To: <towertalk@contesting.com>
> Date: Thu, 30 Apr 1998 17:20:07 -0400 (EDT)
> From: "Charles H. Harpole" <harpole@pegasus.cc.ucf.edu>
> Actually, from field experience, quads are better than beams, in less
> noise, more gain per # of elements, larger capture area, lower angle
> radiation, etc. But, yes they are a bear to put up and keep up.
> I want a 2 el. 40 meter quad in the worst way...... someday!!
> de K4VUD
Discussing quads VS, yagis is often like discussing voo doo or other
occult sciences.
As far as I know, the following is correct:
1.) Antennas develop directivity by forming nulls. If the
system adds a radiation source that places a null in an area that
used to contain energy, remaining lobes get "bigger". A quad is
actually nothing more than two stacked end-loaded 1/4 wl long dipoles
fed in phase. It has a small stacking gain, and gives up just a tiny
bit of "collinear gain" because each radiating element is 1/2 size.
2.) A single horizontally polarized quad element develops gain by
placing rather shallow nulls directly above and below the element.
At the same time the pattern gets a tad wider in the E plane (azimuth
for a horizontal pol antenna).
3.) When the antenna is over ground, vertical pattern compression can
place a deep null in the same areas where the quad design places its
"gain creating" nulls. This will totally negate any gain from
stacking of the two 1/4 long end loaded dipoles that make up a quad.
4.) As the array is made longer and more directive, stacking gain
from two rows of elements decreases. You can see this by
looking at the charts for optimum stacking distances for yagis. The
additional small stacking gain from stacking two rows of half
size elements 1/4 wl apart isn't even there at some heights for a
single element, and when it is there it becomes even less when the
antenna is made longer.
5.) Noise is not blue...signals are not red, and an antenna is not a
light filter. Propagated noise and signals are ALL electromagnetic
waves and a closed loop is absolutely no quieter than a dipole for
propagated noises.
6.) N6NB took a crank up tower on a trailer around to many quad and
yagi locations in his area, and compared the "gain" of quads and
yagis to a reference antenna on the trailer. At the same height, the
quads were never as good as similar length and spaced yagis.
7.) "Capture area" is a useless term.. I think that term came from
something called "effective aperture". Unfortunately (or
fortunately) "effective aperture" has nothing to do with the
physical size.
Effective aperture (what some like to call capture area) is related
to efficiency and directivity, and that's all. A given size antenna
has less "capture area" than any other antenna having more gain no
matter how big and impressive the lower gain antenna is, and a given
size antenna would have more "capture area" than a much larger
antenna with less gain no matter how big and impressive the other
antenna is.
73, Tom W8JI
w8ji.tom@MCIONE.com
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