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

[O. Johns]

Hi Jim,

Well, the ferrite bead may not be self-resonant on 20 meters, but it still provides SOME impedance to the common-mode current.  I found experimentally that it did help.  And lots of people use these snap-on beads to good effect even in situations where multiple turns (which of course would be better) are not practical.

Oliver
W6ODJ


On 25 Jan 2010, at 9:31 AM, Jim Brown wrote:

> 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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