Mobile Antenna Help

Jim Bromley, K7JEB k7jeb at COX.NET
Mon Feb 4 22:31:17 EST 2008

Richard Arland, W3OSS wrote:

> So, my situation is this: I have mounted the
> screwdriver antenna on the rear cross member of the
> roof rack on my wife's Nissan Pathfinder SUV. I have
> tried the antenna on various HF frequencies including
> 80 and 40 meters.  While it does resonate, my 100 watts
> of RF output doesn't radiate efficiently. 

Here's my $0.02 worth:  You need a low-inductance
connection from the base of the antenna (the shield of
the coax) to the car-body metal closest to "real"
ground.  In most cases, this would be the automobile
frame.  By 'low-inductance' I am talking about a piece
of copper strap 4 inches wide.  If you want to use
aluminum, I would double the width.  You would be
ahead of the game if you had several of the these
ground straps, say one on each side and one in the back.
This would reduce the inductance between the feedpoint
of your antenna and "real" ground even more.

You mentioned installing a counterpoise either on the
roof or under it.  I am assuming this means the car roof
is plastic.  This is a good idea.  It should be a solid
sheet of thin metal such as aluminum, or maybe galvanized
steel, and bonded to the ground straps.  Your "real"
counterpoise is the ground underneath your car, out to
about 1/10 wavelength.  You want maximum capacitive
coupling to it and only get that through your car's body
metal.  But a counterpoise will help in that portion of
the antenna's field right at the feed point.

Hope this helps...

Jim, K7JEB, Glendale AZ

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