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Author Topic: RF Feedback Into Mic On 40 meters and 12 meters  (Read 7257 times)
AK4SK
Member

Posts: 150




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« Reply #15 on: October 25, 2013, 09:23:59 AM »

Last night I tried coiling the mic cable as much as I could without shortening it to the point that it was unusable. I even put some ferrites around the coil. I thought I detected a slight change but the problem was still there.

I also tried the stock mic, the problem did not occur at all. I use a much lower SSB mic gain setting for the Koss headset vs the stock mic as the Koss is much more sensitive. I forgot to change the gain back to what it should be for the stock mic so that could have made a difference. I'll try it again with the stock mic and the normal gain setting that I use for the stock mic.
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KA4POL
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Posts: 2028




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« Reply #16 on: October 25, 2013, 12:08:36 PM »

Obviously there is a resonant circuit for 40 and 12. Detuning/attenuating that could be a solution. Try to put all your USB ferrites on the cable. And also try the shielding approach.
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AH6RR
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Posts: 803




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« Reply #17 on: October 25, 2013, 03:38:21 PM »

Last night I tried coiling the mic cable as much as I could without shortening it to the point that it was unusable. I even put some ferrites around the coil. I thought I detected a slight change but the problem was still there.

I also tried the stock mic, the problem did not occur at all. I use a much lower SSB mic gain setting for the Koss headset vs the stock mic as the Koss is much more sensitive. I forgot to change the gain back to what it should be for the stock mic so that could have made a difference. I'll try it again with the stock mic and the normal gain setting that I use for the stock mic.

The Koss head set is made for computer work not RF work and has no RF shielding at all you will end up having to completely re-design the circuitry in the Koss to get it to work properly. I tried to get a Sennhesier Computer Headset($115) to work and I just could not do it. 40M and 15M were affected horribly. I switched to a Heil Proset and never had a problem after that even at over 1KW+. My antenna is a 3element SteppIR with the 30/40 Driven element only 15ft over the shack. The House is on stilts 12ft off the ground and the antenna is on a Glen Martin roof tower.   
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WB6BYU
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Posts: 13336




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« Reply #18 on: October 25, 2013, 03:56:50 PM »

It might be the RF getting into the preamp that is built into the electret element.
If so, a bypass capacitor (perhaps 0.01uF) across the leads of the mic element
may help.

Adding ferrite around a mic cable isn't always as effective as one might wish.
The ferrite should fit snugly, and you get better rejection if you can wrap several
turns around it rather than just a single pass of the coax.  (Also some of the beads
sold for filtering computer cables are more effective at VHF than at HF.)

Probably the best sort of filter you can make would be one that has a mic plug
and jack to go inline, and inside it each wire has a series inductor or ferrite
bead and a bypass capacitor to a common ground.  That's a brute force approach,
but sometimes it is necessary if a mic is particularly sensitive.  (The same approach
can work for telephone lines.)
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AK4SK
Member

Posts: 150




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« Reply #19 on: November 11, 2013, 09:09:15 AM »

Got it resolved. 14 turns around a type 43 toroid ferrite did the trick.

Thanks for all of the help.

73
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