Can a grounded metal mast ever be useful as a yagi reflector?
http://www.qsl.net/dk7zb/2m_veryshort/2ele.htm
apparently uses a metal mast as a sort of reflector.

If the mast is "very much longer" and "very much thicker" than the
other elements, does its distance from the other elements "stabilize"
in the design and not keep changing as we change the other factors.

Maybe this might be useful in making 'foolproof' 3rd world yagis,
where the user merely owns a tape measure and various metal rods. He
already has a reflector much thicker than the 50 ohm split dipole
driven element, so maybe thickness and metal type of the latter
wouldn't be that critical, as at least he could clip it to the right
size (=what?) for 144-146 MHz? Presto, he has a 2 element yagi?

To some extent, yes... or at least so I've been lead to understand.

A local repeater committee I'm part of has been discussing the idea of
replacing our existing Hustler G7 omni with a commercial antenna which
uses a stacked set of half-wave folded dipoles. The dipoles are
normally side-mounted from a metal mast, held out at a distance of
somewhere around 1/10 wavelength.

The antenna designer at the factory told us that we could choose from
any of several different sensitivity patterns, ranging from a
near-cardioid pattern, to an "off-center circular" (something looking
fairly close to what an omnidirectional pattern would look like if you
were to "push" it 2-3 dB to one side, if that makes sense).

The principal difference between the antenna designs for these
patterns, is how close to the mast the folded dipoles are placed. In
effect, the mast is acting as a non-tuned reflector, creating more or
less of a dip or null in the antenna's pattern in the direction of the
mast, and a corresponding boost to the pattern out on the opposite
side.

My impression is that it's difficult to get a deep back-side notch or
null with this sort of arrangement... the front-to-back ratio is low.
It's a useful way to shape the pattern of an otherwise-omnidirectional
antenna to cover a not-entirely-circular service area. I don't think
it's likely to be all that useful for applications needing a
substantially directional pattern, or large front-to-back ratios.

For the sorts of applications you're speaking of (easy and
near-foolproof Yagis made with simple materials under Third World
conditions), the classic "Cheap Yagi" design by Kent Britain WA5VJB
seems idea. They're easy to construct, don't require a split driven
element or a Gamma match or balun, seem to be non-critical in their
design, and seem to measure out quite well.

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