Topband: Shunt feed vs sloper

[Tom Rauch]

Hi Dean,

It helps to look at the extremes. At the best extreme is a 
substantial disc at the top of the tower near 1/4wl in radius, and a 
1/4 wl grounded tower. This should produce minimum current in the 
tower. 

Your model comes somewhat close to that end of the scale, since 
it is a large plumber's delight 20-meter antenna modelled with the 
slopper on 80-meters. 
 
Gradually moving down towards useless performance, we would 
use progressively smaller yagis that with the tower's length 
included make the tower's self-resonance approach the sloper's 
frequency.

The worse possible combination of course is a structure that is self-
resonant as a conventional marconi. That would be a grounded 1/4 
wl tall tower with nothing on the top but the sloper. In that case, we 
would have a very poor system.

Most 160-meter slopers will be nothing like the 80-meter model, 
because few people have 40-meter 5-element full size yagis 
(remember we have to scale it to 160)! They will be more towards 
the poor end of the scale.

It is important that we form opinions based on the actual antennas 
we will use, rather than concluding an 80-meter model using a 
large antenna somehow applies to all antennas, especially those 
for 160-meters. I'd hate for anyone to consider that one situation 
that works means all situations...or even most situations....work.
73, Tom W8JI
W8JI@contesting.com