[Elecraft] K3 RFI Susceptibility

[Jim Brown]

On 12/22/2010 8:15 AM, Bill Davis Jr wrote:
>   I have had similar issues with my K3/10, but I was asking for trouble. I have an involved external sequence switcher and band decoder for all the VHF/UHF/Microwave transverters. Involved is an unshielded wire that runs between my sequencer switcher and the PTT circuit, accessed at a mike cable extension on the K3. The TX function for the K3 is operated by the sequencer. On 6m when running the 250w, the amp  RF gets into the keying circuit. With a bit of lead dressing I have been able to avoid the problem, but I need to do something a bit more "evolved" for the problem.

YES!  Here's the problem -- while that PTT line IS a DC line,  because 
it is a WIRE, it is also an ANTENNA, so it will carry RF current from 
your TX and put it into the K3 PTT circuit (and into whatever is driving 
the PTT circuit). That means we CANNOT treat it as a DC circuit. It is 
an RF circuit, and must be wired with coax (or something like coax), 
with the shield bonded to the chassis at each end.  Now, the RF current 
flows on the coax shield to the chassis at each end, so it does not 
cause problems with the keying circuit.

73, Jim K9YC