Topband: Shunt Fed Tower Info

[Tom Rauch]

> Regarding floating elements be aware that the driven element, although
> insulated from the boom, acts like it is grounded to the boom on 160m
> because of the feedline's coupling to the tower on its way down to the
> ground.  Therefore, the driven element already fully adds to the
> top-loading and there is no reason to "ground" the driven element to the
> boom when you shunt feed the tower on 160m.

The primary job of a GOOD balun is to present significant series impedance
with the feedline for common-mode current.
The baluns does this by inserting a high common-mode reactance and loss
resistance in series with the feedline at the element.
Baluns using ferrite materials having a high loss tangent at 1.8MHz, which
is most baluns, would have severe heating with modest power.

Assuming the balun is a typical current balun using 73 or 77 material, there
are approximately equal parts resistance and reactance in series from the
tower to any floating DE. The balun would present several hundred ohms
inserted at a voltage point, so there is plenty of voltage to drive the
balun. Even a few dozen milliamperes at high voltage and resistance can
significantly heat the balun. Ferrite beads used in baluns like the W2DU
baluns overheat at dissipations less than 1-watt per bead! Force-12 baluns
and Hygain baluns are particularly bad.

I'd be very careful about the balun I used, or add a choke for passing 160
current to the driven element. Even another vertical located away from the
HF antenna can cause enough current to overheat a balun!

73 Tom