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

Tom
I read as far as the word "tho a"
and you made my day, you confirmed what I suspected that all
antennas are based around yagis and not about antennas in general which
is exactly the point I made earlier.But your explanation tho correct
for a yagi is not all encompasing.
If you contend that your explanation over rules the mesh or field
aproach i.e a reflector must always be a longer radiator completely
avoids the essence of the first question i.e long versus shorter
reflectors. When aproached from a meshed
circuit point of view it can be seen that coupling of radiators behind
the dipole generator can easily produce an element(s) of a shorter
length.since as you stated a reflrctor does not reflect
but creates a reactive field in conjuction with other elements in
the immediate field to satisfy Newton's law
The rest of your reply I consider irrelavent to what was previously
stated and thus diversionary to the subject at hand.
If you read the initial post carefully you will note that his question
revolved around the length of reflector (s) which in essence calls for
a different aproach to the traditional format
based around a yagi .It is for this reason that such questions are
raised by those who apply deeper thought to the subject,
ask this question over and over again and cannot be explaned
unless coupling of individual fields are brought into play instead of
introducing personal rules similar to front to back ratios which
is a point ratio of energy efficiency rather than a overall array
energy efficiency. and so on.
Art