[TowerTalk] Gamma Match on 6 meters

[ABowenN4OO]

At 08:31 AM 6/16/2000 -0500, Lee Buller wrote:
>
>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?
=================
There are many sources for gamma match info. One is Hamcalc by VE3ERP.
Personally, I have found that the hairpin match is easier to adjust, but
does require a choke type balun. 

I did wish to try a gamma, however, based on writings by W1JR, who has much
more experience at these designs than I.

For my little 2 element using 5/8" and 1/2" tubing, Hamcalc said to use a
1/4"gamma arm of 9.7" and 30 pf series C.

I put a piece of #12 house wire inside a 1/4" tube and adjusted to those
dimensions, but failed to get a match. No doubt, my .15 spaced reflector
resulted in a somewhat higher feeding impedance. I did not record it
initially.

I then fiddled with arm lengths, wire insertion depth and element length
until I came up with a good match. The MFJ 259 is indispensible for work
like this. Dimensions worked out to be:

53.5" half element length

Gamma arm, 1/4 OD = 13"

9" length of #12 house wire inside the 1/4" tubing. This turned out to be
about 45 pf measured with my Autek RF1.

Two antennas were built to these dimensions using 1" PVC pipe/tees for
boom, element and mast supports. Both performed quite well at about 22 ft
(different locations) during the VHF contest last weekend.

Note that with the insulation used, this arrangement could be a tad lossy
and probably not good for more than 90/100 watts out.

The reflector was 59" (half length) and spaced about 36"

INK N4OO


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