Yes there is a difference, A yagi is a planar beam ie on a single plane
so the height of the array is the same for all elements in the array
which creates a major lobe or beam.
If the array is not planar such as a Quad then the elements are at
different heights
so the true or effective height of a quad antenna which is also a beam
style antenna is approximately the center point of the array or
somewhere between the top and bottom of the quad element.
The point to stand by is that the height of the feed point is
immaterial with respect to the effective height of an array. In another
post I pointed out that no matter which element is fed in a array the
effective height of the array is always the same and thus the TOA is
always the same
Regards
Art



Sal M. Onella wrote:
"CW" wrote in message
ups.com...
Fellows,

I've been wondering for some time now why amateur operators don't build
their Yagi antenna's
so they can be raised and lowered about 10ft in addition to being
rotated.
It seems to me that raising and lowering the height of a Yagi affects
the take-off angle by at least several degrees, meaning that the signal
delivery (target area) would be moved by at least many hundreds of
miles.

I don't know if this helps, but advice for TV DX says that you get
progressively improved performance until the yagi's
height-above-average-terrain (HAAT) is equal to about ten wavelengths.
(Above that HAAT, the signal strength varies up and down with further
increases in the elevation )

I never tested the idea, but if correct and it also holds for HF, there
won't ever be anybody _lowering_ a HF yagi. We would want the most height.
At the 2006 Field Day, one team had multi-band beam at 85 feet and everybody
loved it.

Before anybody tells me there is a difference between a yagi and a beam, let
me thank you in advance. I cannot formulate a sensible distinction
between them and I welcome the knowledge. I presume the terms are related
but not interchangeable.

73