Topband: Length of 'T' Top Hat wires?

[Rick Karlquist]

Joe Subich, W4TV wrote:

>  > Is there any theory to the "25% rule"?
>
> The 25% rule says the ends of the top hat wires should not extend
> more than 25% down the vertical.  The thought is that the wires
> "shield" the main vertical ... that the currents on the top hat
> wires cause cancellation.  Once the tips of the top hat wires get
> 25% down the vertical, the theory is that the cancellation/reduced
> radiation resistance offset any gains from the added electrical
> "height."

It is easy to model this with EZNEC, and the rule of thumb turns
out to be pretty good.  I obey the rule on my 90 ft vertical.
I violate the rule on my 60 ft vertical, because I was in a hurry
and didn't want to fool with a loading coil at the base.  The radiation
resistance (as modeled) goes down but reaches a point of diminishing
returns around 25% point.  Going beyond that doesn't necessary make
the radiation resistance go back up; it just stops going down and
then all the additionial length is doing is saving loading inductance at the
base.  Anecdotally, in the recent 160M contest, the 60 foot vertical
seemed like it worked as well as the previous 90 foot vertical,
which worked really well.

Rick N6RK