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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: (Tom Rauch)
Date: Fri, 8 Feb 2002 19:51:15 -0500
> I found a couple large air variables (600-800pF range) for my 80m/160m
> inverted L's, *HOWEVER* I understand that you could do the same thing
> with a length of coax, like RG8, if you had a rough guess how much
> capacitance to put in series with your gamma.  Consult your coax table
> for the "pF/foot", but I think it runs roughly 10-20pF/ft (or
> whatever), so it would just be a matter of coiling up 30-40' (or
> whatever you think you might need) and and trimming to match  and as
> they say "Bob's your Uncle". 

Coax stub capacitors certainly can work in some applications, but 
we have to be careful how we use them!  

Coax makes a notoriously low-Q capacitor, and using capacitance 
times length in feet is only reliable when the cable is a tiny fraction 
of a wavelength long.

The coax is really a stub, and not a capacitor.

You would have to use a program like TLA, a Smith Chart, or do 
longhand calculations in order to know the correct capacitance.

For example, 40 feet of RG-8 coax actually looks like 1500 
picofarads on 1.85Mhz, yet if we use capacitance-per-foot the 
expected value would be more like 1100 pF or so. The series 
resistance of that "capacitor" is about 1/2 ohm, so Q is only about 
140. In comparison a typical air-variable has a Q of over 2500!

On 80 meters, the same length coax looks almost like a short 
circuit, and has a Q in the single digits!

Another factor is the voltage across the open end of the coax is 
much higher than the voltage that would appear across a 
conventional capacitor. 

This is why antennas that use coax for loading have power handling 
problems, and are less efficient than antennas using conventional 
73, Tom W8JI 

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