Radio noise at high rpm...

[mulderpm]

On the AC make sure the earth is good. Especially between the engine block and the airframe, across the mounting rubbers. Keep all earths, as common as possible. One by the engine mount and the other by the radio antennae is all that is required. Run all earth earth wires to the engine earth on the engine mount. One earth point (not connected to airframe) in the instrument console with a thick wire, 6sqmm or more, to the the engine mount earth. No earth loops please.

QRM, interference, caused by other transmitters in you area will not, not be solved by good earthing

To avoid Radio Frequency feedback on your plane (a squealing noise, usually not audible to the pilot) always, mount the antennae as far as possible from the transmitter & headset wiring as possible. Per example see Air Magics rabbit V antennae. Ooh....and most important ....make sure your antennae is matched, lowest Standing Wave Ratio (SWR) as possible. Less than 1.5 to 1, at 125MHz is good for a 1/4 wave ground plane antennae. Make sure it is ok at both ends of the band. ATC's like the lower end, 118, make sure your radio is ok there, or else your flight may be delayed