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

[Joe Subich, W4TV]

 > Is having the Microham directly on top of the KAT500 Tuner with the
 > microphone cable almost touching the RF out a potential issue? If so,
 > this might be an easy fix for now.

No, not if your antenna system is properly designed and there is no
common mode current on the outside of the feedline.  If you do have a
common mode issue, arrangement of equipment may make minor differences
as the effective line length changes but rearranging equipment is not
going to *fix* an antenna system issue.

Old timers will recognize common mode as "getting bit by the mic" or
getting a "tickle" when touching metal portions of the key <G>.  That
was an extreme level of common mode RF (several volts).  Just a volt
or two can seriously upset modern solid state equipment or impact low
level microphone audio (a dynamic mic has a level generally less than
20 mV).

73,

    ... Joe, W4TV


On 3/11/2014 7:07 PM, tomb18 at videotron.ca wrote:
> Hi,
>
> Thanks for all the replies.
> My Microham is a relatively recent unit.
> I also have a separate power supply for the MicroHam.
>
>
> Is having the Microham directly on top of the KAT500 Tuner with the microphone cable almost touching the RF out a potential issue? If so, this might be an easy fix for now.
>
>
> Thanks
>
>
> On 3/11/14, "Joe Subich, W4TV"  <lists at subich.com> wrote:
>>
>> 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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