[Elecraft] K3: A cure for RF feedback with electret mic

[Jim Brown]

On 3/29/2011 9:53 AM, Brian Machesney wrote:
> The feedback is band-dependent,
> can occur at a power output as low as 10-watts or less and is evidenced by
> artifacts ranging from distorted audio in the headphone monitor to
> uncontrollable, full-power output.

Thanks for the post, Brian.  Band-dependency comes from three important 
variables -- how much RF current is on your feedline at the frequency of 
interest, how much RF is radiated into your shack at the frequency of 
interest (both are antenna-dependent), AND the frequency-dependent 
nature of the Pin One Problem(s) in the radio. The latter depends on 
internal construction and wiring layout within the radio.

When I started using an FT1000MP for contesting, I immediately noticed 
RF feedback on 15M and 75M.  I set up my RF signal generator and did the 
tests for RF Pin One problems on the MP's mic jack.  I found a broad 
peak just below 5 MHz that extended down to 75M, and another peak at 21 
MHz.  I already had chokes on the feedlines, so I added one on the mic 
cable that fixed the problem.  BTW -- I was using an EV RE16 at that 
time, which is a dynamic mic, the antenna was a fan dipole about 150 ft 
from the shack, and I was running 1.5KW!

If I'm not mistaken, the procedure for using the generator to test for 
RF Pin One problems is in the RFI tutorial.  It's also documented in one 
of the AES papers on RFI, which are on my website.  
http://audiosystemsgroup.com/publish.htm

73, Jim Brown K9YC