N4KG comments follow -
On Fri, 7 Nov 1997 21:29:13 -0500 "Jim Bruce"
<jimnv3v@shore.intercom.net> writes:
>Therin lies the problem, no defined standard of measurement that is
>accepted
>and used by all manufacturers. Currently everybody is free to use
>whatever
>they wish and publish the figures without any definition of how it was
>derived. Jim/NV3V
>-----Original Message-----
>I think though it would be nice if we could agree to a standard
>>(NEC?, free space?) gain, and force ALL manufacturers to quote that.
>Then
>>we could be
>>more sure of comparing like with like.
>
.........................................................................
ARRL is currently studying this very question for the purpose of listing
antenna performance parameters in QST advertising.
The proposed standard is to require ALL manufacturers to supply gain
figures for FREE SPACE in dBi (dB relative to an isotropic source).
IF the manufacturers also want to specify gain over ground, then they
must
define the modeling conditions (height above ground, ground conductivity,
etc.)
The Cushcraft numbers for the X7 and X9 are reportedly in dBi (but not so
indicated) above ground, which includes the ground reflection gain. In
spite
of their indicating that the antennas were at 1 wavelength above ground,
several readers have overreated to the gain numbers without stopping
to realize that the 6 dB ground reflection number was added in. Using
antenna gain in dBi in FREE SPACE will hopefully eliminate this reader
confusion!
If you wish to make your own antenna gain measurements, please use
a known Yagi (something that can be easily modeled) as your reference
antenna and NOT a DIPOLE. The gain of a dipole varies with height
above ground and the signal strength from a dipole can be altered by
reflections from objects BEHIND the dipole. Using a Yagi with modest F/B
minimizes potential errors from rearward reflections and height
variations.
Merely add the KNOWN reference antenna gain (from your computer model)
to the measured gain on your "antenna range" and you have the gain of the
antenna under test. If you are looking for concensus, N6BV at ARRL and
Roger Cox at HyGain have both concurred with this suggestion.
de Tom N4KG
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