[TowerTalk] Gamma Match on 6 meters

[Bill Coleman AA4LR]

On 6/16/00 9:31 AM, Lee Buller at k0wa@southwind.net wrote:

>
>I am building a 6-meter beam for FD and have never used a gamma match as a
>feed on a beam.  Although I have looked at numerous designs over the past
>couple of weeks, the gamma match calculations elude me.  I cannot find how
>they come up with the "L" and the "C"

I looked for this information, and I was able to come up with a "rule of 
thumb" only for the case using dipole elements and a matching arm of the 
same size. The recommendation was 10% of the length of the dipole.

>Going to the Cushcraft web site, you can see how one is built, but it
>doesn't show dimensions very well.  I understand the contruction and the
>theory behind it, but I do not understand how they come up with the
>dimensions.  Is there a fomula?  Or, is it just try it until it works?

Well, the portion of the antenna and the gamma match can be looked at as 
a transmission line. The impedance transformation of the match is 
affected by the length of the match, element, and the relative diameters 
and spacing of the match and element. With this many variables, it's hard 
to have a hard and fast rule. (Except for the case where the match and 
element are the same diameter, then the spacing doesn't matter so much)

I made some gamma matches for my attic mounted delta loops. I used a 
"rule of thumb" value for C, which I stole from several designs in the 
Handbook. I started off with 10% of the length of my loop (which turned 
out to be far too much) and experimented using an MFJ-259 analyser.

The analyser made short work of this type of trial-and-error design. I 
highly recommend one for any sort of antenna work. 

The hard part of creating a match this way is there are several 
configurations that will create a 1:1 VSWR match, but one will have the 
broadest VSWR bandwidth. 

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com