In article ,
Cecil Moore wrote:

What we found at the CA shootouts is that when the bottom
section runs closely parallel to the vehicle body, as it
does with a trailer hitch mount on an SUV, the field
strength is much lower than if that bottom section is
in the clear, e.g. mounted on the roof of the SUV.

That seems to match up with various peoples' experiences that I've
heard. Close spacing in this way makes the signal weaker, and also
seems to make the antenna more difficult to tune/match properly.

This also makes sense from an engineering point of view. The
closely-parallel spacing of the bottom section and the metal vehicle
body would form a transmission line of sorts. This transmission-line
section would not radiate much (or efficiently) - its radiation
resistance would be quite low. As a result, the antenna's feedpoint
impedance would be lower than otherwise (requiring a more aggressive
impedance step-up of some sort to match a 50-ohm line).

The coil and whip would be above the body, and would still be able to
radiate, but you'd be left with something akin to a bottom-loaded whip
with no high-current radiating section, rather than a center-loaded
radiator with a low-loss high-current radiating section below the coil.

In effect, a close/parallel mounting of this sort would seem to
sacrifice much of the radiating power of this type of antenna.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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