radio noise

I'm getting a high freq. noise when I press the PTT with the
headphones and mic plugged in. With the mic unplugged it 's ok.
With a handheld mic it's ok. Tried a different headphone, still
noisy. I grab the mic wire and the noise goes away. Continuity cks
ok in mic wire. What am I missing? Wires are shielded.

Ed
at wits end

Bryan Martin wrote:

All of your audio shields should be grounded only at one end, ...

And finally, your audio jacks should not be grounded,
they should be isolated from ground by plastic washers and grommets.

This is all correct, but it may not fix the OP's problem. Sounds like
transmit RF might be getting into the low-level audio wiring, in which
case the shielding may do the trick.

If you have a bad antenna installation (poor antenna
grounding/counterpoise/internal antenna, high VSWR on the coax running
from the COM to the antenna, or if the coax is bundled too close to some
of the audio wiring), the transmit RF can couple into the audio wiring,
and get rectified inside some of the audio equipment (audio panel,
intercom, or mic stage of the COM itself), causing a "howl" or "squeal"
only while transmitting.

If this is the case, checking/fixing the antenna to reduce VSWR and
resulting radiation off the coax is first priority, rerouting coax more
distant from audio wiring is second, shielding the audio wiring is
third, and putting some RF bypass caps or RFI suppression beads on audio
inputs is fourth...

On Sep 25, 11:03*pm, MikeMl wrote:
Bryan Martin wrote:
All of your audio shields should be grounded only at one end, ...
And finally, your audio jacks should not be grounded,
they should be isolated from ground by plastic washers and grommets.

This is all correct, but it may not fix the OP's problem. Sounds like
transmit RF might be getting into the low-level audio wiring, in which
case the shielding may do the trick.

If you have a bad antenna installation (poor antenna
grounding/counterpoise/internal antenna, high VSWR on the coax running
from the COM to the antenna, or if the coax is bundled too close to some
of the audio wiring), the transmit RF can couple into the audio wiring,
and get rectified inside some of the audio equipment (audio panel,
intercom, or mic stage of the COM itself), causing a "howl" or "squeal"
only while transmitting.

If this is the case, checking/fixing the antenna to reduce VSWR and
resulting radiation off the coax is first priority, rerouting coax more
distant from audio wiring is second, shielding the audio wiring is
third, and putting some RF bypass caps or RFI suppression beads on audio
inputs is fourth...











I wired up a new shielded harness and installed a new RST antenna kit,
and guerss what, it works great. Thanks for all the input. I've
just acquired a new skill. Next month the MIDO inspector comes out
for the AW inspection. Everything looks like a GO.

Ed

Ah, you used an RST antenna kit. There's your problem, right there!

{;-)

Jim

--
"It is the mark of an educated mind to be able to entertain a thought
without accepting it."
--Aristotle


I wired up a new shielded harness and installed a new RST antenna kit,
and guerss what, it works great. Thanks for all the input. I've
just acquired a new skill. Next month the MIDO inspector comes out
for the AW inspection. Everything looks like a GO.

Ed