Q: How do I enter an antenna’s beamwidth for use
in TAP?
A: The beamwidth of an antenna is typically measured
as the aperture between the half-power points (3dB down) of the
major lobe. For example, if the major lobe gain of an antenna
is 9dBd, and the gain at +/- 10°
from that axis is 6dBd, the beamwidth of the antenna would be
20° .
The antenna beamwidth is a convenient measure of an antenna’s
directionality. A narrow beamwidth indicates that the gain drops
sharply away from the azimuth of the major lobe. A wide beamwidth
indicates more uniform gain over a broader aperture of the major
lobe.
TAP propagation programs use antenna patterns consisting of
the gain on specified azimuths, so if you know the actual azimuth
gains, you can enter these using the antenna editor. However,
sometimes an approximation can be made using the known antenna
gain and a beamwidth value. If you want to assume that the antenna
pattern varies linearly across the beamwidth with no power outside
of this aperture, you can create an antenna record for use with
TAP to meet that criterion. (Obviously this is a crude approximation
of an antenna, and the actual azimuth gain values are much better.
The method described below is only for approximations if better
information is not available.)
As an example, consider the hypothetical antenna described
above with a 9dBd gain and 20°
beamwidth. You could approximate this antenna in TAP by specifying
a 9dBd antenna gain, with the following azimuth gains entered
with the TAP antenna editor:
TAP would then use this "pattern" to interpolate
the gain between 350° and 10° as a value between 6 and 9dBd (linear
interpolation based on the azimuth), and any azimuth from 11° to 349°
as having a gain of -30dBd. (The -30 value is arbitrary. The
intent is to use a gain value that is substantially lower than
the major lobe.) If you are entering gain as Relative Field in
the TAP antenna editor, the major lobe value would be entered
as 1.000, the field at the half-power points (10°
, 350° ) as .707, and the values
outside of the beamwidth should be .0001 (a relative field value
of 0 is not allowed.)
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