"Joel Koltner" wrote in message
...
I've taken college classes in antennas and hence have a pretty good feel
for some of the mathematics behind it all, but I've found that at times I
don't have good, intuitive explanations for various antenna behaviors --
and I'm not at all good at being able to look at some fancy antenna and
start rattling off estimates of the directivity, front to back ratio,
etc. -- so I wanted to ask a simple question on a two-element phased
array:

First, start with one antenna. Feed it 1W, and assume that in some
"preferred" direction at some particular location the (electric) field
strength is 1mV/m.

Now, take two antennas, and space them and/or phase their feeds such that
in the same preferred direction the individual antenna patterns add.
I.e., we're expecting a 6dB gain over the single antenna (but only at that
location). Since we start off by splitting the power to each antenna (1/2W
to each), that initially seems impossible, since 1/2W+1/2W = 1W -- should
imply the same 1mV/m field strength. But this is an incorrect analysis,
in that powers don't add directly. Instead, the fields add... hence, each
antenna alone will now produce 707uV/m (at the one particular location in
question), so the two together produce 1.414mV/m which is the same as if
the single antenna had been fed with 2W. Hence the 6dB gain we're after!
(This analysis also implies there must be other locations that now receive
1mV/m in order to conserve energy.)

Is that correct? "Powers don't add, field strengths do" is obvious
enough, but definitely leads to some slightly non-intuitvely-obvious (to
me) results. By extension of the above, though, it becomes obvious that
(in theory) one can build an array with any desired amount of gain, the
beamwidth just has to become narrower and narrower, of course.

Thanks,
---Joel



yes, all true. and that is where many of the arguments on here begin,
trying to add powers instead of fields, voltages, or currents. and yes,
theoretically you can keep making the beamwidth narrower and get more and
more gain, that is one reason lasers are so intense with such low power,
they have extremely narrow beamwidths.