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Old April 17th 08, 06:56 PM posted to rec.radio.amateur.moderated
Michael Coslo Michael Coslo is offline
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First recorded activity by RadioBanter: Jun 2006
Posts: 828
Default RFI mitigation tips for mobile Ops

Hey all,

I installed a Bugcatcher in my vehicle last year, and thought I might
share some tips from my experience.

1. Power supply.

Run separate wires for power. Don't try to run an HF rig off a cigar
lighter outlet. I don't care if is an FT-817.....

2. Ground, ground, ground

Getting the "other half" of the antenna to work well is probably the
most important thing you can do. As well as providing the "ground", it
eliminates a lot of little antennas that can give you RFI headaches.

I used coax shield with lugs soldered onto the ends to bond everything.
Keep them as short as practical. Mine were around 4 inches, the longest
around 6 inches. In most cases, the bond can be made by using bolts that
are already on the vehicle. In a few cases I had to drill and bolt. Be
careful of where vehicle wires are already running.

Bond the hood on both sides, all the doors, the hatch if there is one.
Don't forget to bond the exhaust pipe in at least 2 places. The exhaust
even looks a little like an antenna.

Bond the radiator, especially if it is one of the newer ones that have
plastic ends. There is usually some place on the metal part that has a
bolt you can attach to.

Bonding the engine is mandatory. If your vehicle has a frame, like mine
does, bond the frame to the body at several locations.

I probably ended up with 30 different places when I bonded.

3. Ferrites are your friends

I used clip on ferrites. These worked well, and were something around a
dollar a piece.

Clip them on:

Alternator wires, battery wires, radiator fan wires, gas pump power
wires, radio speaker wires, ignition coil wires (some engines now have a
separate coil for each cylinder. If you can reach your wiper motor and
washer pump motor clip one on there also.

If you can get a ferrite on the dreaded main computer module it would be
a big help also. I just happened to have a big clip on from an old IBM
computer that fit around the cable of my computer unit.


4. Look for other sources.

After all the corrections I made, I suddenly found S9 level noise on
HF. I looked to see if ferrites had fallen off, or a bond failed. I
bought a cheap transistor radio to sniff out the problem.

Turns out that my inverter for my laptop is really noisy. I unplugged
it, and the problem went away.

In the end, my mobile HF system is less than S2 (that was as low as I
could test it, seemed to be just around ambient. The only noise source
that I haven't beat yet is the cruise control, and I can just avoid
using that. Finally, I have to remember to keep the car stereo turned
off while transmitting, otherwise I get some strange stuff coming out of
the back speakers.

Oh, and by the way, the antenna appears to work "very well", at least
I've gotten good signal reports on 40 and 20, and reasonably good ones
on 75 meters also. It seems to make use of the limited efficiencies we
see with short HF aerials.

Don't know if anyone else has anything to add, but I thought I'd share this.

-73 de Mike N3LI -