From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: RFI susceptability of new cars?
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Destructive Testing Systems
Date: Fri, 8 Jan 1993 15:49:15 GMT

In article <13412@bnr-rsc.UUCP> everett@nmerh97.NoSubdomain.NoDomain (Jerry F. Everett) writes:
>My RFI susceptability is so bad I took the rig out of my GM car....Read on!!
>
>
>My problems with vechile RFI have been a bit different. 
>
>First a overview of my system:
>Chev Cavalier 1990 (There is noooo waayyyy I am going to drill a hole 
>and believe me when I tell you I am a truely fanatic ham. It is my first car.)

Believe me, real hams have the electric drill whirring before their brand
new Porsche has stopped rolling into the driveway. :-)
A properly installed NMO mount does not hurt resale value of a car. Just
put a cellular antenna on it when you're ready to sell it and advertise
it as "cellular ready."

>Radio: Kenwood TM-241A 2 Meter rig. Problems only on 50 watts. 
>Same power as Bill.Antenna: Antenna Specialist, ON GLASS mount, for 2 meters.

Well here are your second and third problems, a Kenwood radio and an on glass
antenna. :-)
Ok, I'm biased against Kenwood VHF rigs, but the on glass antennas really
are a problem for RF on the transmission line. Tying the little tab to
metal helps a bit, but don't be surprised if the antenna works just as
well if you remove the whip and just let the coax radiate.

>Config:  DC is taken from the fuse panel. (I know it should be on the battery
>but it is really tough to attach the leads to the battery Terminals)
>Coax runs from under the AM/FM radio, through the enclosed  moulding which 
>runs up the side of the windshield to the roof, back along the roof (under 
>the roof fabric) to the middle of the back windshield. The antenna is mounted
>as close to the metal of the roof as possible.

Ok, fourth and fifth mistakes. You should take power from the battery,
*both* leads. You can do this on a GM side post battery by using ring
terminals under the heads of the connecting bolts. Grease them well after
torquing down the bolts to prevent corrosion. GM says to route coax to the 
rear of the vehicle through the wiring channel under the molding strip at 
the bottom of the driver side door. Avoid the passenger side kick panel, 
that's where the computer lives. And naturally avoid the entertainment
radio.

>The SWE is around 1.5:1 last time I checked.

That's a tad high, but acceptable for a normal antenna. For a on glass
antenna, it probably indicates a problem. Try adding a few feet of cable
and taking another reading. If it's different, you've got hot coax problems.

>PROBLEM:  When I use 50 watts:
>        - The dome light comes on.
>        - The AM/FM radio generates a load hum. (RF getting in somewhere)
>        -  My "shift up" light comes on.
>        - On occasion there is a clicking/light_thunking sound from the doors.
>(Sounds similar to when you press the power door lock button when the doors 
>have already been locked.)

All classic signs of a radiating coax inside the vehicle. As a test, try
replacing the on glass antenna with a dummy load. Use a cable between the
dummy load and the radio that parallels the on glass antenna cable. All 
these symptoms should go away. If so, you *know* it's the on glass antenna
that's causing your problems. A magmount in the center of the roof would
be much better. Drilling a hole and installing a NMO would be better
still.

Gary
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