[amsat-bb] Re: Polarity questions

Angus angus at young5769.freeserve.co.uk
Sat Sep 20 22:57:46 PDT 2008

Hmmmmm might get shot down in flames for this but.

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 

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.


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