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Stacks and SWR

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Subject: Stacks and SWR
From: (L. B. Cebik)
Date: Wed, 12 Feb 1997 10:50:58 -0500 (EST)
Jay's message about the 26.6 dBi, 62 dB F-B beam suggests to me that
perhaps some e-mail readers may have skewed my columns.  The correct
version (for the Free Space) model is
 TO angle      Gain (dBi)     F-B ratio (dB)     Beamwidth      Feed Z
  (degrees)                                       (degrees)      (R+/jX)
   ---           8.1           26.6                62            25.5 - 0.1
If it is skewed, the first column of every model is TO angle (--- = 0 for
free space), the second is gain (8.1 dBi), the third is F-B (26.6 dB), the
next is Beamwidth to -3dB points (62 degrees), and the last is R +/- jX
(25.5 - 0.1 ohms).  I regret any reading difficulties that the morass of
readers may have caused some folks.

This model is based on one developed a number of years ago by K6STI, but
simplified to 1" dia Al, with some element adjustments to peak
performance.  It is a fairly easily replicated design with nothing fancy.
Shorten the driven element to use a beta/hairpin match; lengthen it to use
a gamma match (with minor adjustments for everything being attached to
gthe metal boom).  If you are wedded to dBd, subtract 2.1 dB from every
gain figure.

NEC models, of course, presume flat uncluttered ground, and it would take
a different sort of program to model terrain.  That means that even the
first decimal place may be superfluous in practical terms, but noting it
is an aid to watching trends in numbers, where rounding to integers might
obscure the trend.  For example, see the trends for the upper/lower
impedances for stacks.

Hope this clarifies.



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