Gerard O Connell asked:

Anyone any ideas as to why I get transmit noise when airborne only?

Gerard, a little more data, please. When on the ground and getting no noise,
is that with the engine/alternotor running or not?

Here's some things to consider:

Your radio is making it's highest current demand during transmit. If the
alternator is on line and being turned at a high enough RPM for normal
output (e.g., in flight), it will most likely be the predominant source of
the current since it's output voltage is, of necessity, set slightly higher
than the battery voltage when under load. On the ground, when the alternator
isn't turning (or turning too slowly to provide much output) the battery
will supply the load. Because of this, the effective current return paths
could be different depending on whether the battery or the alternator is
supplying the load. Try turning off the alternator in flight to see if
anything changes.

Secondly, by their nature, an alternator's rectifier bridge will produce
some amount of ripple and RF noise. The battery usually surpresses the
ripple voltage quite well (the battery acts like a humongous filter
capacitor), however, this noise voltage is only maximally surpressed right
at the battery - the further you get from the battery (on the
alternator/battery wiring) for your radio power and ground, the more chance
for this noise being injected into your comm circuits. So take a hard look
at your wiring, both power and ground, for your radio(s) with the above in
mind. You might try a dedicated ground direct to the negative battery
terminal for the affected unit.

The above is just an observation. I certainly don't pretend to understand
all the odd-ball things that can arise when we all wire our airplanes
differently. Case in point: I had always had a terrible noise intrude when
transmitting on the lower frequencies (like typical ground control, say
121.7 for instance) that seemed totally random - some times it would happen
and some times not. Took me a month or more to notice that it only happened
if I had unplugged the second head-set from it's jacks. Plugged in - no
noise, unplugged - YUK! Never did figure that one out so I just never fly
without the other head-set installed - go figure.

Good luck,

Dan Schaefer
Early LNC2