Topband: A question of efficiency

[Tom Rauch]

Hi Charles,

All of this may be academic because the dominant loss in 
your system may very well be losses external to the antenna 
that you have no control over.

<My current 160M transmitting antenna consists of a vertical 
radiator 82
<feet tall together with an "inverted U" linear loading 
section at the
<base.

As a general rule, for the same size conductors, a loading 
coil has less loss than linear loading. This is because 
mutual coupling between turns allows less conductor length 
to be used to have the same reactance.

A typical air-wound loading coil of modest reactance with 
large conductors has a Q of 500-800, while a loading stub 
might range from 40-500 in Q.  It's a common myth, 
popularized by antenna manufacturers, that stubs or linear 
loading has less loss.

<(A) Bringing the antenna to resonance with the loading 
section, and then
<matching to the line with the L-network?
<Or,
<(B) Matching the non-resonant antenna directly with the 
L-network, omitting
<the loading section altogether?

It probably makes very little difference unless the L 
network has abnormal loss problems into one impedance and 
not the other. Either way, ground losses will probably 
dominate the system. Either way, you have a base loaded 
vertical and the current flowing into the ground (the major 
source of loss) will be the same.

You might gain a small amount by top loading since that 
would reduce ground currents, but that would complicate band 
switching. In any event any change caused by top loading 
would be fairly small if you had a very good ground system.

73 Tom