----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: <tentec@contesting.com>
>
> Capture area for HF antennas is a rather useless
> unless perhaps part of the design for really large
> V-beams and Rhombic antennas and, again, I
> am not really sure you can view the wave front
> onto such types as coherent, or truly in-phase
> over the entire set of wires comprising these
> antennas. My thought now is, that the term is
> not really applicable to these sorts. Perhaps
> others will know for sure!
>
> 73, Jim KH7M
If you know the power density (in watts/meter^2) of
an incoming plane wave, effective aperture is a very
useful concept for determining the amount of energy
captured by a receive antenna. In other words if I tell
you that the the local field due to an incoming plane
wave is 10 microwatts/meter^2, and I know the
antenna has an effective aperture of 1 meter^2, then
the received signal will be 10 microwatts (assuming
the main lobe of the receive antenna is aligned with
the incoming wave and matched for polarization and
the receive antenna is impedance matched to the
receiver).
If on the other hand, I just tell you that the antenna
has +3dBi gain in the direction of the incoming plane
wave and that the incoming plane wave has a power
density of 10 microwatts/meter^2, you won't be able
to tell me the level of the received signal (not enough
information).
BTW, for a free space link, the power density of a
plane wave (watts/meter^2) at some distant receiver
is easily calculated by dividing the effective radiated
power of the transmitter by the surface area of a
sphere or radius, R, where R is the distance from
the transmit antenna to the receive antenna.
73 de Mike, W4EF.................................
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