[TowerTalk] Re: gamma vs T match

[Jan Erik Holm <sm2ekm@telia.com>]

Comments below.

Tom Rauch wrote:

> > 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.
>
>
> I never really noticed a pattern problem, but then how do
> you A-B test between a gamma and a balanced feed?
Hard to measure on a short (few elements) yagi. Instead
if you take a long boom yagi, forexample 15 el 144 MHz
and measure the side loobs you will notice that when
using a gamma match the side loobs are not egual in strenght.
That tells me that the front loob wouldn´t be uniform
either, i e gamma match feed is not 100% balanced feed.
Then if you insted swap the gamma for a T-match feed
the side loobs will become equal, i e tells me that the
feed now would be balanced. This I have confirmed during
lots of measurements on VHF yagis.
> I noticed a voltage (and thus current) on the shield
> problem.
>
> Power levels and cable lengths have a lot to do with it. In
> the USA we have an extreme power limit. They allow us to use
> 1500W output and most amplifiers will run a bit more. The
> higher the power and more congested the environment the more
> frequently problems show up.
>
> I found my problem because of TVI to neighbors.
>
> Think about this Jim. If the voltage from the element to
> boom mechanical and electrical junction was truly zero, how
> would any current flow in the other non-excited half of the
> element? Even if we are happy, it doesn't mean the system is
> good. It was just my luck that 1/4 wl away from the gamma
> the cable shield had enough voltage to arc to the tower leg.
> I don't like machines or antennas that work because I'm
> lucky.
I agree Tom. I also did say before that a gamma match is not
a perfect balanced feed. However I still consider it to be a
simple a reliable way of feeding shorth boom yagis.
I have currently 12 HF yagis using gamma match feeds and have
absolutley no problem. I have not been able to detect any
currents/voltage on the coax shields. Dont get me wrong, I´m
not saying that there isn´t any, it´s just that I can´t detect
any during normal operation so if there is some it doesn´t harm.
Also I have tested the system at MUCH MUCH higher power levels
then 1500 watts.
I just tought it wasn´t fair to ditch the gamma compleatly. Sure
from a engineering standpoint it´s not too elegant and these days
when building new yagi´s I probably would do T-matches instead.
>
> 73 Tom
73 Jim SM2EKM




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