Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

Posted by Jim Brown-10 on Jan 25, 2010; 5:31pm
URL: http://elecraft.85.s1.nabble.com/K3-transmitted-RF-feedback-with-condenser-mics-tp4450419p4455286.html

On Mon, 25 Jan 2010 09:14:21 -0800, Oliver Johns wrote:

>Try putting a snap-on ferrite bead ON the actual boom of the
>electret headset microphone.  Put it right at the end, just before
>the microphone bulb.  Maybe the RF is getting into the electret or
>its FET and is being rectified there.

Ferrite beads are tuned to about 150 MHz. Clamping on onto a cable
simply adds that 150 MHz resonance to the common mode circuit.
That's great if the RFI is on 2M, but next to useless if it's on
20M. To make a choke work on 20M, we must wind multiple turns
through the ferrite to more the resonance down to 20M. For a typical
ferrite clamp-on, that's about 5 turns. You can't do that on the
boom,  but you can do it on the cable.

The original poster of this question emailed me this morning telling
me he had solved his problem by re-cabling the coax between his rig,
amp, and antenna switching, so the source of his RF was probably a
badly terminated coax connector.

73,

Jim K9YC


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