Tho the Yagi has been good to communication over
the years it has in fact retarded the advance of antennas.
The biggest reason is the misinterpretation of the term "curl"
where it is termed as a two dimensional vector instead
of a three dimensional vector. The two dimensional term
came about by adding vectors to a static charge where
the vector values are zero i.e time was removed from
equations by reducing the vector to a point for the sake
of mathematical expedience but with a known direction.
This aproach has fastened into the minds of academics that
radiating elements should always be in parallel form and
which the Yagi has perpetuated. The error in this aproach
is that when a time varying field is applied to a static charge
is that the addition of vectors move from a two dimensional
form into a cartesian three dimensional form. With this
concept in mind which is an extension of a Gaussian
antenna aproach, it should be seen that parallism tho good
in terms of manufacture is not the ideal array arrangement
and in fact verticals may well be better off place at an angle
somewhat less than 90 degrees. Ofcourse as always
antennas are compromises and the biggest drawback here is
the lack of symetry for only a small advance in efficiency.
Food for thougtht gentlemen if you have an open mind
Art