"Boozo" wrote in message
...

"Tam/WB2TT" wrote in message
. ..

"Jimmie D" wrote in message
...
Been cleaning out the garage this weekend and came across an old attempt
at build a 432Mhz helix antenna. As I remember I gave up on it because I
was having trouble building the matching section which was a 11/4wl long
fin that attches to the helix as it apprached the feed point. It was
proving difficult for me to fabricate. I was wondering if the fin could
not be attached to the counterpoise instead of the helix, adjusting the
space beteen the fin and the helix to get a match.

Jimmie.

One way is to run a single wire next to the reflector from the feedpoint
to the transmission line. If the Z0 of the helix is 120 Ohms, space the
wire from the refleector so that at that end it forms an impedance of 120
Ohms. Space it at the coax end so the impedance is 50 Ohms. I think that
theoretically the impedance is supposed to change is some logarithmic
fashion, but linear reduction of distance works. If the reflectopr is not
solid, fasten a metal strip to it to get a smooth impedance. I can't
remember the minimum length for the conversion line; something like a
wavelength. Google helix antenna matching.

Tam/WB2TT

Pardon my ignorance but how can I work out what the Z0 of an antenna is?
As you say above, "If the Z0 of the helix is 120 Ohms".

I'm not totally stupid but could never work it out.
It isn't as simple as connecting an ohm meter to it......is it ?

Boozo.

Yoou need to find design equations for it. I think it was a function of
helix diameter and pitch. It is in either the Radio Amateurs Handbook, or
the ARRL Antenna book. Probably any book on antennas. Info should also be
available on the 'net.

You could measure the SWR. You know it is more than 50 Ohms; so, if the SWR
is 3:1, that means 150 Ohms rather than 17.

Tam