[amsat-bb] Re: Polarity questions

Robert Bruninga bruninga at usna.edu
Sun Sep 21 06:43:41 PDT 2008

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
> 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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