[amsat-bb] Re: Helix matching

i8cvs domenico.i8cvs at tin.it
Fri Sep 27 08:05:01 PDT 2013

----- Original Message -----
From: "Robert Bruninga" <bruninga at usna.edu>
To: <amsat-bb at amsat.org>
Sent: Friday, September 27, 2013 2:57 PM
Subject: [amsat-bb] Helix matching

Why don’t we see simple ¼ wave 75 ohm lines used as a first step in
matching a 140 Ohm Helix to 50 ohms?

The ¼ wave line would give an easy 112 ohms and then it shouldn’t be that
hard to do a little additional construction matching to get to 140?

(building a 2.4 GHz pop-out helix that compresses flat for launch)


Hi Bob, WA4APR

I have built a 10 turns and a 15 turns Helix for 70 cm using the above
matching system wich works very well.

The characteristic impedance of my 1/4 wave matching line made of
coaxial copper tubing is 88 ohm because SQR(150 x 50) = 88

The only adjustement necessary to get a VSWR close to 1 was to slip
the reflector back and fort over the boom only a few centimeters to
get the right position.

In a separate email I have sent to you the building drawing of my
Helix antenna for 70 cm and a picture of my feed matching system
for 2400 MHz.

For 2.4 GHz I believe that the simplest system to match a Helix is to
solder many small strips of copper at the beginning of the first turn
of the Helix over a lengt of 1/4 turn and adjust the distance of the
strips from the reflector plate until the VSWR becomes close to 1
as possible and this is the system I use for the feed of my 2400
MHz 1.2 meters dish (see photo).

73" de i8CVS Domenico

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