The absolute signal levels in dBm that are published in the comparison report
are consistent with pattern look angles that are significantly attenuated
relative to expected boresight gain. In my mind, any suggestion, implied or
otherwise, that boresight performance of the antennas under test can be
extrapolated from field strength measurement made deep inside a pattern null
is dubious. This is not to say that the tests weren't carried out with great
diligence and care. I believe they were. I am only concerned about the strength
of the link between the test data and actual skywave performance.
I seem to remember from previous work that diffraction fences can be used on
outdoor door antenna ranges to more closely approximate free space conditions.
Perhaps a modified comparison protocol could be developed that utilizes such
range
techniques.
Mike, W4EF...........
----------
From: Larry L Moore[SMTP:km6iu@jps.net]
Sent: Friday, June 18, 1999 9:54 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Antenna Measurements
<<File: ATT00001.html>>
N0AX and K7LXC have made some substantantual and laudable efforts, over
the past few years, to make comparison tests on numerious commerical
antennas. The results have been presented at Dayton and are available
commerically from K7LXC on his web site.
The Mosley people have recently raised a question [ in an ad in the most
recent QST ] about the testing that I think needs to be answered. It
needs to be answered not because Mosley antenns did not test well but
because all of us should have confidence in what is published if we
propose to use it in making technical decisions. I too have question the
test results based on my limited antenna design experience using YO and
other programs..
First several observations;
1 There are numerious computer [NEC based ] antenna design programs
being used by amatures and professionals alike. They are generally
accepted to be valid and to produce resonably accurate results [ by the
scientific community ].
2 The computer programs will predict the exact gain of dipoles [ and of
course multi-element yagi's ] placed over ground at any elevation above
ground. A yagi, or dipole, placed above ground will produce lobes [and
nulls ] of gain [with the number and elevation of maximums and minimums
depending on the antennas height above ground]. The ARRL antenna book
details this information.
3 When one evaluates a dipole or yagi placed above perfect ground [and
1/2 to 1 1/2 wave lengths above ground ] useing these programs they
predict a null [ zero gain ] at 0 degrees elevation above ground.
4 It is generally accepted that yagi antennas have the same recieve and
transmit patterns of gain.
The question I have is as follows;
If these computer programs are producing valid data then what parameters
is one measuring between 2 yagi antennas, each mounted 55 ft high [ ie 0
degrees elevation angle between them ], located 1 mile apart and
seperated by a sea water inlet [ ie near perfect ground ] ???
The computer programs say both antennas would have no gain at this
elevation. Is this true?? If you are measuring gain what is the
mechanism providing it ?? Can one have confidence that data taken this
way will represent how the tested units are likely to perform in
practice ??
There are people on the reflector that have the experience and the
technical background to answer these questions [ I am not one of them ].
Answers can be sent directly to me but I think they would be of interest
to all subscribers. It is hoped responses will be directed to technical
fact and not contain antidotial or ones opinion of one antenna
manufacture vs another.
Larry Moore km6iu
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