On 11/30/2018 5:20 AM, Jim via RFI wrote:
I’m running 100 watts to HamSticks on mag mounts on the roof, no other
particular RFI proofing other than ferrite common mode chokes at the antenna
feedpoints.
I use mag mounts on VHF/UHF, and all that I've seen have either no
connection or poor connection between the coax shield and the base. Any
mobile antenna depends upon the car body for a counterpoise, and the
only connection between a mag mount and the body is capacitive, between
the base and the body. If there is no connection to the base, there is
no counterpoise, which makes the antenna not work as well it should.
And, it should be noted, there isn't much capacitance, even with a big
mag mount, and the feedpoint is pretty low impedance, so a small
capacitance is not a great connection. Without a choke, the coax could
be the counterpoise, but with noise pickup.
I bought my current vehicle, a 2006 Toyota Sequoia (big SUV) in 2005 in
preparation for my move from Chicago to W6. I bought it in Nov, the
beginning of winter in Chicago, so K9IKZ invited me to the loading dock
of his biz to help install wiring for a mobile rig. I had successfully
used HamSticks on my Volvo S80 with a mounting to the trunk lid (bonding
around the hinges to get to the rest of the body), and we probed the
Sequoia around the luggage rack looking for contact with the body.
Thanks to paint, bolts holding the rack showed no conductivity between
them, but we did find one that made contact and used it to bond the coax
shield. We also pulled big red/black glorified zip cord from the battery
through the firewall to power the rig.
I thought everything was cool until firing up the rig on 17M SSB and
noticing that the ventilation fan speeded up when i transmitted. Minor
annoyance, which I ignored. On my first trip west, in a section of I-80
passing through the desert in NV, I fired up on 20M SSB with a CQ and
got several replies. But I also noticed that my speed had slowed to 25
mph -- RFI to one of the car's computers had kicked it into "limp-home"
mode.
Since I was very busy moving, I never took the time to fix the problem,
and, wanting to preserve my happy marriage, don't play ham radio when
driving with the XYL, so have had no motivation to fix it.
73, Jim K9YC
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