On Mon, 12 Apr 2004 19:36:11 -0700, Dan Sawyer wrote:
>Tom,
>
>I am trying to increase the efficiency of a center loaded vertical by
>creating a good ground system.
>
>Based on models the antenna 'should' have a have a total impedance of
>less then 10 Ohms. Yet it reads close to 50 Ohms. I assume the 40 plus
>Ohms is related to ground resistance. Is this a reasonable starting
>assumption??
If you study the replies that you have been given (and you have been
given some very good ones), it all boils down to this:
1) Assuming that you are measuring the impedance correctly at the
feedpoint and you get a measurement that says it is 50 ohms resistive,
the radiation resistance is probably roughly what your model is telling
you, and the rest of the resistance is split between the loading coil
and the ground system.
2) Reducing those losses is not easy -- it takes a LOT of copper in the
ground system.
3) Now, measuring the Z at the feedpoint is not easy -- there will be
some feedline between your instrumentation and the feedpoint, and that
feedline will transform the impedance in a rather complex way based on
its length and the impedance that the antenna presents to the line as
compared to the characteristic impedance of the line. A Smith Chart is
one of the few good ways to study this transformation. The ARRL Antenna
Book shows you how to do this.
4) Measuring the ground impedance at RF is not easy. As Tom has told
you, there are some fairly well documented ways to do this at power
frequencies, but they aren't easy either, and they don't translate well
to radio frequencies.
5) Accept the fact that anything you do in this area is going to be a
rough approximation, even if you are a really good engineer, and do
some really serious measurements with some really serious test
equipment. As Tom has told you, there are so many variables in real
world antenna systems (and their ground systems) that it's really hard
to get them all into your measurement (or a design).
To help you understand this better, do a search on the ARRL website for
the many excellent technical pieces on vertical antennas and ground
systems. There is a five-part series on ground systems! Also, study
some of the things written by W2FMI, Jerry Sevick, about his
experiments with ground systems for short loaded antennas.
Jim Brown K9YC
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