Jeff Liebermann wrote:

I'm not so sure. I couldn't find any specific references to this
effect in several books I skimmed. Same with internet searches. If
true, then the concept of converting solar power in an orbital
satellite, converting it to microwaves, beaming it down to an antenna
array in the middle of the desert, and converting it back to
electricity, isn't going to work if the array re-radiates half the
power. That's going to ruin quite a few nifty science fiction stories
and innovative business plans.

I also note that the common microwave path analysis calculations don't
take re-radiation into account. For example, if I start with an EIRP
of perhaps XX dBm from a transmit antenna, -YY dB of path loss, and ZZ
dB receive antenna gain, the power delivered to the receiver (ignoring
coax losses) is calculated at (XX - YY + ZZ) dBm without any mention
of the -3dB that would need to be subtracted if half the receive power
is re-radiated from the rx antenna. It would seem that the common
formula and web forms for link calculations are -3dB off.

I trust your judgement in such matters and you have far more expience
than me, but something seems wrong or I'm missing something. Can you
point me to any books or refernences? I just skimmed Chapter 2
(Fundamentals of Antennas) in "Antenna Engineering Handbook" by Jasik
(1961) and found no obvious mention of this effect.

I could add more references to the ones already mentioned, but you
should be able to find it in most antenna texts. Look in the index under
aperture and scattering cross section. When dealing with path loss
calculations, the effective aperture is used, and this has the
reradiation already accounted for. In fact, the reradiated power has its
own descriptive unit, the scattering aperture. A good and brief
description of these can be found in Kraus' _Antennas_, p. 29ff, and
many other texts.

You're right that the antennas used to receive beamed power will catch
only half of it at best. But many, many business plans have been
developed and billions in stock sold for schemes which are much less
plausible. For starters, how about the current idea of hydrogen "fuel",
"made from water"? (For those not acquainted with the harsh reality of
thermodynamics, it takes more energy to extract hydrogen from water than
you'll get back when you burn it. Charlatans notwithstanding, there's
just flat no way around this little fact.) Then there's SDI. . .

Roy Lewallen, W7EL