[TowerTalk] The simple Gamma Match?

[Dino Darling]

It's what I have and as pointed out, all elements are shorted to the
boom.  I don't want to re-invent the wheel here, this is just a once a
year Field Day antenna.

That said, we have considered cutting the driven element in half
inserting some fiberglass, use a DX Engineering BEB-2 clamp, re-scale
the steps for proper length and test that.  If it was going up for more
than 24 hours we would probably go that route.

For now, I just want to understand how a variable air capacitor is made
out of two sections of tubing in order to get me 85.5pF or provide a
range of say 50pF to 150pF.

I KNOW how to use coax to mechanically make a gamma.  I KNOW how to
mount it.  I know how to short it.  What I DON'T know is how long to
make the tubes in order to provide me the desired amount of
capacitance.  The best response so far was 20pF per foot.  But per foot
of what?  3/4 tubing?  1/2" tubing?  Where is the formula or something
concrete so I can...

1. Reverse engineer what I have so I can find out what I have.
2. Re-build what I have so it works, because right now it doesn't.
3. Learn how to build a tube air variable capacitor for future projects
and personal education.

It's not in the Antenna Handbook and Dave Leeson's book doesn't take
long for me to begin to glaze over.  I've dug through Dave's book but I
see only where it makes reference to gammas.  The actual formula must be
buried somewhere in there.

I'm willing to put in the effort and willing to RTFM, but I need some
help!  Thanks and 73!

Dino  -  K6RIX
dino at k6rix.com


> -------- Original Message --------
> Subject: Re: [TowerTalk] The simple Gamma Match?
> From: K7LXC at aol.com
> Date: Thu, May 18, 2006 8:00 am
> To: towertalk at contesting.com
>
> My question is why a gamma match in the first  place? It's an unbalanced
> feed. My preference is a hairpin which is easier to  build and balanced.
>
> Cheers,
> Steve    K7LXC
> TOWER TECH