[TowerTalk] Top hat on mobile usefull?

[Tom Rauch]

> If you can mount the top hat 6" or more above the center
loading coil
> the additional capacitance will reduce by approxmately
23-33% the number
> of turns required to resonate the antenna on 80/40m. Less
resistance =
> better performance.

Sometimes, but not always. As a general rule the hat has to
be near the top of the radiator.

Under some conditions when the hat is installed close to the
loading coil and/or if the hat is mounted low on a long whip
it can actually not increase field strength or even decrease
field strength!

Two things happen.

The low hat has the effect of shunting current off that
would flow to the top of the radiator, so it can reduce
effective height.

Any capacitance appearing directly across a loading
inductor, even though it reduces turns required, will
actually INCREASE effective coil resistance.  The increase
in effective loss resistance in the inductor can be
significant even though bandwidth and number of turns in the
coil decrease.

All those bug catcher sheet metal hats mounted right on the
loading coils at the bottom of a long whip? Bad idea.

73 Tom