Hi John,
> Here's another solution for improving the efficiency of short
> verticals that doesn't seem to be widely known. Add a second (or even
> a third) identical short vertical in very close proximity to the first
> and feed them all in phase. The phased system radiates as a single
> vertical with improved efficiency. I believe this technique is
> documented somewhere in ON4UN's book. The mutual coupling between the
> close spaced verticals drives the radiation resistance up, which
> enhances efficiency.
If the book says that, it is not correct.
Originally proposed in 1920, you can find this system analyzed in
Jasik's "Antenna Engineering Handbook" 1st edition page 19-9.
Ground loss remain constant for a given area of ground system and
antenna, because the sum of currents from each drop lead flowing
into that fixed size ground system remains exactly the same no
matter how many down leads are added.
The only improvement occurs when multiple drops are so far apart
zone currents at the base do not overlap each other, which means
each ground system has to be smaller than optimum. The end
result however is no better than making a single large ground
system the exact size of the sum of the tiny systems!
One case where this would help is when a driveway would be in the
middle of an area, and you couldn't cross the driveway with radials.
You could do a dual-drop antenna, with one drop on either side of
the driveway, and separate "half" ground systems on either side
that are not connected. In this example efficiency would be
identical to a single vertical in the middle of the driveway with a full
radial system that covers exactly the same physical area, but you
can still have a driveway.
This is an advantage for you because you already have a system
for 80, and can make that system act like a SINGLE vertical the
same height would behave if placed in the exact middle of your
array with the same physical area of ground system. That would be
better than feeding only one element at the edge of the ground
system. But it gains nothing over a single vertical loaded the same
way with the same area ground system, except convenience.
A four-square works the same way. The center two elements
combine to effectively make one element in the middle of the array.
That is why we can feed a four-square with a 1:1:1:1 current ratio
when a three element array requires a 1:2:1 ratio! The center two
elements (being in-phase) form one "radiation fat" element.
If the same physical size restraints of in-phase close spaced
elements are put into a single vertical, the loss results are always
identical. That is true with a folded monopole, it is true with close
spaced verticals in-phase. They are the same.
There is no free lunch.
73, Tom W8JI
W8JI@contesting.com
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