> I've been following this thread on input swr VS.
> cable length.
> I've seen this many times but what has apparently not been
> considered is
> the capacitance per foot of the cable, which is in the
> range of 25-30 pf
> with the RG8 series of cables. That means 20 ft. of cable
> has around 600
> pfs.
The cable also has distributed inductance and when
terminated in the surge impedance the capacitance vanishes.
It's a transmission line, not a capacitor.
Even when you make a "capacitor" from coax by leaving one
end open the capacitance at radio frequencies is never the
capacitance per foot times the number of feet. It is close
only when the cable is very short in terms of wavelength.
Consider 11.6 feet of RG-8/U cable. If open circuited the
impedance at the input end is:
28MHz 3200 - j16
21MHz 1.6 j50
14 MHz .53 j0
Now where does it look like ~28pF per foot, except where the
cable is very short in terms of the wavelength. When we
terminate it the resistance, capacitance, and inductance
variations become less and less.
Another good rule to remember is the impedance of a source
can never affect the impedance or SWR of a load. Never. No
impedance or SWR change downstream (towards the source from
the measurement point) can affect the SWR or impedance
upstream.
73 Tom
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