich »
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