Out of every long and nasty thread, comes a little useful
information. This is useful for stacking yagis, as well as sorting out
the BS from fact with quads and other antennas.
(W1JR mentioned a comparison where the original quads that
kicked off this "2 dB" stuff were measured at a VHF conference,
and found to be well below claims. I notice that post got swept
under the rug!!)
Where does the gain of a single square quad loop come from? It
comes from stacking gain.
When two sources of radiation are fed in-phase, and as we move
them apart, they form two nulls in-line with the sources. Since
radiation is suppressed in those directions, it has to go somewhere
else and that's where the gain comes from.
How can we determine the maximum possible gain? By looking at
the amount of gain mathematically possible if two totally non-
directional sources are fed with that phase (in this case zero
degree shift) at that distance. It really isn't a major problem open to
a lot of debate!
If you look in the ARRL Antenna Handbook under broadside arrays,
or in almost any engineering text on arrays of antennas, you'll find
the math. In many cases it is all worked out, and there is a graph
of gain vs spacing. If you look at two sources separated 1/4 wl, the
gain is one dB.
BUT, that gain ONLY applies when the elements do not already
have a null in the direction where the new null is being forced. The
antenna has to remove significant energy from some direction in
order to have gain in another. If we have a null or shortfall in
radiation in the direction where the stacking tries to force a null, we
get LESS than the theoretical maximum gain.
So the theoretical maximum gain is about 1 dB, and if things aren't
perfect we get LESS than that amount.
What makes it less-than-perfect? Making the array longer does!
That makes each "element" that is stacked have less radiation
above and below the stack. There is less energy for the stacking
effect to "null", so the stacking (broadside) gain has to be less.
We are, in effect, forcing a null where the antenna already has a
shortfall in radiation. We just don't have as much energy to remove,
and so we don't have as much gain to obtain!
That's why, when we stack large yagis, the larger and more
directive the yagis are the further apart we have to stack them! A
larger and more directional yagi has a null above and below it, and
so if we stack at a distance that forces a null in an area where
there is no energy we get no gain! The pattern does not change.
Same thing is true over earth, where certain heights force nulls
above the antenna. If we mount the antenna at a height where there
is a wide vertical null, stacking will not ad any noticeable gain.
Move the antenna 1/4 wl higher, and we now see maximum gain
difference.
A quad is nothing more than a stack of two short element yagis.
Nothing magic, nothing free.
Make the quad longer and the theoretical "stacking gain" of 1 dB
decreases. Place it at a mean height of 1/2 (or multiple)
wavelength and the 1 dB is minimized.
I can fudge things around in models and make that 1 dB "appear",
but in the real world it will always be less than that. In the real
world, assuming the antenna is properly tuned, expect zero to
something less than 1/2 dB.
The longer the quad, the less advantage there is. Over earth, the
advantage is less.
We may not like it, but that's how it works in EVERY case as long
as both antennas are properly tuned and constructed.
This is why we never see quads in EME arrays, or other
applications where actual performance is critical. It isn't that they
won't work, they just aren't worth the bother.
73, Tom W8JI
w8ji@contesting.com
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