art wrote:
I disagree unless yoiu are specipically adressing the yagi design
which
is an explanation in terms of vectors.However an element radiates a
field
not a vector. To 'maximise' the redirection of rear field generation
requires
multi "reflectors" or a dish to capture all the rear radiation.

A yagi works exactly as I described. It is nothing more than a
parasitically excited end-fire phased array. The beam forming mechanism
in a Yagi is nothing even remotely similar to the beam forming in a
wide area array like a dish or a broadside-collinear array.

Tho a
dish is used
for micro wave frequencies it can be simulated by multi reflectors
aranged in
parabolic form. This method is not as mechanically feasable as the Yagi
but does
illustrate the effectiveness of a "refletor" versus a "director" in
terms of "efficiency"
or "effectivenes" ala, the two element yagi..when viewed as a mesh
cuircuit assembly.

Not true.

The gain in a dish comes from the wide area of surface that is excited
in phase. The dish surface looks like multiple dipoles all excited in
exactly the same phase. Gain is not high because a reflector is "more
effective", it is high because a wide area of radiation (multiple
wavelengths wide) can be used to focus the forward beam.

This is why USIA Curtains for SW broadcast have substantial gain, as do
bedspring arrays at VHF and UHF. Dishes are much more closely related
to broadside-endfire arrays than any other antenna, and work on very
different principles than a Yagi.

The Yagi relates closely to an end-fire array, and that includes the
reflector.

This is why you do not see any yagis with multiple in line reflectors
and very few with trigional or sheet relectors, and why you do not see
dishes with directors. The workings
are entirely different.

73 Tom