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January 19th 04, 02:41 PM

(Richard Harrison) wrote in message ...

Terman has a comment on page 906 of his 1955 edition regarding
"Close-spaced Arrays-Super-gain Antennas. A review of the behavior of
broadside and end-fire arrays make it appear that in order to achieve
high gain it is necessary that the antenna system be distributed over a
considerable space. However, the antennas of Figs. 23-35 and 23-39
obtain enhanced directivity by employing antennas that are closely
spaced. Moreover, it can be shown that an end-fire (like a Yagi) type of
array that is short compared with a wavelength can theoretically achieve
any desired directive gain provided enough radiators are employed and
they are suitably phased. Such antennas which give great gain using
small over-all dimensions are referred to as super-gain antennas."

Read on. There is a fly in the ointment. Terman says:

" A characteristic of all close-spaced arrays is that as the ratio of
size to antenna gain is reduced, the radiation resistance also goes
down; this is illustrated by Fig. 23-36. The result is a practical limit
to the amount of gain that can be achieved in compact antenna systems,
since as the radiation resistance goes down the fraction of the total
power dissipated in the antenna loss resistance goes up. The Yagi
antenna of Fig.23-39 andf the corner reflector represent about the best
that can be achieved----."

This is the fly I refer to when he keeps talks about "lossless
matching" for small antennas or arrays..

So, Art may be on to something to some extent.

Not anything really new though. There is no free lunch. Many have
tried to find it, but it's almost always spoiled by the time they
do...:/ I've modeled close spaced arrays that had loads of gain, but
to feed them efficiently in the real world is not going to be easy.
I'm not sure what the most efficient fed "very small" antenna is.
Maybe a magloop? Dunno...But even a magloop's efficiency will be lucky
to be over 70%?? or so. Not exactly what I'd call a lossless feed. MK

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