[TowerTalk] T-match vs Gamma-Match Trade-off

[Frank Donovan]

I've used gamma-matched Yagis for many years. I still have three
gamma-matched 20 meter Yagis and my remaining 18 Yagis
are T-match fed.   All have been trouble-free for over 10 years.

Over a fifteen year period, I had numerous problems running
1500 watts into gamma matched Yagis, including an incident
where a feedline on a 10 meterYagi arced to the boom about
8 feet from the driven element, a 40 meter feedline that arced
to the tower leg about 30 feet from the driven element, and at
least two capacitors that arced on the 40 meter Yagi.

A 30 second micro-burst in 1993, with winds in well excess of
120 mph, provided an opportunity (!) to rebuild 16 of my Yagis.
After the damaged antennas were removed, I discovered that
several of the still functioning gamma-matches had water in
the capacitiors, a sure indication of failures yet-to-come.

In addition to major upgrade in the mechanical strength of the
Yagis, I decided to eliminate the troublesome gamma matches.
I implemented T-matches on all of the new Yagis, because
I was convinced that they could be built in a manner that would
prevent matching network failures.  My T-matches are of
conventional design, with no capacitors, 1/2 wavelength
RG-213 baluns, with particular attention to long-term
water-proofing of all electrical connections.

An significant benefit of T-matched Yagis is that they present
a DC short circuit.  This is very beneficial for routine
maintenance checks of the Yagis feedlines, from the shack
to the T-match.  I keep records of the precise DC resistance
measurement (a fraction of an ohm) for each feedline.  Any
change in DC resistance (and the companion TDR
measurements) is a sure indicator of feedline or matching
problems.

73!
Frank
W3LPL
donovanf at erols.com






----- Original Message -----
From: Jan Erik Holm <sm2ekm at telia.com>
To: 'towertalk' <towertalk at contesting.com>
Sent: Sunday, December 05, 2004 2:45 AM
Subject: Re: [TowerTalk] elevated short vertical dipole orquarterwave
monopole?


> Strange. I´ve used gamma matched yagis for 35 years
> or so and also used (at times) very high power, never
> had a failure or any problem with it.
> Sure a gamma match isn´t a perfectly balanced feed
> method but IMO it´s simple and reliable.
>
> 73 Jim SM2EKM
> --------------
>
> Tom Rauch wrote:
>
> >>Hey, while we are on the subject, how about gamma matched
> >>yagis. I have a friend who swears by using a gamma match
> >>on his yagis with no feedline choke.
> >
> >
> > When I had a gamma matched Wilson 20M yagi, the common mode
> > voltage on the coax was so high the coax actually arced
> > between the tower leg and the coax shield. I was only using
> > a 4-1000A amplifier, so it isn't like I had a lot of power
> > either.
> >
> > That's my only experience with a gamma matched Yagi.
> >
> > I'd bet the whole antenna is pretty much unbalanced.
> >
> > 73 Tom
> >
>
>
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