What is the available power, Pa in picowatts, that can be extracted from a free
space field of Ea microvolts per meter?

A reference I have says it is:

Pa = Ea squared multiplied by Ae (the antenna effective area) all divided by
2Zo, where Zo = 377 ohms, the impedance of free space.

My question is why 2Zo rather than just Zo. Is it because only half of the power
in the field can be extracted and delivered to a load?

Ron, W4TQT

The fact that you can extract only half the power from a wave with a
perfectly efficient antenna is already built into Ae. That is, Ae = P/S,
where P is the power delivered to a conjugate load and S is the power
density of the wave.

I suspect that in your reference, Ea is the *peak*, not RMS, field
strength, and that's why the factor of two.

Incidentally, while you can extract only half the power from a wave with
a perfectly efficient antenna, it is possible to extract *all* the power
from a wave. An anechoic chamber extracts essentially all the power from
waves generated within it.

Roy Lewallen, W7EL

Ron wrote:
What is the available power, Pa in picowatts, that can be extracted from
a free space field of Ea microvolts per meter?

A reference I have says it is:

Pa = Ea squared multiplied by Ae (the antenna effective area) all
divided by 2Zo, where Zo = 377 ohms, the impedance of free space.

My question is why 2Zo rather than just Zo. Is it because only half of
the power in the field can be extracted and delivered to a load?

Ron, W4TQT

My question is why 2Zo rather than just Zo. Is it because only half of the
power
in the field can be extracted and delivered to a load?

===============================

Yes. With the current induced in the receiving antenna it doubles as a
radiator and re-radiates half the power back into the field. But only when
the receiver and antenna are Z-matched.

If not Z-matched then more than half of the energy is re-radiated and less
than half of it is accepted by the receiver.

It's a simple case of a conjugate match between a generator and a load which
applies whether the antenna is resonant or not.

In series with the antenna's radiation resistance there's a small conductor
loss resistance and a corresponding small loss in power efficiency. The
radiation resistance changes when in an off-resonance condition and so does
efficiency.

But apart from the change in efficiency, provided a conjugate match is
maintained, the 50 percent receive/re-transmit condition always holds and
even when well away from resonance a receiving antenna can have high
efficiency.

It is required only that radiation resistance be considerably greater than
conductor loss resistance. Which is rather obvious whichever way one looks
at it.
---
Reg.

On Wed, 08 Sep 2004 23:56:20 -0700, Roy Lewallen hath writ:
The fact that you can extract only half the power from a wave with a
perfectly efficient antenna is already built into Ae. That is, Ae = P/S,
where P is the power delivered to a conjugate load and S is the power
density of the wave.

I suspect that in your reference, Ea is the *peak*, not RMS, field
strength, and that's why the factor of two.

Incidentally, while you can extract only half the power from a wave with
a perfectly efficient antenna, it is possible to extract *all* the power
from a wave. An anechoic chamber extracts essentially all the power from
waves generated within it.

As well, the universe, in all it's vastness, eventually "...extracts
essentially all the power from waves generated..." by _any_ source. HI!HI!

Jonesy, W3DHJ
--
| Marvin L Jones | jonz | W3DHJ | linux
| Gunnison, Colorado | @ | Jonesy | OS/2 __
| 7,703' -- 2,345m | config.com | DM68mn SK

Allodoxaphobia wrote:


Incidentally, while you can extract only half the power from a wave with
a perfectly efficient antenna, it is possible to extract *all* the power
from a wave. An anechoic chamber extracts essentially all the power from
waves generated within it.


As well, the universe, in all it's vastness, eventually "...extracts
essentially all the power from waves generated..." by _any_ source. HI!HI!


No it doesn't. Free space can't extract any energy whatsoever from EM
waves travelling through it. Consider :-
The cosmic microwave background hasn't had it's power extracted by
the universe in all the time since the big bang. Conservation of
energy applies everywhere.

Just my opinion ;-) but it's allodoxOphobia.

best

Andy

Andy Cowley wrote:
Allodoxaphobia wrote:
As well, the universe, in all it's vastness, eventually "...extracts
essentially all the power from waves generated..." by _any_ source.

No it doesn't. Free space can't extract any energy whatsoever from EM
waves travelling through it. Consider :-
The cosmic microwave background hasn't had it's power extracted by
the universe in all the time since the big bang. Conservation of
energy applies everywhere.

True so far, but just you wait till the *BIG CRUNCH*. You don't
really think that our present Big Bang was the first one ever,
do you? :-)
--
73, Cecil, W5DXP


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Cecil, W5DXP wrote:
"You don`t really think that our present Big Bang was the first one
ever, do you?"

Suppose that in a time line of infinite length, with no beginning and no
end, there are really no unique events? Everything repeats sooner or
later, does it?

Best regards, Richard Harrison, KB5WZI

Anything which repeats itself is amenable to Fourier Analysis.

If there's a once-only event then Heaviside's Operational Calculus is
appropriate.

It's not more difficult. It's just a matter of getting used to it.

=================================

"Richard Harrison" wrote -
Cecil, W5DXP wrote:
"You don`t really think that our present Big Bang was the first one
ever, do you?"

Sheeesh ! "Fourier Analysis of the universe". Now that's HEAVY!

I'll take a pass...
Steve

"Reg Edwards" wrote in message
...
Anything which repeats itself is amenable to Fourier Analysis.

If there's a once-only event then Heaviside's Operational Calculus is
appropriate.

It's not more difficult. It's just a matter of getting used to it.

=================================

"Richard Harrison" wrote -
Cecil, W5DXP wrote:
"You don`t really think that our present Big Bang was the first one
ever, do you?"

Steve Nosko wrote:
Sheeesh ! "Fourier Analysis of the universe". Now that's HEAVY!

I'll take a pass...
Steve

"Reg Edwards" wrote in message
...

Anything which repeats itself is amenable to Fourier Analysis.

If there's a once-only event then Heaviside's Operational Calculus is
appropriate.

It's not more difficult. It's just a matter of getting used to it.

=================================

"Richard Harrison" wrote -
Cecil, W5DXP wrote:

"You don`t really think that our present Big Bang was the first one
ever, do you?"




No! Fourier analysis of the universe could improve the S/N ratio of
Usenet--get all the cranks to go away until they've evaluated the integrals....

Cheers,

Phil Hobbs