

Re: Explain the RG62 thing to me, please...

On Fri, Apr 06, 2001 at 09:05:06AM +0100, Howard Long wrote:
> A turnstile is two 75 ohm (not 50 ohm) dipoles assuming there's no external
> effects, like ground or reflectors.
> If you have two antennas wired in parallel, to make the 50 ohms total, each
> antenna needs to be 100 ohms, just like putting two 100 ohm resistors in
> parallel (the 90 degree CP phasing is irrelevent at this point).
> Q. So how do you make 75 ohms into 100 ohms?
> A. Stick in a 1/4 wave coax matching section...

Hmm. The phasing you describe (RG-62 to both dipoles, with additional
RG-59 section to one) is quite a bit more complicated than that
described in the ARRL antenna and satellite books.

The satellite book just has two dipoles connected directly together
with RG-62 and driven with 50 ohm coax. That's the arrangement I use
here, and it seems to work well. I don't really know what the
circularity is like yet, but SWR is about 1.2:1 or so. I used
4/10 lambda spacing between the elements and reflector to improve
the SWR and also the pattern. The impedance of each individual
dipole was about 100 ohms, so the 93 ohm coax is the closest.

The antenna book has a 75 ohm feed line, with a 1/4 wave matching
section of 50 ohms, to the first dipole, with a 1/4 wave phasing
section of 75 ohms (RG-59). I don't think this intended to give
circular polarisation.

