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The Gamma Match

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There's always the question: "How will I feed this thing?" when homebrewing an antenna. For a mobile antenna there are additional requirements:

That it be Real Sturdy. Top highway speeds are, after all, equal to hurricane force winds.

That it present as little cross-section as possible. Again, hurricane force winds...

The gamma match pretty much covers these concerns.

Here's a head-on, from above, view of my gamma match:
Gamma match -- front view from above
The coax connector bracket came from my Aluminum Angled-Pieces Bin and was cut and trimmed to size. It mounts to the halo using two of the halo-to-mast mounting bolts. The coax connector came out of my RG-8 Coax Connector Peanut Butter Jar and is an 'unusual' chassis-mount style in that it has a threaded body plus star washer & nut for the mounting. I have no idea where I scrounged it it -- I have two of'em. (If I had it to do over again, I might go for an N-connector versus SO-239/PL-259. The weatherproof qualities of N-connectors might be a better choice here.)

And, thinking like a raindrop, I used a hot glue gun to run a bead of glue around the upper, exposed halo joints at the mast mount to 'persuade' the rain to run off the assembly. I think you can see a little of that in the picture above.

You do not want to seal the whole thing -- I think that would be impossible. And, I think it would create problems: you'd probably end up getting moisture trapped inside -- not what you want! I haven't yet drilled drain holes (weep holes) on the underside -- but, I probably will.

There is also a blob of hot glue run right 'round the coax connector butt -- after the gamma rod was soldered on.
Gamma match -- underneath view
The gamma match consists of a section of #8 copper wire (found in my Scrap Copper Wire Box) and a thick-wall 1/4" aluminum tube (from my Small Diameter Aluminum Tubing Bucket.) More on the actual assembly of these two items below. The I.D. (Inside Dimension) of the gamma rod from the halo tube is 1 1/16" (versus Dick's 1".)

The distance from the coax connection to the shorting strap's inside edge is 7 1/8" (versus Dick's 8 11/16".) My shorting strap is a 1/2" wide strip cut from some old telecomm equipment's front panel (found in my Scrap Aluminum Sheet Bin.) I choose a piece that was stiff, yet not 'brittle' -- thin, yet not 'fragile'.

I carefully formed the wrap for both ends of the strap by using under-sized surrogates for the halo and the gamma rod (a deep-well socket and a screwdriver shaft, if I remember correctly.) The screws, washers, and nuts for the strap are stainless steel.
Gamma Match -- in "action"
Olde Trick taught me by my dad: The copper wire is 'prepared' by clamping a longer-than-final-cut piece in a vise and chucking the other end in a 1/2" or 3/8" drill. Whilst pulling 'firmly' on the drill and wire, run the drill to create about 1 full twist per inch in the length of the wire. This stiffens up the wire and helps straighten it out.

Okay. Now I wouldn't just shove the copper wire into the aluminum tubing and call it a gamma match, right? Right! I wrap the gamma rod with Teflon plumbers tape -- full length. For my purpose here, I buy the thickest and widest Teflon tape I can find in the plumbing dept. of my hardware store.

Start at the outer end, so that when you cut for "perfect length", the tape does not come unwrapped so easily. Furthermore, as you wrap the Teflon tape around the #8 copper wire, 'hover' over the wire every 1" to 1.5" -- wrapping a thicker layer that just snugly slips into the gamma tube. What we want is a gamma rod that fits into the gamma tube tight -- without flopping around.

Another matter: Sealing the gamma match. After I had it all tuned and ready to lock down, I ran a bead of hot glue around the gamma rod at its insertion into the gamma tube. I also plugged the other end of the gamma tube with hot glue. Then I slipped a thickish (clear) piece of heat shrink tubing (from my Heat Shrink Tubing Peanut Butter Jar) over the the insertion point and shrunk it down tight.

And, I ran a light bead of 'automotive emblem adhesive' around the outer end of the gamma tube and slipped a plastic cap (from my Rubber and Plastic Cap Peanut Butter Jar) over it. You can see this in pictures 1 and 2, above.

I believe the gamma match assembly is something you want to get sealed tightly.

So, how can you know what size gamma rod and gamma tube you would need? My experience is to start with 'things' at least 30-50% longer than you think you will finally end up with. You can always cut a gamma tube or gamma rod shorter -- It is very difficult to cut them longer.
It's:
1.  cut - trim .... cautiously
2.  measure -- test
3.  GoTo Step 1.

I used an MFJ-259 "Antennalyser" to do my gamma 'adjustments' (and, later, to do the final tune-to-freq. adjustments of the halo gap.)

My first attempt at a gamma match (before I did this one) employed a piece of 1/2" Cable TV HardLine. I had scrap left over from some 1/2" and 3/4" lengths that I had scrounged from the local cable company -- as the feed lines for my 6M yagi (the 1/2" stuff) and my 2M yagi (the 3/4" stuff.)
[ See my Cable TV HardLine write-up. ]
However, the gamma capacitance needs to be somewhere around 25-35 mmF and that required (or, would've required) a gamma assembly that was Just Way Too Long -- using 1/2" Cable TV HardLine
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While adjusting the gamma match to be 'somewhere in the ballpark', and without having done any 'fabrication' at the gap to properly tune the halo, I found that the 'natural frequency' of the antenna was up around 66 Mcs. Wow! 16 Mcs. high!
hmmmmmm.......
If I had to do it over, I would probably cut the U-sections of the lawn chair to be longer. I would maybe add an inch to each U-section -- making the width overall 27+" versus the 25+" that I now have.

OK, then... We have a gamma match.
But, the sucker is tuned Way High in frequency. How did I tune it to 50.200 Mcs.?
Let's move on to: "Tune to Frequency":



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Last updated: 21-Feb-10
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