Jeff Liebermann wrote:
[misc.consumers.frugal-living dropped from distribution list.]

Roy Lewallen hath wroth:


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


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.



Antennas For All Applications by John D. Kraus and Ronald J. Marhefka
Third Edition
Page 746, Paragraph 21-15

"Prec=(Rr/(Ra+Rr))Pa

where

Rr=receiver impedance, ohms
Ra=antenna radiation resistance, ohms


For a perfect match, Rr=Ra, so that

Prec=(Rr/(Rr+Rr))Pa=0.5Pa (W)

and the receiver gets 1/2 the power collected by the antenna. The other half
is reradiated."

Also see "TV and Other Receiving Antennas" by Arnold B. Bailey published in
1950 by Rider Publishing. Beginning on page 235 near the bottom:

"In radio receiving antennas the predominant resistance is, strangely
enough, largely due to the fact that no electrons can move on the antenna
surface *without also sending radio energy back out into space*. So here we
have the paradox of a receiving antenna, having the prime function of
collecting or extracting energy from space, but unable to do so *without
itself returning radio energy of like kind* into space. The amount which it
returns is one-half of the total that it extracts under properly matched
conditions. In a good installation, with the antenna properly connected to
its receiver load, the receiving antenna will be able to *deliver to its
load one-half of the energy* it extracts from the oncoming radio wave but,
by necessity, *must return the other half to free space*. A receiving
antenna, then, is itself a *new source* of radiation. This is not so
surprising, since *any* reflecting surface, as we have seen, establishes a
new source of radiation"

(Note: The emphasis in the book was italics)

Cheers,
John