"Roy Lewallen" wrote in message ...
Don K wrote:
"John - KD5YI" wrote in message news:XLQrg.2896$bd4.372@trnddc01...
Rod Speed wrote:
A receive antenna has no EIRP, it doesnt radiate any real power.
Actually, when properly matched, it radiates half the received power.


How do you get that?
If the receiver input impedance is matched to the antenna, all the
received power is absorbed. There is no reflection. There is no radiation.

If the receiver matching is for optimal noise figure, there may be
some reflection and reradiation, but there's nothing pinning it to
be half the received power.

John is correct. A receiving antenna, when matched, reradiates half the power it receives. An
impinging field induces current in the antenna. This causes radiation, just like the current in a
transmitting antenna. As it turns out, when the antenna is matched, the amount of power radiated
equals the amount of power delivered to the load, and that's the best you can do. If you'd like a
more in-depth and mathematical explanation, you can find it in any antenna text, often discussed
as "scattering".

If a receiving antenna did absorb all the impinging power, it would be a lot easier to make a
shield or a stealth aircraft.

Roy Lewallen, W7EL

Maybe to some extent, it's a matter of nit-picking over what
"received power" means. You can think of an antenna as having an
effective aperture size over which it captures all the energy
crossing that cross-sectional area. To me it's logical to think
of "received power" as the power that actually gets scooped
up and delivered.

Effective aperture increases with antenna gain. Obviously something
like a dipole has a relatively small effective aperture. But the
effective aperture of a high-gain horn antenna for instance, will
approach its actual physical cross-sectional area.

For instance, look at Figure 13 in this pdf.
http://www.coe.montana.edu/ee/rwolff...WBAntennas.pdf

Don