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

[Joe Subich, W4TV]

 > Yes, that condition *does* sound like RF-in-the-Shack - on only some
 > bands and only at higher powers is the clue to that conclusion.

The key to understand this is to remember that the common mode RF
creates a standing wave on the outside of the coax as if it was a
random length radial which may be open circuited, short circuited or
terminated in a complex impedance.  Due to the "random radial" effect,
the standing wave has RF voltage peaks and troughs.  If a peak happens
to occur on a sensitive piece of equipment, e.g.,  the mic preamp in
an audio processor or transceiver, you will most certainly have "RF
feedback" on that band.  However, if the voltage trough happens to
occur on the equipment you will never notice the common mode RFI.

Similar issues occur with computers - if the "radial" happens to
include the computer as it will with a computer connected to and
controlling a transceiver and the voltage peak occurs on the serial
or USB cable, you may lose control of the transmitter in transmit.
If the voltage peak happens to occur on a wired keyboard, the keyboard
may "go crazy" or lock up in transmit - the same thing can happen with
a wired mouse.

The worst effect can be physical damage to electronic devices if the RF
voltage gets excessive.  If the peak RV voltage exceeds the operating
voltage of the solid state devices (5V for typical USB ports, 3.3V for
low power devices), that negative swings of the RF can cause permanent
reverse voltage breakdown in the solid state devices just as if one had
connected the power supply backward - fortunately the currents are
small and the pulse durations are limited but common mode RF has been
known to damage hardware so it should not be ignored.

73,

    ... Joe, W4TV


On 3/12/2014 12:09 AM, Don Wilhelm wrote:
> Yes, that condition *does* sound like RF-in-the-Shack - on only some
> bands and only at higher powers is the clue to that conclusion.
> Refer to the RFI document by K9YC
> (http://audiosystemsgroup.com/RFI-Ham.pdf) and construct the common mode
> current chokes that he details.  One should be at the antenna feedpoint,
> and possibly another may be required at the shack entry point (on all
> feedlines).
>
> You may get by putting clamp-on ferrites on your mic cable, but the best
> solution is to keep the RF out of the shack with the addition of
> effective common mode chokes in your antenna system.
>
> 73,
> Don W3FPR
>
> On 3/11/2014 4:39 PM, tomb18 at videotron.ca wrote:
>> 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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