> Optimum stacking distance is a function of your
antenna
> pattern. Antennas with narrow beamwidths need wider stack
> spacing to optimize gain.
That's right.
It's all a matter of pattern multiplication. To have
significant gain increase in one direction or angle, you
have to remove significant energy from another direction. We
used to do this long hand by looking at patterns of
individual antennas and arranging them so energy was removed
from major lobes.
You have to model the stack over real ground, and adjust
spacing over real ground. Rules of thumb do not work well,
and neither does setting optimum stacking spacing in
freespace.
You can prove this with simple dipoles or quad antennas.
Let's assume we have a quad.
A horizontally polarized quad is two stacked 1/4 wl long
dipoles with 1/4 wl stacking height. The second element
forces a shallow null straight up and down. At heights above
earth where the individual sources in the quad element have
a deep null at 90 degree elevation, like 1/2wl mean height,
the quad has no gain over a dipole element. That's because
the element spacing is trying to remove energy where there
is already a deep null.
At heights where vertical null turns into a significant
lobe, like 3/4wl mean height, the quad element shows maximum
gain (about a dB or so) over a dipole element.
This is also why a multielement quad with several elements
has no real gain advantage over a Yagi at any height. The
use of multiple elements forces a vertical null, and the
second set of elements stacked 1/4 wl away (created by the
quad element's second current points) try to force a null
where there already is no energy.
Your yagis would behave the same. At some mean heights they
would have much more gain at a given stack spacing than at
other heights. A spacing that provides maximum gain in
freespace would likely not be the optimum over earth,
because the patterns of individual sources change over
earth.
>From what I have seen, very few stacks have optimum spacing
and height. Most HF stacks really only provide the advantage
of moving nulls around, rather than adding substantial gain.
For optimum gain..... the narrower the lobes of each antenna
or combination of antennas, the wider the spacing must be.
The more arrays in the stack, the wider the spacing must be.
The real difference most people see is they simply steer the
elevation nulls in the forward lobe to less harmful angles,
and that is what causes the "gain".
You can see this effect in the gain curves of stacked
dipoles at:
http://www.w8ji.com/stacking_broadside_colllinear.htm
Someday I'll correct the spelling of collinear in the link,
hi hi.
73 Tom
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