Date: Tue, 06 Jun 2006 13:52:03 -0600
From: Joe Wilkowski <k8fc@k8fc.com>
Subject: Re: Topband: Shunt feed short tower
Earl, I respectfully disagree to some extent with your
comments. Although I agree that omega is a good way to match a
physically short tower I would not give up on the simpler gamma match just yet.
The best matching system for feeding a grounded element, unless we
are very lucky or very careful in planning the installation, is a
simple gamma capacitor and shunt feed conductor with the tap point
adjusted to find 50 ohms resistive. It is my belief that virtually
any reasonable height tower can be gamma matched with in some cases
many trips up and down but it can be done. Obviously the variables
that determine bandwidth (gamma wire diameter) and tap point (~50 ohm
) must be dealt with. Every one of these installations in my
experience has been different. In addition, it is imperative that
long before ANY matching attempts of the vertical grounded tower that
an EFFECTIVE ground and radial system must be in place prior to
constructing the match.
While losses are not significantly increased when using an omega
match with reasonably sized components, matching range is not
significantly extended either ! I understand that omega matching can
definitely be used on short towers but is it worth the trouble
especially with the voltages on the omega caps when running power ?
Ratings are still a problem when the system requires the series
capacitor to have a small capacitance value (high reactance) and now
with the omega match we have two capacitors that have to handle
essentially the same high voltages when the series operating
capacitance value is small! At best, the omega match saves us a few
trips up and down the tower while searching for the precise gamma
shorting strap position between the gamma wire or cage and the tower.
If we are already at the top of a tower with the gamma tap point, and
if at that point we find resistance after the reactance is canceled
is too high, an Omega will not help. The omega can only match loads
LESS than 50 ohms resistive by stepping the resistance up! It can NOT
step or transform antenna feed resistance downwards when using
capacitors, and neither the omega or gamma can match capacitive
antenna loads. I agree that this situation (K0VD's) would probably
not meet this condition of the real resistance being too high but I
am not comfortable that omega is the best route either.
There are still an number of things that can be attempted including a
small wire cage to facilitate matching. The gamma match capacitor can
only cancel reactance, it can not modify the resistance presented to
the feedline. It is the most simple form of matching, and has the
lowest operating Q and loss of any matching system. Adjustment of
resistance (not the reactance) requires adjusting the diameter,
spacing, or length of the gamma section. Looking over the data
provided, the first thing that I would do is improve the
radial/ground system employed at this location then move on to the
matching challenge.
My 2c worth.
/joe k8fc
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