Owen Duffy wrote:
But... in that article which recommends 28' of 300 ohm ladder line you
say "To improve the 75m SWR, try installing a 1000pF capacitor (mica
or doorknob) in parallel across the ladder line at the ladder line to
coax junction. Remove the capacitor for all other bands."

Yes, but installing the cap will raise the resonant frequency.

What I have done on my G5RV is find the point where the SWR circle
intersects the 1/50 conductance circle in the capacitive reactance
^^^^^^^^^^
region on 3.8 MHz and install a 1000 pf parallel cap there. My series
section line is 22.5 ft. of Wireman #554 at that point. The 50 ohm
SWR is reduced from about 5:1 to 1.3:1 on 3.8 MHz.

This implies you are trying to "tune out" the shunt capacitive
reactance at a point on the line where the shunt resistive component
is 50... but you need the opposite sign of reactance reactance (so
that the susceptances subtract), you need an inductive reactance in
that case.

Sorry, I misspoke. Where I said "capacitive reactance region"
above, it should have been "inductive reactance region".

If "your G5RV" has a feedpoint impedance of 36-j324 (that seems
reasonable), your 22.5 ft. of Wireman #554 will transform that to
21.53-j53.33, and the VSWR in 50 ohm line connected at that point
would be 5.3.

"If" is the important word. My G5RV is obviously different from your
values. It's made out of insulated wire and I'm not sure it is exactly
102 feet long.

Flawed explanation aside, the only way that 22.5' works is if your
feedpoint Z is quite different to 36-j324.

And it is obvious that's the case. The "450" ohm ladder-line is 22.5 ft.
long and a 1000 pf capacitor resonates it on 3.8 MHz. Whatever the
feedpoint impedance needs to be to cause those conditions, that's
what it is.
--
73, Cecil http://www.qsl.net/w5dxp