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[TowerTalk] Mobile Antenna Mounting

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Subject: [TowerTalk] Mobile Antenna Mounting
From: (Tom Rauch)
Date: Sat, 5 Jun 1999 22:08:28 -0400
Hi Dave, 
> Also, I have thought of mounting the screwdriver portion (COIL) of the
> antenna horizontally on the roof rack, then run a flexible wire strap to a
> 102" whip mounted vertically in the middle of the roof rack.

The stray shunt C from the strap to the roof will greatly increase 
circulating currents in the coil, and lower feedpoint resistance 
greatly. At the same time, radiation efficiency will fall way off. Very 
little current will appear in the whip. 

Base resistance will look excellent, bandwidth will look narrow, but 
the antenna won't radiate well at all.   

> I've made some field strenght measurements of a HB screwdriver which is
> mounted on the back of  my motorhome and it looks like very little if any
> RF radiates from the portion of the antenna beneath the coil.  The coil
> has some radiation and the whip produces the highest voltages measured.
> When I first started with the RV ant. it was mounted on the rear bumper
> and performed

Wrong test instrument, and wrong test method..You are measuring 
the electric field. It is the acceleration of charges that causes 
radiation, but we can't isolate radiation fields from induction fields 
because the test equipment we have can not sort them apart.

In order to measure the radiation field, you have to be back about a 
wavelength. That's the distance it takes for the induction fields, 
both the electric induction field and the magnetic induction field, to 
decay to negligible values from a small antenna.

The sniffer you have is measuring one of the induction fields, 
probably the electric field. That's because the induction fields 
ALWAYS vastly overwhelm radiation field levels close to the 
antenna. They simply decay faster with distance.

> very poorly.  As I moved the ant. up the performance improved. However, I
> couldn't notice any significant improvement once the coil portion of the
> ant had cleared the roof level which again causes me to wonder if the
> lower portion of the mobile ant. radiates any significant RF.
> All comments welcome -- thanks,
> dave
> p.s. If anyone is not familiar with the screwdriver ant here are a
> couple of
>        web pages with pictures, ""; and
>        "";
> --
> FAQ on WWW:     
> Submissions:     Administrative
> requests: Problems:                
> Search:                  

73, Tom W8JI

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