----- 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

On 9/27/2013 8:57 AM, Robert Bruninga wrote:

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?

In my limited experience, it's easier to diddle around with a strip of brass or copper at the feed end of the Helix..

For example:

http://www.amsat.org/amsat/articles/g3ruh/117.html

I remember building a 1/4 wave coaxial matching section out of a piece of copper pipe with brass rod down the center. I think it was from a design in the ARRL antenna book as published in the late '70's. It was a pain to build since I had to sand down the brass rod to get it to the right diameter, and I was never really sure how well it worked, since I did not have any antenna instrumentation at the time.

-Joe KM1P