[Elecraft] Heil Pro 7 headset with KX3 - RF feedback?

[Drew Arnett]

Hi,

In the KX3 for Field Day department, we plan to use it for the GOTA
station for phone on HF.  (Yes, will have antennas not on top of each
other and bandpass filters on ALL radios.)  Would be great to setup
with a headset and foot switch.

Last weekend, I used my KX3 barefoot on 6 and with a
transverters-store.com transverter (also barefoot) for 2.  6m antenna
was 2 elements about 15 ft above the ground and 2m antenna 6 elements
at 10 feet above the ground.

I worked a couple of contacts on 2m phone with the headset.  Later,
when I tried to use the headset for phone on 6, seemed like I was
having a feedback problem.  Cabling was the stock cabling that comes
with the Heil Pro 7 headset and a footswitch.  Monitor was on and set
to 4.

So, ALC didn't indicate anything at the normal level (60), and
increasing it very abruptly caused horrible sounding audio.  (Yes,
acts like positive feedback.)

I have a small electret mic on a 1/8 plug thing I bought some time ago
from the computer store.  (Meant at the time for adding mic to
laptops.)  I have a 1/8" audio extension cable for it.  I plugged that
in instead of the headset mic and it seemed to act normally.

Is this purely an RF from the TX antenna getting into the mic audio
problem or is it something else?

Best practice to avoid this problem?

On my todo list:
- ask here
- look at KX3 schematic to see if it has RF ingress mitigation on the mic jack
- ask Heil
- unplug the Heil mic element and gator clip my cheap electret mic on
instead and try on 6m in driveway with identical antenna/mast/radio
setup
- look for or assemble an RF filter to inline with the mic jack (tough
to do with this little time
before field day)

Thanks and best regards,

Drew
n7da

ps.  Other than this, KX3 was great in the VHF test.  Worked great
with the transverter on 2 and was a lot of fun running barefoot with
the sporadic E opening Saturday afternoon & evening.