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Re: [RFI] Mobile amps and vehicle issues

To: "Howard Lester" <hlester@mmto.org>, <rfi@contesting.com>
Subject: Re: [RFI] Mobile amps and vehicle issues
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Thu, 12 Jan 2006 16:28:20 -0500
List-post: <mailto:rfi@contesting.com>
> What about the power leads from the radio to the battery?
Are any particular
> routing precautions advised?  As for the antenna, it'd be
a quarter wave mag
> mount on the trunk - nothing fancy.

Hi Howard,

That's actually the kind of thing that causes problems with
current on coax shields! You have a poor antenna ground.

A requirement of any unbalanced two-conductor transmission
line is that current leaving the ground must equal current
flowing up into the antenna, and the voltage from the shield
termination be zero when compared to the area the
transmission line runs through or connects to.

What we have with a mag mount is a tiny area that has
capacitance to the car body trying to carry just as much
current in the path to the body as flows up into the whip.
Since the mount has impedance to the car body, a voltage is
created. That voltage drives current into the coax shield
that flows along the coax shield. No matter how good the
coax is or how bad, this unintentional current dominates the

Sleeving the coax some small distance away from the mount
with suitable beads would help reduce that current.
Manufacturers should warn people about poor antenna grounds,
and that should include mag mounts. It's better to not have
the current follow antenna cable than it is to move the
wires, because there is no guarantee it will decrease to
acceptable levels by the time it reaches the power leads.

It wouldn't take a great deal of time for a manufacturer to
write a tutorial on what to do and not do with an antenna.

Does the ARRL measure RF on power leads of mobile rigs?
Anyone know?

73 Tom

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