"art" wrote in message
Really Cecil I am trying to get people to think
about elements containing inherranr directional properties so that
uneeded radiation is harnessesd for useful purposes but they are
shutting their ears.
__________________

Art,

The dipole elements (of all lengths) in a Yagi _do_ have their own
directional properties, and generate their own radiation patterns -- the
fields of which add/subtract in space as a function of their relative
magnitudes and phases to produce a net field that varies around the
radiation sphere centered on the antenna.

From reading between the lines, maybe you are relating antenna efficiency to
the free-space field strength that the antenna produces at a given distance
and direction in the far field when a given amount of r-f power is applied
to its input terminals, compared to the field produced for the same
conditions by a reference radiator such as a 1/2-wave dipole, or an
isotropic source.

For this definition it is reasonable to expect that both the test and the
reference antenna have negligible conductor and dielectric loss, and that
they both present a return loss of 30 dB or better to the transmission line
leading to the transmitter. All of that is practical to achieve. Also note
under these conditions that a return loss of 30 dB means that 99.9% of the
power applied to the antenna is radiated (somewhere), so if that is the
meaning of antenna efficiency, it is high indeed.

By this definition, the efficiency of a Yagi in its direction of maximum
field is very high, and does not indicate that sub-optimal choices were made
for its mechnical layout. In fact, the inventors of this antenna and many
others have spent much time and effort with physical and electrical models
of the Yagi to optimize its patterns and gains. The result of all that
finds that the director(s) should be shorter than the driven element, that
the reflector should be longer, and that using more than one reflector has
minimal effect. That is the "bottom line," and speculation to the contrary
won't change it.

Using this Yagi design and this definition of efficiency, a standard,
6-element Yagi has a main lobe peak efficiency of about 250% compared to a
1/2-wave dipole, and 316% compared to an isotropic radiator, which
correspond to radiated power ratios of about 610% and 1,000%, respectively.

IEEE Standard 145-1983 gives the standard definitions of terms for antennas
(gain, directivity, efficiency etc).

RF