Jim, my first exposure to "capture area" was in a text from 1935. Then as now,
we were taught out of texts ten to twelve years behind the state of the art.
Aperture came along in the mid to late 1940's. Along with klystrons,
magnetrons, and plumbing.
For all intents and purposes, the capture area of a simple HF dipole is a
sphere with a radius one fifth the wavelength. Or a half sphere of lambda/5 for
a simple ground mounted vertical, for that matter. Of course, more complex and
more directional antennas have capture areas that are distorted into something
similar to the radiation pattern given by antenna modelling programs. Big one
way, small the other.
It's often more useful to consider the "effective area" of an antenna - using
the common formula for phi - than the "capture area." The latter gives a result
that's much more easily visualized - roughly 0.130 lambda squared for a half
wave dipole, or two to four times that for a simple dipole/reflector beam.
That's about 13 square meters of effective area for a resonant dipole for 30
mHz, or 3300 square meters for a 160 Meter dipole. In free space of course!
73 Pete Allen AC5E
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