[RFI] Re: RFI Problem....

dgsvetan@rockwellcollins.com dgsvetan@rockwellcollins.com
Wed, 20 Mar 2002 10:24:57 -0600


It is difficult to say exactly what is happening in the circuit of your
vehicle's turn signal system.  Given the vintage of the car, I would guess
that the circuit is the usual thermally-cycled flasher that depends upon
current draw of the lamp load to set the flash rate.

A few years ago, I participated in some vehicle RFI testing in which
several modules of a vehicle were monitored for reaction to RF fields.
Results varied (for a number of reasons, and the test results are
proprietary, so I can not discuss them), but turn signals circuits were
among the affected.  Bear in mind that ALL of the RF stimulation to the
test vehicle was external to the vehicle.  That is my way of leading into
comments about your antenna.

I don't believe that electrical load imposed on the vehicle when you are
running 5 watts of output from your rig is sufficient to alter the flasher
characteristics.  However, if the field from your antenna couples into one
(or both) of the rear tail light assemblies or cabling, that's another
story.  Honestly, unless you are suffering abnormal bulb burnout or the
changed rate really annoys you, I would not worry about it a whole lot, so
long as no other important functions of the vehicle are affected.  You do
not mention how the antenna is attached to the trunk.  Is it a mag mount,
or does the antenna make contact with the metal of the trunk via a trunk
lip mount (secured with screws) or something like an NMO mount set into a
hole in the trunk lid?

While mag mounts are very convenient, they have a drawback:  the RF current
around the antenna and in the coax between it and the rig may flow in
unpredictable ways.  If the antenna is directly fastened to the vehicle
body, the antenna ground currents can flow primarily on the outer skin of
the body, and the return current in the coax (between rig and antenna) will
flow along the inner surface of the coax shield.  When RF gets coupled onto
the outer surface of the coax shield, then you have the potential for upset
in unexpected ways and places.  The coax appears as a radiating element.
This is why, in my opinion, antennas should be body-mounted whenever
possible (assuming a metal body, of course).  Getting power from up at the
battery is better than from an under-dash clip-on.

Good luck.

73, Dale

"Ace" <n8qnp@bigfoot.com>@contesting.com on 03/19/2002 05:35:54 PM

Sent by:    rfi-admin@contesting.com

To:    <rfi@contesting.com>

Subject:    [RFI] RFI Problem....

I have a problem which I have not seen addressed anywhere.

I have a 1980 Toyota Tercel, with a Yaesu FT-1500M mounted
in the passenger compartment and a quarter wave antenna mounted
on the top of the trunk.

When I am sitting at a light with the turn signal on and I transmit
even at five watts, I notice that the turn signal flasher speeds up. I
have the antenna cable running from the back of the car under
the door panel and up under the dash to the radio. The power cable
is tapped off of the battery. Any ideas as to where the RFI is coming
in at?

Ace, N8QNP
Dayton, Ohio

RFI mailing list