Yup, your method will work...
A bit more accurate way is to use your antenna analyzer (or an RF generator)
as an rf voltage source and a VOM with an RF voltage probe... (you can make an
RF probe in minutes  see the ARRL handbook)
For those who have not done this before just remember one fact  the driven
end of a quarter wave stub reverses the impedence seen at it's far end.. So, if
the far end is infinite impedence (open circuit) it will be zero ohms impedence
at the driven end...
First cut the coax to a computed .25 wave, plus a bit, then by leaving the
far end open circuited and frequency sweeping for the frequency that shows zero
volts at the driven end gradually cut the coax so it is a quarter wave (zero
volts @ 90 deg) on your chosen frequency...
By doing it this way you eliminate the variables that the coax may not have
the velocity factor claimed by the manufacturer, etc...
Then measure the length and compute your inches per degree and do your final
cut to your desired degrees...
****************************************************************************
The other way is the same method  but to calculate at what frequency your 71
degree stub will be 90 degrees electrically... Then use the method above to
directly cut the stub to the 90 degree point at that offset frequency  which
means it will be 71 degrees at the desired frequency...
As example, we want a 71 degree stub at 3500kc.. At what frequency will that
stub be equal to 90 degrees? The proportion is:
71 90
 = 
3500 X
Cross multiply 90 x 3500 = 71 times X
or 315000 = 71 X
or X = 315000 / 71
and X = 4436.6 kc
So, cut your stub for 90 degrees at 4436.6 and it will be 71 degrees at
3500...
You guys can check my math for brain farts, etc...
And, yes I know about 0.7 volts loss across the diode and offsets and
oscillator pulling and hysteresis, etc.. But this is ham radio not the physics
lab and this will get him as close enough for variable frequency ham use...
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