What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

I believe your data may be correct. Electrical length Lamda at 10.1 MHz is 97.24
feet, and Lambda/4 is 24.35 feet. And 15.7/24.35 = 0.645 for the VF. Although
this VF is in the vicinity of what you'd find in coax, if there's a lot of
dielectric surrounding the conductors, this value could be correct.

I believe I recall reading an admonition (in a grid-dip or analyzer
manual) that measurements of open-circuited stubs were rather tricky
to do accurately. There's always some amount of capacitive coupling
at the open end of the cable which leads to it being less than a
perfect open circuit, and thus won't necessarily look like a perfect
short at the other end of a quarter-wave line. This can throw off the
calculations a bit.

Might be better to short the far end, measure at twice the desired
frequency, and trim until you find the short being perfectly mirrored
at the end of the resulting half-wavelength line.

--
Dave Platt AE6EO
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