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[Towertalk] mobile antenna matching problem

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Subject: [Towertalk] mobile antenna matching problem
From: (David Jordan)
Date: Tue, 23 Jul 2002 07:24:39 -0400

You could solve your problem really fast if you had a MFJ 259 antenna 
analyzer.  It's not clear whether what you have in the way or resonance 
or input impedance.
At 13.1mhz you have your 1:1 50ohm match but the antenna may not be 
resonant at that freq.  The analyzer will tell you what you have.

The presumption that no ground and no RFI is OK if false. You need the 
bonding for the antenna to perform.  The skin effect of the sheet metal 
of the vehicle is critical to the optimization of the mobile antenna's 
performance. Bond the bracket to the frame rails or sheet metal of your 
vehicle and take you measurements again.  I've seen as much a 10-15db 
difference on receive signals from making such a change!

Don't detune your antenna in search of a 1:1 match.  Use the analyzer to 
determine the resonance and adjust the antenna for the freq. you prefer, 
then build a matching device to match the coax impedance to the input 
impedance of the antenna. The instructions with the mobile whip should 
have directions to build a simple matching device. Usually a shunt coil 
across the input with the input coax tapped across the coil till 1:1 is 


Bob Puharic wrote:

>I put a new antenna mount on my honda crv today. the mount is about 3'
>angle aluminium stock, with a hamstick mounted at the top. the
>aluminium is held against the spare tire by 2 long bolts, and the
>bolts are anchored on the back of the tire mount by 2 other pieces of
>when i used this antenna on my blazer, it was a perfect match at 14.2
>mhz.  now it matches 1:1 between 13.2 and 13.7 mhz. lengthening and
>shortening the whip does nothing, since the swr climbs to over 5:1 in
>the 20 meter band. 
>although the swr is high, signal reports find the audio is clear with
>no signs of distortion (so no rf getting into audio). i did not try a
>ground but the lack of distortion and rf seems to indicate this
>probably wouldnt help.
>anybody have any ideas? thanks all...
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