[TowerTalk] Gamma vs balanced feed meathod

[Frank Donovan]

Hi Bill!

I've used dozens of gamma, beta and T-matches and I've never been able to
discern any observable difference in electrical performance.  I've settled
on the T-match for all of my big Yagis because I've developed a much more
"bullet-proof" construction technique for it than I was ever able to
develop for the gamma or bata matches.

I abandoned the gamma match because it has a reactive component in series
with the transmission line (the gamma capacitor) that can easily upset the
phase balance between stacked antennas and because high voltages are
present across the capacitor that can cause unanticipated catastrophic
failure unanticipated failure when moisture is present.

The beta match is a better choice (perhaps the best choice electrically),
but I didn't care for the balun voltage breakdown issues and the
difficulty of adjustment (the variables are beta "hairpin" length and
element length).

I settled on the T-match because the there is no series capacitor subject
to high voltage breakdown, its 1/2 wave balun is nearly loss-free and
the 1/2 wave balun is not subject to catastrophic breakdown as the
beta-match balun can be.  I also developed a construction technique
that is mechanically and electrically bullet-proof.  The T-match does
have disadvantages but I've found them to be managable:

   - No good electrical model yet exists for driven element length in a
     T-matched system.  But if you lengthen the driven element 3% beyond
     the length modelled in YO you will be pretty close.

   - The most important T-match variable is element length.  This can be
     a little inconvenient, but manageable...

73!
Frank
W3LPL
donovanf@sgate.com

On Mon, 28 Apr 1997, Bill Fisher, W4AN wrote:

> Did I read something about the gamma not yielding as clean a pattern as a
> balanced feed meathod?  Has there been any testing done on a yagi fed with a
> gamma as opposed to other matching systems?  Seems to me it shouldn't matter.
> 
> 73
> 
> Bill, W4AN - Optimistically building 10M antennas!


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