Hi all..
Antenna's develop "gain" because of cancellation of radiation in unwanted
directions.
In order to have gain, the system must force a null into what once was an
area of appreciable radiation.
That is the only thing that increases amplitude of a desired lobe.
The way we used to look at this (before computers) was pattern
multiplication. Take the pattern of each individual antenna, and modify it
by the pattern produced by stacking two isotropic sources the same distance
with the desired phase. That's what you get.
That's why...as the individual antennas or cells in an array gets more
directive..stacking distance must increase for optimum gain improvement.
For example, two dipoles in free space achieve optimum gain (about 3 dB)
when stacking distance is about 5/8 wl. Change those dipoles to yagi's, and
the stacking distance for optimum gain also increases. Make the yagi's more
directional and optimum stacking distance gets even larger. The stacking
gain decreases, because the array is already highly focused and has a
deeper and deeper null in the direction where stacking tries to place a
second null.
For two stacked cells of two Yagi's per cell, optimum stacking distance to
another cell (and from yagi to yagi in a cell) gets larger. Stack a new
antenna with the direction and spacing that places a null where the array
already has a null, and nothing changes! EME guys have this figured out.
There is little doubt most of the "gain" people see is from null-filling or
null-shift rather than actual gain.
> way to predict what he might actually get. Also I was not
> addressing any possible benefits due to space diversity in the
> elevation plane or increased capture area, etc.
"Space diversity" does nothing but establish a new pattern, unless there is
some voting system that automatically selects the optimum phasing or
antenna. During times of slow QSB, the operator might do that manually.
Capture area is one of amateur radio's premier myths. Capture area is more
correctly called "effective aperture" and relates only to antenna gain. It
has nothing to do with antenna size except how that size affects gain. A
simple dipole can have more capture area than a large antenna one hundred
times its size, if the large antenna has less gain.
Any antenna with more gain has more "capture area" than an antenna with
lower gain, no matter what physical size the antennas are.
73 Tom
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