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

[Jim Lux]

At 01:33 PM 6/8/2004 -0400, Tom Rauch wrote:
> > >Let's submerge our screwdriver mobile antennas in a
>cryogenic
> > >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"?