Fellows,

I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.
It seems to me that raising and lowering the height of a Yagi affects
the take-off angle by at least several degrees, meaning that the signal
delivery (target area) would be moved by at least many hundreds of
miles.
It would also affect the average wave incidence angle with the
ionosphere, so one could tune (or peak) the signal significantly with a
few feet of height adjustment. I am aware of stacked Yagi's being used
by some contest stations, where variable phasing feed techniques
between the upper and lower Yagi can (and does) affect take-off angle.
I'm just suprised at how little literature and practical use of this
technique exists. It can't be all that difficult to build a Yagi that
can be winched, or slid up and down the side of the tower by several
feet.
Does anyone have a better theoretical understanding of the possible
signal strength change from an ideal one-bounce propagation when a 20m
Yagi is varied in height from .9 to 1.0 wavelength in height?
How would a 0.1 wavelenght height change compare to a 30 degree
rotation angle change?
I know that some of the difficulty in quantifying the benefits of such
a scheme is that the refractive layers of the atmosphere change in
altitude regularly. So take-off/refraction/range calculations become
cumbersome. But perhaps some fixed assumptions would allow some general
statements about the typical gains vs. antenna heights for a fixed
range.

Thanks for any input on this apparently unusual technique.

73,
CW-AI4MI