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Author Topic: Homebrew beams with gamma match  (Read 1598 times)
KE6WNH
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Posts: 131




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« on: August 31, 2009, 04:27:43 PM »

Recently, I built a 70 cm and a 222 mhz beam from aluminum rod and tube, but substituted a capacitive match for the inductive phasing loops called for in the original designs. The 70 cm needs a little tuning, but the 222 gave a good SWR right off the bat. I learned a few things about making gamma matches:
1. it helps if you have access to decent machine tools. I had to use a lathe and a drill press to make the connector between the capacitive tube and the feed point. This consists of a .5x.5" piece of aluminum hex stock with one axial hole and one perpendicular hole drilled in it, plus the brass pin which is soldered to the feed point\'s center conductor. This little coupling is probably why more homebrewers do not make gamma matches.
2. the antennas will need to be at least 10 feet above ground to give a good SWR and forward power reading. KE6VRK fiddled with the SWR meter while I stood on a chair in my back yard and held each antenna aloft. I\'ll need to work on the 70 cm some, but the 222 worked so well that \'VRK asked me to build him one.
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G3RZP
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Posts: 4962




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« Reply #1 on: September 01, 2009, 02:39:45 AM »

All equipment homebrewing is MUCH easier with suitable machine tools - and test gear!!!

Since getting a lathe, a mill, a shear, a brake, a small brazing hearth, grindstone, pillar drill and two benches with vices (one 2 inch, one 8 inch) the mechanics have got much easier. For the electronics, a shop with a vector voltmeter, a spectrum analyser and tracking generator, distortion factor meter, 2 oscilloscopes,6 RF signal generators, a function generator, frequency counter, two AF generators, pulse generator, AF and RF power meters, two DVMs and an old VOM (that doesn\'t go silly with RF around), a VTVM with a high impedance 300 volt RF probe and other odd parts, I figure I can do most jobs now.

Incidentally, you can often pick up some bargains in the mechanical line at Dayton. For $20, I got a Mitutoyo 10 inch vernier height gauge the other year.
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WB2WIK
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Posts: 20666




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« Reply #2 on: September 01, 2009, 09:00:19 AM »

There's a great design for a 3-band cubical quad here:

http://commfaculty.fullerton.edu/woverbeck/vhfquad.htm

I've seen this antenna, and used it, with Wayne N6NB, who is the designer and builder of the model shown on line and I even suggested he go into business manufacturing them, as it's very unique: Works very well, small, lightweight, transportable, non-critical, requires no matching network of any kind.

If you like homebrewing VHF antennas, try this one!

WB2WIK/6
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