Topband: shunt fed towers

[Guy Olinger, K2AV]

From: "John" <ljdoyle at suscom-maine.net>


>read recently that a tall tower say 1/4 wave with large antennas atop of it 
>really does not work well as a 160m antenna...what do the experts say?i 
>have been using one of those for years and i think they work like 
>gangbusters but i'm not an expert so it may be that for years i've been 
>using the wrong antenna...and should shorten mine to say 70 feet or so ....

--------

What I have read here and elsewhere is an urban legend substitution of easy 
feed-point for performance, where "works well" means SWR instead of far 
field signal strength. Not all high performing antennas are a natural match 
to a piece of 50 ohm coax.  Perhaps most of them are not.  The easy-feed 
designs are popular just for that, easy-feed. And there are details that can 
detune an expected result whose debugging is irritating indeed.

Some tower heights do not lend themselves to easy feeds and involve 
constructing something that cannot be bought already made at the local ham 
outlet.

And, as always, the three most important components of a high performing 
excited tower on 160 are radials, radials, radials.  Poor radial systems can 
cause the feedline shield to be a major loss factor in the performance.

A 125' tower with some (what amounts to relatively minor) beam top loading 
over a 60  times 1/4 wave radial field will be an excellent performer.  If 
you don't search the archives for stories on how people successfully fed 
such systems, you may be sore tried to grab a 50 ohm point to make it easy 
for your coax.

You may not be able to read it on your MFJ (or ...) due to a BC station at 
some considerable distance.

You may have serious issues with the coax feed shield acting as a major 
player in the system.  Etc, etc, etc.

But get all the issues ironed out, and it will be a killer antenna.

73, Guy.
K2AV