|Subject:||[TowerTalk] The simple Gamma Match?|
|From:||Dino Darling <email@example.com>|
|Date:||Wed, 17 May 2006 17:25:24 -0700|
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...
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...
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
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