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Old March 7th 09, 02:27 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jun 2006
Posts: 1,374
Default Definition of antenna receive gain?

Daniel wrote:
. . .
I *have* done one experiment as a kid: I placed a dipole permanent
magnet under a piece of paper and poured iron filings on the paper to
see the pattern created when the filings arranged itself in the
magnetic field. So I kinda have some understanding how field phenomena
can behave. I just didn't connect it to antennas.


Your magnet experiment showed you one thing about a *static* field.
Time-varying fields behave much differently, so it's a mistake to think
that you now understand radio waves.

Is it more correct
to say that a field is established between the two antennas rather
than saying that something is transmitted in a direction from one end
to the other? (Of course on another level, *information* is
transmitted in a direction, but I am referring to the level of the
electromagnetic field).


No, it's not. One antenna creates a field whether or not the other
antenna is there. The field propagates at the speed of light away from
the transmit antenna.

. . .


As far as I can understand, the notion of "effective area" is quite
artificial and only meaningful if one wants to fit a complex wave
phenomena into a form where one can think of it in the naive way I
have expressed in my earlier posts... Anyways, thanks for a great
reply!


Yes and no. "Effective area" or "effective aperture" describes the cross
section of the impinging field which contains the amount of energy the
antenna captures and delivers to its load. It's most useful in the
analysis of antennas that are very large in terms of wavelength, such as
parabolic reflectors and horn antennas. For those, the effective
aperture is on the order of, and closely related to, the physical area.
When dealing with simpler and smaller antennas, though, there isn't any
direct relationship between effective aperture and physical size, which
leads to a lot of misunderstanding.

Roy Lewallen, W7EL
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