[TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower

[Tom Rauch]

> Tom,
> What if the coax is in SERIES with the feedline, like in a gamma
> match, instead of using the coax as a 'tuning stub'? This was my
> impression of what the questioner meant. Don N8DE

Hi Don,

All problems are exactly the same.

I'm not in the least saying coax does or will not work. I'm just 
mentioning we should be mindful of what it really is, and that is a 
stub!

The longer the coax is when compared to a quarter wavelength, the 
more problems we will have if we consider it a pure "capacitor". 
After all, an open stub just slightly more than a quarter wavelength 
is an inductor!

The maximum voltage is higher, the "Q" we consider with loss is 
lower (since equivalent series loss resistance is higher), and the 
"Q" we consider with bandwidth is higher (frequency bandwidth is 
narrower) than in a traditional capacitor.

On the other hand coax is inexpensive, can work OK in some 
applications, and we all have coax laying around. 

The ARRL's program TLA written by Dean Straw (given away with 
the ARRL Antenna Handbook) is a good program for looking at 
transmission lines. It agrees almost exactly with measurements I 
have made, to the point I use it now more than test equipment! 
It even confirms virtually all loss in coaxial transmission lines at HF 
and lower VHF is from conductor resistance and not dielectric 
losses!
73, Tom W8JI
W8JI@contesting.com 

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