[RFI] Hybrid Car RFI

[Jim Brown]

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