Ok, I have a radio issue that has been plaguing me for a few weeks, I have taken every step possible to try and resolve it and nothing has worked. At this point I have asked every LAME and Radio/Avionics outfit at Jandakot for advice, or they have had a go at locating or resolving the issue.
I am out of ideas and so I am looking for suggestions.
Firstly here is the issue.
There is an intermittent white noise that only appears on VHF (ADF is clean) that reduces a received radio transmission to 2's or 3's, however it is extremely directional and only happens if the station making the transmission is on the nose of the aircraft, if whilst receiving the poor signal I request a radio check they get me 5's If I swing the nose of the aircraft 20 degrees either side of the station the signal becomes perfect again. This happens whether the transmitting station is 500 meters or 10 miles away. It only happens when the engine RPMs are above 2200. There is no noise at 2150rpm and then at 2200rpm it is there in full intensity, it does not increase in any way (pitch/strength) as RPMs increase.
The aircraft has 2 separate radio's connected to 2 separate antennas, they both behave the same way.
Sometimes the noise is there and then it is not. It can be gone for a whole day and then return for the first flight of the next day. I can not correlate it with temperature or humidity or rain or time of day or anything, I have tried! The aircraft is working every day for multiple short flights and the problem comes and goes with annoying regularity, you just never know when it will re-appear. It started about 1 month ago, prior to that is was not present. It did not coincide with any maintenance or event.
The Aircraft is a Aero Subaru (Fuji FA-200) Lycoming IO-360B1B with a McCauley B2D34C53 constant speed 2 bladed prop, this is very similar to the powerplant found in a Piper Arrow. The Fuji is a 4 Place all Metal (Aluminium) low wing aircraft with fixed undercarriage.
Here is what has been tried so far:
Idea 1, the radio antenna is on the rear fuselage, perhaps the front of the aircraft is blocking the signal. Even though the SWRs checked out as perfect we replaced the antenna with a new higher gain one, it made the problem worse! Com 2 with the old poorly positioned antenna now gets a weaker white noise interference than com 1 meaning it is 3's when comm1 is 2's
Idea 2, It is poor earthing of the engine or mags. All the aircraft earthing points cleaned and shielding repaired everywhere. No Change
Idea 3, it is bad plugs. All plugs replaced with new plugs. No Change
Idea 4, the alternator. Engine run with alternator field disconnected (so not energised) and master bus disconnected. No Change.
Idea 5, Static build up, all static bonding points and wicks cleaned and tested. All OK. No change.
Idea 6, Noise filters on the alternator and master bus. Both filters replaced with new units. No Change.
Idea 7, A Loose connection. Every connection under the dash tested and tightened. No Change.
Idea 8, Capacitor in Mags breaking down. Mags overhauled accumulator/capacitors replaced. No Change.
Idea 9, Turn and Bank Coordinator causing noise. Fuse pulled whilst engine running. No Change.
Idea 10, Another Nav Aid or System. Everything except Com1 de-energised (including Alternator) so running off battery. No Change.
Idea 11, Prop making static charge, prop cleaned with solvent and re-painted. No Change.
Idea 12, Radio faulty, radio swapped for new unit, No Change.
Idea 13, Magnetised Crankshaft, but this could not come and go, so discounted.
I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio. The noise does not change with pitch, it is not a whine, it does not get stronger or change frequency. It is just white noise.
If anyone has any ideas, please let me know. No-one seems to have any idea what could be causing this.