Stealth Pilot wrote:
radio is an icom a200
headset is a clone of a dave clark, an australian altronics C9073

when taxying and early into the flight when I press the ptt the radio
squeals in the headset.
after a while in the air the squeal vanishes and the radio setup works
just fine.

I changed headsets today while taxying out and discovered that the
squeal vanished with the headset. The new/different headset worked
fine.

I thought it might have been moisture/condensation and have wd40'd the
connectors without changing the problem one iota.

anyone an idea as to what would cause this problem?
all equipment appears to be in sound working order with no damage.
When you are on the ground the antenna pattern is modified by the
nearby ground, allowing more of the RF field to be incident on your
headset and wiring. Sometimes, also, if your antenna is on the bottom,
the nearby ground can change your antenna's SWR, which may result in
antenna currents flowing on the outside of the coax. You might try
ferrite beads on your mic leads close to where they enter the radio,
and also a large clamp-on ferrite on your antenna's coax near the
antenna connection.

Stealth Pilot