In a message dated 4/12/00 11:22:56 AM Eastern Daylight Time,
[email protected] writes (excerpted):
<< The performance of the Decca Ant. is quite interesting, the "gain"or at
least the advantage over the normal amateur installation appears to be at
least 20db.... This is tremendous and I dont see how it can be attributed to
the height alone, conventional wisdom says that a short vertical has the same
radiation pattern as a full sise one,and even 300 feet is far short of full
sise. So where do we look for the explanation? >>
This is not at all difficult to explain.
The radiation pattern has very little to do with the difference in
signal strength, because as Laurie states, it's not that much different for
300 feet. But, none the less, that difference in height does make a serious
difference in the efficiency of the antenna system! Remember a discussion
that appeared here previously...it is not enough to compare two antennas
based on pattern gain. The net gain or loss of one antenna relative to
another must take into account both the vertical and azimuthal patterns, AND
LOSSES.
The radiation resistance of a 300 foot mast is significantly greater
than for a 50 or even 100 foot antenna. Add the fact that losses in the
commercial-grade ground system will be lower, and that the installation is
probably more clear of local environmental loss contributors (trees, etc.),
and you have a tremendous improvement in efficiency over what most amateurs
can hope to achieve. If the 20 db figure is a good approximation of the
observed difference, the radiation resistance resulting from the increased
antenna height is a major component of the total improvement.
73,
John
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