----- Original Message ----- From: "Robert Bruninga" bruninga@usna.edu To: amsat-bb@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)
Bob, WB4APR
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