In article ,
Dan Jacobson wrote:
D It might well be worth your while to experiment with a simple
D somewhat-directional antenna. A single reflector, located perhaps
D .1 to .2 wavelengths on the mountainside side (sorry :-) of your
D current vertical, cut to perhaps 5% longer than a half-wavelength,
D could be used to shape your antenna's pattern into something
D vaguely cardioid.

Yes! Say, can one just cut the reflector 5% longer than the antenna?
The antenna is some complex double 5/8 wavelength job.
If the reflector is shorter than the antenna, it becomes a director?
What if the reflector is just a pole jabbed into the ground, thus
grounded and longer downwards than the 5%? What if the pole is "very
much longer than the antenna in both directions"?
How about the thickness of the reflector? Only as thick as the antenna
itself (but wait, the antenna has two thicknesses, top thin, bottom
thick), or just grab any iron pipe (wait, metal type and shape
important?)

Well, I think you'd probably need to do a bunch of modelling in NEC2
or similar to get a really solid analysis, for any given antenna and
reflector.

The "5% longer" rule of thumb applies to antennas which use halfwave
radiators - it works with Yagis, and Cebik has an analysis which
indicates that it works with Yagi-like beams based on vertical J-poles.

I suspect that the issue is rather more complicated for antennas which
use 5/8-wave or stacked/collinear radiators. The phase relationships
between driven and parasitic elements in such antennas are probably
different enough that this simple rule might not apply.

I believe that you could probably get a fairly effective
pattern-warping by using a non-tuned reflector which is significantly
longer than your collinear radiator. Try cutting a piece of EMT or
other conduit (or aluminum pole, or whatever) so that it's a few feet
longer than your antenna, and then install it 6" to a foot on the
undesired side of the radiator... have one end of it sticking up at
least a foot above the top of the radiator and the other extending
down below the bottom of the radiator.

Some commercial multi-bay stacked dipole antennas use the mast or
tower in just this fashion, to convert a circular omni pattern into a
cardioid or off-center near-circular pattern. Varying the antenna-to-
mast/tower spacing changes the shape of the pattern.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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