Tim said:
Did you find this page on Wikipedia:
http://en.wikipedia.org/wiki/Radio
I always wondered why transmitting antennas act like
receiving antennas and vice versa. "Reciprocity" certainly
works, but I've never seen a good explanation of the
process.
Here's my own explanation of antennas. I've yet to
encounter similar things elsewhere, so I can't compare it
against textbooks for accuracy. (The textbooks go about
things differently.)
---
One way to understand antennas is to look only at the
EM fields and waves surrounding them.
Suppose we could *see* EM waves. If we illuminate
an antenna with a parallel beam of radio waves, so
the antenna is encountering a pattern of plane waves,
what would this look like?
Well, first we'd notice that EM waves behave much like
light, and an antenna would both reflect the waves and
also cast a shadow. Even if the antenna only scatters
the incoming waves without absorbing any, there'll still
be a region of shadow behind the antenna. We might say
that the antenna "punches a hole" in the planewave
pattern, leaving a long fuzzy slot in the waves moving
past the antenna.
Physics has a simple description for such a process, and
it applies both to light and radio. Whenever a small
opaque object casts a shadow, we can describe the object
as being a wave-emitter ...where the emitted waves are
out of phase with the incoming waves. The small object
scatters waves in all directions, like a concentric
bullseye pattern. During the wave-scattering process,
the object absorbs incoming waves and then re-emits them
in a spherical pattern. And downstream from the object,
the two waves partly cancel, forming a shadow region.
In this shadow the sphere-pattern of scattered waves is
being subtracted from the incoming planewaves. Or in
other words, there is an interference pattern in the
waves surrounding the object, with one of the minima
forming the shadow in the wave pattern behind the object.
The above explanation contains a central concept for
understanding receiving antennas:
In order to absorb waves, a receiving antenna *must*
emit waves.
This might sound impossible. But as long as no energy
comes magically from nowhere ...as long as the total
energy passing out of a closed surface surrounding
the antenna is zero or negative, we're not breaking the
rules.
In order to receive, antennas must transmit.
See what's coming next?
Because a receiving antenna interacts with incoming EM
waves via an emission process, therefore the physics of
transmitting antennas is a subset of the physics of
receiving antennas. If we can figure out how an antenna
can emit waves, then we'll know how the same antenna
can act as a wave-absorber.
The explanation isn't complete yet, since our antenna
could very well be a perfect conductor which only redirects
waves without absorbing any. A superconducting antenna
would still radiate a sphere-wave pattern and still cast a
shadow, but it could only send out as much energy as it
absorbed, so it wouldn't receive any EM signal on average.
The above explanation makes lots more sense if you can *see*
what's going on. Get a couple of Moire transparencies, one
with fine dark parallel lines, and another with a bullseye
pattern of concentric circles with the same spacing as the
parallel lines. Overlap them to produce the interference
pattern, then slide them a bit so you get a minimum or
"shadow" which extends behind the center. This shows
how an opaque object blocks waves by (re)emitting a wave
pattern which produces a shadow by wave cancellation.
http://amasci.com/graphics/antenna2.gif
In the shadow, where did the missing waves go? Clearly
they're part of the maxima lobes going off in other
directions. But if our antenna was actually absorbing
energy on average, the shadow would be a little bit bigger,
or the forward-scattered waves a little bit smaller.
The missing energy would end up inside the radio receiver
sitting at the center of our antenna.
((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))
William J. Beaty Research Engineer
[email protected] UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph425-222-5066 http//staff.washington.edu/wbeaty/