Unless I am missing something, the technique below will give you the free-space length, not the "in coax" length which can be 66% of that length.
If you want to know the velocity factor of a coax (known or un-known) try getting two eaqual lengths of coax which are physically 1/4wavelength long i.e. 50cms for 2mtrs. Short out ONE end (join the inner and outers together with a short a link as possible) and do this for both pieces of coax. We are going to make a coaxial dipole so these two shortened ends go on to a piece of feeder (try and keep this feeder as short as possible also but the dipole needs to be somewhere clear but does not to be 100 feet up either). Yes you will need to support the dipole elements as they will just hang down so perhaps a piece of garden cane or some other RF invisible
material.
Trim back both OPEN ends of the dipole until you get the lowest possible VSWR (on the centre frequency you want to use) and you should have very close to an electrically 1/2wave dipole for the coax you used (each side of the dipole will be a 1/4 wave long). You can use these figures for making a 1/4wave delay line or if your making a polarity control box you will need to know the lengths of both 1/2 and 1/4 of the coax you are going to use, the dipole will be considerably shorter than a normal wire dipole for the same frequency because we have used the velocity factor to help
reduce its
length.
If you think that the feeder you have used is radiating either do perhaps 6 turns around a small 1" tube to make a balun at the feedpoint or alter the length of the feeder slightly, if theVSWR alters when you change the length of the feeder then the feeder is radiating.
My two pennys worth, should help you get very close to the figures you need without the use of a GDO or an antenna analyser.
regards Gus
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