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

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Subject: [TowerTalk] (the) Actual Capacitor for Shunt-Fed Tower
From: (Don Havlicek)
Date: Fri, 08 Feb 2002 22:22:10 -0500
I think you still misunderstand what I'm saying.
What if the coax is in series with the feedline ??
For example:

The center conductor of the coax 'capacitor' connected ONLY to the
center conductor of the feedline, and the only the shield AT THE OTHER
END of the 'capacitor' connected to the gamma rod.

Surely this would be different than connecting the coax 'across' the
feedline-gamma connection???

Teach me, please!

Tom Rauch wrote:
> > 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

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