[Amps] Aircraft Generator RFI

[Rick Mainhart]

Good morning Mark,

I've worked on a similar RFI issue in the mid-1990s on two different 
steel and aluminum ships that had issues with other 
electrical/electronic interference on HF; but I believe the principals 
(and the solutions) are the same.

You will need an RF field strength meter to verify:

Measure the RF field strength INSIDE the aircraft adjacent to the 
VHF/UHF antenna penetration ... next to the coax ... with the 
transmitter keyed. You will see a very high RF level. If you can, 
disconnect the coax from the antenna and connect to the dummy load and 
remeasure. I believe you will find little to no RF detected.

Alumimum, as we all know is a very good RF conductor, as well as a great 
RF reflector. What you have is an antenna monopole (let's call it a 
probe) penetrating the skin of a rather big (and expensive) aluminum 
shield box. The coax shield appears to be bonded at BOTH ends to the 
inside aluminum surface. So ... we have a probe sticking through a tiny 
hole in this aluminum shield box, and the shield connected to the inside 
of the shield box.

The RF current path needs to go from the probe to the inside of the 
aircraft, and the only path it has is one very tiny hole (that is 
probably much less than 1/10 wavelength diameter), and any wiring that 
penetrates the aircraft (such as navigation light wiring). The result is 
high RF current on all interior wiring, and the interior of the aircraft 
... all to complete the RF current path to allow for radiation. (holes 
less than 1/10th wavelength is a good rule of thumb for RF chamber 
ventilation hole size to keep RF in, and let the hot out).

You also pick up a lot of noise from aircraft systems on your radio as well.

Where your antenna penetrates the aircraft skin, it is most likely in a 
fin or other aerodynamic radome. This antenna, in all likelyhood, has an 
aluminum mounting base that is fastened to the aircraft skin and bolted 
in place. The outside skin of the aircraft was either painted or 
anodized before the antenna was mounted, and the only connection between 
the antenna ground connection (continuing the RF shield from the coax to 
the aircraft) is on the INSIDE of the aircraft.

You will want to mark the outline of the antenna on the airplane 
exterior, remove the antenna, clean the paint or anodizing off the 
antenna mount contact area on the outside of the aircraft. Chromate 
passivation IS conductiive, where anoziding and paint typically are NOT. 
Once you've prepped the area, I would suggest reviewing the antenna 
manufacturer's installation instructions, particularly where they 
mention surface-to-surface corrosion prevention. I'm not Airframe/Power 
Plant certified, so I will not direct you to use anti-seize compound 
here. Reinstall the antenna according to the provided directions, and 
finish sealing the interface between the antenna radome and aircraft 
skin as appropriate.

Oh, and if someone other than the manufacturer painted the antenna, 
replace it. Too many paints are NOT suitable for radome use, and will 
provide shielding and attenuation to RF (something we don't want). If 
you REALLY need a paint match, you have to work with the antenna 
manufacturer to have THEM paint the radome an appropriate color.

This procedure does not require any TSO modifications, is well within 
the skills of an A/P mechanic (well, aircraft artist actually!), and 
will quite possibly get others with the same aircraft happy as well.

After installation, repeat the RF field strength test. You will see a 
great improvement, if my experience is worth anything.

Hope this helps.

73
Rick
NM3G