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
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
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 Tom73 Jim SM2EKM
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