> 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
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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