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
Tom Rauch wrote:
>
> > 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
> capacitors.
> 73, Tom W8JI
> W8JI@contesting.com
>
> ________________________________________________________________________
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