Modeling a single or 2 element quad and a dipole and 2 element yagi in
free space shows the "why" of the difference. In the vertical plane
bisecting the elements, a quad element squeezes the vertical whereas
the dipole does not, and the 2 element yagi squeezes the vertical very
little. That extra vertical squeeze by the quad elements is the basis
for difference in gain of a 2 element quad and 2 element yagi. This
advantage quickly disappears as the number of elements grows.
A 2 element yagi's lack of suppression of the pattern straight up and
straight down is why 2 element yagis don't do as well as expected in
stacks unless *designed for* both the stack and height above ground.
They interact heavily, even if one or the other is not fed. Bill's
stated equivalence of a 2 element quad and 3 element yagi is easily
confirmed in a model.
73, Guy
Bill Tippett <btippett@alum.mit.edu> wrote:
Tony, I think we are saying the same thing. A 3 el
Yagi (on a 1.8 X 8' = 14.4' boom) should be very close
to a 2 el Quad on an 8' boom. Regarding anecdotal
comparisons, I would simply note that none of the
large multi-multi stations use Quads above 80 meters
(KC1XX, W3LPL, K3LR, K9NS, etc.). If there was really
much difference, don't you think they might use Quads?
Back to Don's original question, the model for a
2 el Quad will not be that much different from the model
for a 3 el Yagi supplied as a part of the HamCAP front-end
for VOACAP. We're talking very minor differences which
will be lost in the other VOACAP uncertainties.
73, Bill W4ZV
P.S. I built a 4 element monoband Quad for 20m in 1960.
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