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

[Jan Erik Holm]

Agree, as I wrote in a previous post T-match sure is a lot
better from a engineering standpoint. On a performance
basis I dont think anyone can see any difference at least
not when it´s up to short yagis, i e yagis without side
loobs.
However I guess my point was to point out that the gamma
match isn´t a totaly unusable thing, it didn´t seem fair
to ditch it compleatly. I still consider it to be a very
quick and simple way to feed a yagi antenna.
I have 12 HF yagis with gammas, some over 20 years old,
never had a problem. Also friends in the area got gamma
matched yagis and we never seen those problems you guys
line up.
But as I did say previously, these days I probably would
do T-match yagis instead since a gamma match isn´t a
perfectly balanced feed.

73 Jim SM2EKM
---------------------

Frank Donovan wrote:

> 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
>>>