[Elecraft] K3: Very distorted audio on transmit but only on 20m and only with the KPA500 on

[Joe Subich, W4TV]

VSWR is not an issue - the issue is imbalance in the antenna system.
The problem is well known as the "common mode RFI" issue due to a lack
of effective decoupling of the antenna and feedline.

A *much more common* issue is users who fail to follow instructions and
connect microKEYER II (or any other accessory that handles microphone
audio) to the same power supply as the transceiver.  With "modern" rigs
which use 12V finals, the peak power supply current can be greater than
25 amps.  When the transceiver uses light cable for power (anything
less than #10 wire) and "friction" connectors like Molex or Power Pole,
the voltage drop in the power supply return can be significant.  That
drop causes some current to flow through the accessory and "pump" the
ground reference to the accessory.  With just 0.2 Ohms in the connector
and ground return, that means at least 0.5V *in series* with the audio
input!

If you are using any device that handles microphone audio, either power
it from the K3's +12V accessory jack or use a *separate* isolated power
supply.

73,

    ... Joe, W4TV


On 3/11/2014 5:58 PM, tomb18 at videotron.ca wrote:
> Ok, thanks, that's what I thought. A little more complicated since I have a MicroHam microkeyer II in the circuit but probably fixable.
>
> 73's Tom
>
> On 3/11/14, "Donehrlich at q.com"  <Donehrlich at q.com> wrote:
>> Almost certainly RF getting into the mic circuit. Try a different mic .. or use the rear panel mic connector as a check. You probably have high SWR into the antenna feedline (or coax) after the tuner on 20 meters and that is why you only see problem on 20M. High SWR on the feedline (Note: your rig can be seeing low swr .. it is AFTER the tuner where you have high SWR) results in high current or high voltage that then couples into other shack wiring. Of course, the higher power from the KPA500 pushes those voltages/currents to levels that are more likely to cause problems. Been there, done that .. many times.
>>
>> Don K7FJ
>>
>>
>>> Ok,
>>>
>>> So my audio is quite distorted as I can hear in my headphones with the monitor. But this only happens on 20m and only with the KPA500 in operate.
>>> Sound like RF? or is something else going on?
>>>
>>>
>>> Thanks
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