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Loops for mobile operating was Re: [TowerTalk] bandwidth vs efficiency

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Subject: Loops for mobile operating was Re: [TowerTalk] bandwidth vs efficiency
From: Jim Lux <>
Date: Tue, 08 Jun 2004 10:55:25 -0700
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At 01:33 PM 6/8/2004 -0400, Tom Rauch wrote:
> >Let's submerge our screwdriver mobile antennas in a
> >container.  An additional advantage would be that no bugs
would nest
> >in there!
> >
> >73,
> >
> >   George T. Daughters, K6GT
> reducing the loss in the coil will make the bandwidth
narrower.  You run
> into this when you contemplate making high efficiency
compact loops.  Sure,

The mobile system is dominated by ground loss. In my large F250 supercab truck the grounding loss was about 10 ohms on 80 meters. Increasing loading coil Q by a factor of three doesn't have near the effect on bandwidth that changing the VALUE of reactance does. If I add a large hat so as to reduce inductance by three-fold, bandwidth is roughly three times larger. This can occur even while efficiency increases substantially, because the ground loss swamps out resistance changes in the overall system.

My narrowest 160 meter mobile antenna is also the least
efficient by a considerable margin.

Loops, being a closed system, are different. Unless of
course they are near a very lossy media.

There are some commercial mfrs (in Australia, land of pervasive HF mobile) of compact loops that look much like a luggage rack, and there are some journal papers on turning the whole vehicle + a big whip into a fairly large loop. (coupling to the "A-pillar" of the vehicle, with a transformer)

A loop is going to couple to the surroundings, just as a dipole/monopole would, but the coupling mechanism might be different, and the dominant loss mechanisms might be different (high H field, low E field with the loop).

I wonder if there's some qualitative statement that could be made about the relative efficiencies of the two. They're both "physically small radiators", but very different in some ways.

Is it the "ground loss" that's dominating, or the "vehicle body loss"?


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