Hi Dino,
Realize that you're looking for more technical data on building a gamma
match from the materials you have, but I found this via Google...
http://www.ve3sqb.com/
Scroll down to the "for the experimenter" section (or do an in-page
search for the word "gamma") and you'll find a link to a simple Windows
app that VE3SQB wrote. Using this app with a target freq of 21.225MHz
(center of 15M), I got the following info:
Spacing: 4.759" (from driven element, center to center)
Gamma tube length: 7.276" (outer tube)
Tap point: 31.904"
Insertion length: 5.654 (overlap of outer and inner tubing).
The app suggests that the gamma tube (outer tube) be 1/4 the driven
element size.
With these numbers, it looks like your assumption that the outer tube is
too long is correct - it should be cut down to 7.276". After
"assembling" the capacitor, the tap point will be near the end of the
inner tube. I wish there were a way to test this before doing the
actual cutting, but you may not have that option. How about cutting the
tubing in half (to about 9 inches) and trimming from there?
Also take a look at Tom's (W8JI) info about the gamma match and how
length and diameter affect the network.
http://www.w8ji.com/omega_and_gama_matching.htm
Hopefully, both of these links will get you in the general ball park.
If these numbers work, send a "thank you" note to VE3SQB...he deserves
the credit for the handy app.
73,
- Aaron Hsu, NN6O (ex-KD6DAE)
{nn6o}@arrl.net
{aaron.hsu}@nbcuni.com
No-QRO Int'l #1,000,006
. -..- - .-. .- ".... . .- ...- -.--"
-----Original Message-----
Sent: Wednesday, May 17, 2006 5:25 PM
Subject: [TowerTalk] The simple Gamma Match?
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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