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[TowerTalk] Re: [Antennas] Steve's antenna study

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Subject: [TowerTalk] Re: [Antennas] Steve's antenna study
From: cebik@utkux.utcc.utk.edu (L. B. Cebik)
Date: Sat, 18 Jul 1998 22:10:34 -0400 (EDT)
> A more accurate number is 2.15 dB, but that's quibbling.  2.15 dB is the
> gain of a dipole (broadside) over a non-existent, isotropic antenna.  An
> isotropic antenna is a fictional point source which radiates equally well
> in all directions.  

Actually, 2.15 dB is the gain of a hypothetical resonant dipole with
lossless infinitely thin wire over the hypothetical isotropic source in
free space.  Both are equally theoretical concepts, but that does not
disturb their utility when correctly used.  If one is using a computer
modeling system, then the difference in gain between a given Yagi and a
given dipole--both models--will be the same whichever base is used.  But
even in computer modeling, one gets the best idea of a directional
antenna's gain by comparing its model with a model of a comparable
dipole--that is, one likely to be used where the beam is placed in the
modeling coordinate system.

A "real" dipole of wire that has a finite diameter and a finite loss
factor over equally real ground has a variable gain in terms of either dBd
or dBi, depending on material losses, height above ground, etc.  When such
an antenna is used as a comparator for other antennas, it needs to be
completely described, and--of course--the antenna(s) compared need to be
used under exactly the same range conditions.  The comparisons most likely
would lose any ambiguity if labeled something like dBdr, for dB gain over
the range dipole actually used.

For other than very general suggestive guidance, translating models into
range tests or range tests into models is at present not
especially feasible. Modeling presumes a level coherent ground, and
terrain analysis must be separately done.  Many modeling programs invoke
only the RP(0) output, which yields only far field data and does not take
into account both surface and far field combined (the RP(1) output).  Many
models do not account for the physical properties of matching devices--and
some cannot accurately model them.  Range testing includes all of these
factors inherently.  These are only some of the difficulties.  So far, I
am not aware of any standard for detailed translation from one mode of
analysis to the other--although that is not an impossible project.

This translation difficulty does not empty modeling of utility, since it
yields designs that can be and have been effectively built--designs that
have originated with modeling and optimizing software.  Nor does the
translation difficulty empty range testing of its obvious utility.  It
only tells us that going from the terms appropriate to one into the terms
appropriate to the other is not yet a precise operation.

-73-

LB, W4RNL

L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
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37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
                URL:  http://web.utk.edu/~cebik/radio.html





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