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
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