Hi Dave,
I just read this from W4LC who hit the nail on the head:
Quarter wave 2-element (driven & reflector)
dipoles, stacked a
quarter-wave
apart and fed in-phase?
> > Most of the benefits of quads are far over-rated. They do not,
> > contrary to folklore, have "2 dB gain over a yagi".
> >
> >
> [Dave D'Epagnier] As I recall, because Quads are almost always made
> with conductors with much smaller diameter than Yagis, the full
> theoretical gain advantage is usually not realized. FWIW, I've never
> owned a quad but if they are quieter antennas that would be a big plus
> to me. Maybe some day...
Remember they are only quieter in some cases where corona is an
issue. That would almost always be only during inclement weather,
and even then in specific situations.
It is not a good general rule.
As for the gain, picture what a quad element is.
It is two 1/4 wl long dipoles that are "end loaded" by the non-
radiating bent ends. These two short dipoles do not give up much
gain, because they are end-loaded and most of the ampere-feet are
in the radiating area. They only give up a tiny bit of gain.
Now you have two of these dipoles, but they are stacked 1/4 wl
apart.
If you look at the stacking gain in free space of two antennas
spaced 1/4 wl apart (like on page 8-35 of the 18th Antenna
Handbook), gain is about 1 dB. You'll find, if you model the
antenna, that indeed a square quad element in freespace has only
about 1 dB gain over a dipole!
(If it was a perfect loop instead of a square, gain would be higher
but still somewhat less than 2 dB.)
Now if you add more elements in front of that one, the more
directional you make the antenna the less gain advantage the
"stack" you'll get using the quad configuration. If you place the
antenna over earth, you'll see at some heights gain advantage is
zero and at some heights about a dB or so.
The problem with a quad isn't loss in the wire, it's the fact the
darned thing has no real gain in an ideal case (freespace single
element)...and anything you do to sharpen the pattern only makes
it worse.
We wouldn't stack two yagis 1/4 wl apart and bend the elements
over so the tips touch and expect to see gain, so what the heck do
we think a wire would have gain just because it is bent the same
way?
Where does the 2 dB come from?
It comes from a mistake.
A couple of fellows measured some quads on 440 MHz and
assumed the measurements were accurate. That data made it into
books. Unfortunately they overlooked some errors, and the results
were flawed. But to this day people still quote the flawed
conclusions, 2 dB gain over a yagi the same boom length.
Now you see why I pick at measurements and insist they include
and describe potential errors (and there are ALWAYS potential
errors). Once something is in print, it remains almost forever even if
wrong.
73, Tom W8JI
w8ji@contesting.com
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