[Amps] RF in Vehicles

Thu Apr 20 23:55:58 EDT 2017

I had a similar issue while working as a contract engineer on Sandia’s participation in the TWP-ICE program a few years ago. We were flying a composite aircraft from Mojave, CA to Darwin, Australia and the ground crew had installed the required HF rig and the nasty longwire antenna. Keying the mike in any of the used aircraft bands caused various instrumentation problems, especially with the Fuel Fumbler. I was consulted and it was pretty quickly obvious that the antenna mismatch had RF on the feedline. It was solved with the requisite number of coaxial ferrite beads.

The composite nature of the aircraft made for some interesting interactions between the aircrafts various emitters and our fairly sensitive (and emissive) payload. I spent a while with wire mesh, nickel paint and copper strapping to solve a number of them.

My suggestion (which someone else has already posted) would be to start with a dummy load on the back of the rig, moving to a dummy load on the end of the feed. After that, an impedance analyzer on the feedline connected to the antenna. 
The problem has to be isolated. Oddly, I’ve never done this sort of work on a metal skinned plane, only mixed composites.

73 de K0IW
George K. Watson

> On Apr 20, 2017, at 7:07 PM, Carl <km1h at jeremy.qozzy.com> wrote:
> 
> RF can flow on both sides of the aircrafts skin and it appears you have a typical common mode issue. If you would be a bit more specific about the frequency, since VHF/UHF covers from 30 mHz to 3 gHz, a solution can be suggested.
> 
> Carl
> 
> 
> ----- Original Message ----- From: <qrv at kd4e.com>
> To: <amps at contesting.com>
> Sent: Thursday, April 20, 2017 7:14 PM
> Subject: Re: [Amps] RF in Vehicles
> 
> 
>> He was speaking in shorthand.
>> 
>> Of course the radio is the "source" of the RF.
>> 
>> Presuming that the coax is properly connected to the radio, the
>> radio is properly grounded, and the antenna is resonant then the
>> problem would seem to be that something is coupling the RF from
>> the antenna into the wiring to the Supervisory Control Panel ...
>> or, the RF is somehow getting back though the power supply line.
>> 
>> If the antenna is non-resonant, for some reason, then the coax
>> can unintentionally become part of the antenna system ...
>> 
>> 1KW at VHF/UHF is a ton of RF anywhere - but in the closed system
>> of an aluminum container - wow. You don't have to misplace much
>> of that power to cause mischief.
>> 
>> Given your altitude I wonder about the need for so much RF power,
>> but that's your business.
>> 
>> Just one non-engineer Ham's thoughts ...
>> 
>> 73, DavidC KD4E
>> 
>>> So I have this airplane.  Not a Cessna 150, but not a B-52 either.  Jet with with about 30 KW available as the power source.  The transmitter consists of a phased combiner with four PA stages adding up to well over a KW in the VHF/UHF range.  The antenna is a basic monopole that uses the complete mounting base as a ground plane.  This arrangement is mounted on the aircraft centerline.  The aircraft itself makes a fairly good Faraday Shield in that it has a solid surface of aluminum that is overlapped and bonded to the internal structure.  Never-the-less, there are several wiring bundles that connect to this transmitter assembly for command and control that are fed inside of the aircraft and thus by-pass the "Faraday Shield".
>>> 
>>> When the transmitter is turned on, one of the internal generators in the aircraft immediately turns off.  The generator is a 3 phase 115 VAC 400 Hz model that uses an internal permanant magnetic generator at 40 volts AC 800 Hz, that is in turn regulated to control the field of the generator.  This is controlled by a Supervisory Control Panel that controls the field and also monitors for over/under voltage and frequency.
>>> 
>>> Clearly the Supervisory Control Panel is being impacted by RFI thus shutting the generator down.
>>> 
>>> The comment was:  "This makes no sense. Unless something is very wrong with the radio,  there should be no RF on power or control wiring. From an EMC point of  view, the ANTENNA and it's counterpoise (metallic parts of the vehicle)  are the source of RF, NOT the radio. Clearly, whoever at Toyota wrote this doesn't have a clue."
>>> 
>>> Ok, so what's wrong with this airplane?
>>> 
>>> Mark Bitterlich
>>> WA3JPY
>> 
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