[TowerTalk] The simple Gamma Match?

[Dino Darling <dino@k6rix.com>]

A short history and then to the question....

What a nightmare!!!!

In 2004, AA6PW won a 3-element 15 meter beam manufactured by WiMO and 
donated by World Radio...
http://www.zx-yagi.com/

This antenna is your standard 3-element mono band beam with all 
elements shorted to the boom and fed with a gamma match.  Nothing 
fancy!  We assembled this antenna for the first time in 2004 for 
Field Day.  It would NOT match up!  We checked and double checked the 
dimensions that were converted over from metric.

What the MFJ analyzer was telling us is that the antenna was too 
short.  The elements each have 8 full inches of overlap and two holes 
spaced 7" apart to secure the element.  We ended up pulling the 
elements to just a couple of inches of overlap and used only one of 
the pre-drilled holes.  This lengthen all the elements and the 
antenna worked OK for the 24 hours.

We took 2005 off and for 2006 Field Day I said I wanted to model this 
antenna and see if I could find out what the problem was.

I had just purchased the 20th addition of the antenna book and used 
the included software called YW.  I measured all the elements and 
input the data.  I even got help from Dean Straw on modeling the 
brackets correctly!  The antenna was not great, but according to YW, 
should perform OK even with the lousy F/R.

Next came the matching network.  YW has a utility that allows you to 
enter info for a BETA match or a Gamma.  I entered the data for the 
gamma and found the measurements were different than what the 
horrible written manual said.  Another few e-mails with N6BV and he 
came up with the corrected size of the gamma (tube capacitor).

We attended a yearly event here in Southern CA called, "Antennas in 
the Park".  We use this family picnic to build Field Day antennas and 
eat!  This year we re-built the ZX15-3 per the optimized instructions 
(which were real close to the original dimensions) and STILL couldn't 
get the gamma to provide a match!

Here is Bob in test configuration.  I snapped a few pictures between 
running up and down the tower...
http://www.k6rix.com/zxyagi/

Now to the question: HOW DO YOU BUILD A GAMMA CAPACITOR OUT OF ALUMINUM TUBING?

What I'm really trying to do is verify if the assembly I have is even 
correct.  I know how to build one and insulate it properly, but don't 
know where you get the results for the correct lengths of the two 
tubes to make a capacitor.

MY guess is that the larger tubing is too long (like it was intended 
for a 17 or 20 meter beam instead, and mis-packaged...the 
instructions do not give material lengths and the original location 
of the shorting bar at 21" to center of boom is IMPOSSIBLE)!  We ran 
the shorting bar out to 24.45" per YW and tried to adjust for 
85.5pF.  I KNOW the antenna elements are resonant and all dimensions 
are perfect.  The problem is the feedpoint matching network.  I could 
use a 1/2" piece of tubing and a small tuning capacitor and 
everything would probably work fine, but I want to fix what I have 
here already!

I should point out that the pF at 21.225 were close to 275 instead of 
the 85.5 we were looking for.  I tried pulling the inner rod out and 
it dropped, but not far enough.  We had a PERFECT match at 21.400 however.

SO, I have three pieces of material and I want to build a gamma assembly...

Tubing #1
0.785" OD x 19.625" long.

Insulator
0.670" OD x 19.625" long.
The insulator fits snug inside tubing #1.

Tubing #2
0.480" OD x 31.625"
Tubing #2 fits snugly inside of the insulator.

The shorting bar holes are 4" center to center.

I need educating!!!!

Dino - K6RIX
dino@k6rix.com
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