Topband: Shunt feed short tower

[Tom Rauch]

> Earl, I respectfully disagree to some extent with your
> comments.  Although I agree that omega is a good way to 
> match a
> physically short tower I would not give up on the simpler 
> gamma match just yet.

If the tower is not reasonably close to resonance a gamma 
will not do the trick.

> The best matching system for feeding a grounded element, 
> unless we
> are very lucky or very careful in planning the 
> installation, is a
> simple gamma capacitor and shunt feed conductor with the 
> tap point
> adjusted to find 50 ohms resistive.

A short tower far from self-resonance as a Marconi is 
similar to a Hairpin Monopole. Earl's statement seemed 
accurate to me.

Although I'm not positive where the transition point is, 
when a folded element  (like a shunt fed tower)  is 
electrically too short it requires a capacitor across the 
folded element with a single capacitor to the feeder.

You can see impedance ranges of various configurations at:

http://www.w8ji.com/omega_and_gama_matching.htm

The main problem we run into with a very short antenna far 
from resonance is the R is too low, so even if we add enough 
C to hit resonance in the folded section SWR is terrible. I 
guess  another way of saying that is there won't be a 50 ohm 
point.

With a shunt/series C, we can manipulate the impedance 
upwards. I can take a 30 foot chunk of 450 ohm line and 
match it on 160 that way.

The best thing to do is model the antenna if in doubt, 
although I've never found Earl's charts to be wrong.

73 Tom