# Re: Polarity questions

• Subject: [amsat-bb] Re: Polarity questions
• From: "Robert Bruninga" <bruninga@xxxxxxxx>
• Date: Sun, 21 Sep 2008 09:43:41 -0400

```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
> 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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