Antenna re-radiation Was: ping Wayne - new thread
 

"Wayne" wrote in message
...


"John S" wrote in message ...

On 7/16/2015 4:46 AM, bilou wrote:
"John S" wrote in message
...
The other thread is getting too long for my mouse wheel to handle.

Do you have any other questions that we might be able to solve
together?

John N1JLS
Hi. (Sorry I have not seen the other thread)
I have one:
Does a receiving aerial reradiates more or at best the same power
it delivers to the receiver ?
Or on the contrary would a perfectly matched antenna be invisible on
radar ?
My own experience of the second proposition is that it is not at all the
case.
73's

According to Kraus, a matched antenna absorbs half the incoming power and
re-radiates the other half.

As for best re-radiation, consider the reflector element of a Yagi. It is
shorted (no load in the middle).

It is difficult to make metal invisible. Notice that stealth aircraft have
a special rf absorbing coating and they take care to reflect what they
don't absorb away from the incoming rf.

73

This sounds interesting, and I'm surprised it didn't get picked up for
general discussion.
I'm late responding to this post because I've been "off the grid" on
vacation for a couple of weeks.

So "bilou"....why not post the question as a new thread?
Hi
Here I am quite late too :-)
Now let us imagine a perfect waveguide with a pair of
coaxial /guide transitions (antennas) at each end.
Luckily we don't have a minimum of 3 dB loss.
For me the most satisfying explanation is that antennas behave quite
differently
depending of what is between them. :-)
What happens if ,by design, we use a frequency very much higher than the
cut off frequency of the guide.
There is also the regularly reapearing projects of transmitting
power from a geostationnary solar panel to ground by microwave.
I have great difficulties to understand how it could be efficient.

Antenna re-radiation
 

"bilou" wrote in message ...


"Wayne" wrote in message
...


"John S" wrote in message ...

On 7/16/2015 4:46 AM, bilou wrote:
"John S" wrote in message
...
The other thread is getting too long for my mouse wheel to handle.

Do you have any other questions that we might be able to solve
together?

John N1JLS
Hi. (Sorry I have not seen the other thread)
I have one:
Does a receiving aerial reradiates more or at best the same power
it delivers to the receiver ?
Or on the contrary would a perfectly matched antenna be invisible on
radar ?
My own experience of the second proposition is that it is not at all
the
case.
73's

According to Kraus, a matched antenna absorbs half the incoming power and
re-radiates the other half.

As for best re-radiation, consider the reflector element of a Yagi. It is
shorted (no load in the middle).

It is difficult to make metal invisible. Notice that stealth aircraft have
a special rf absorbing coating and they take care to reflect what they
don't absorb away from the incoming rf.

This sounds interesting, and I'm surprised it didn't get picked up for
general discussion.
I'm late responding to this post because I've been "off the grid" on
vacation for a couple of weeks.

So "bilou"....why not post the question as a new thread?
Hi
Here I am quite late too :-)
Now let us imagine a perfect waveguide with a pair of
coaxial /guide transitions (antennas) at each end.
Luckily we don't have a minimum of 3 dB loss.
For me the most satisfying explanation is that antennas behave quite
differently
depending of what is between them. :-)
What happens if ,by design, we use a frequency very much higher than the
cut off frequency of the guide.
There is also the regularly reapearing projects of transmitting
power from a geostationnary solar panel to ground by microwave.
I have great difficulties to understand how it could be efficient.

I don't have much experience with re-radiation. But, I can understand how
yagis might like the concept :)

Antenna re-radiation
 

"Wayne" wrote in message
...

I don't have much experience with re-radiation. But, I can understand how
yagis might like the concept :)
Yes it is a tricky undocumented subject.
Most authors say nothing about it.
Some as quoted by John S agree on the half power re-radiated at least.
For the amateur trying to put a maximum of antennas in his limited space
it can lead to false comparisons of performances.
For example you try a dipole close to a long yagi and then discard the yagi
on the ground that the dipole WAS almost as good :-)

Antenna re-radiation Was: ping Wayne - new thread
 

On 8/16/2015 11:08 AM, bilou wrote:

There is also the regularly reapearing projects of transmitting
power from a geostationnary solar panel to ground by microwave.
I have great difficulties to understand how it could be efficient.

I'm lost. What is the problem? If you are worried about the losses in
transmitting power to the ground, I think you are missing the point.
The only thing that matters is the amount of power delivered, vs. the
cost of the system. In other words, you are comparing apples to
oranges. You may have losses in the transmission of power, but all
power systems have losses. The important part is the system, not one
isolated portion of it. The point of the orbital PV cells is to greatly
increase their power output. As long as those gains can make up for
other system losses *and* the increased cost of the system, the system
costs less per watt.

I believe the only real obstacle to orbital PV power is concerns about
the safety of beaming the power back to earth without possibility of
frying people.

--

Rick

Antenna re-radiation Was: ping Wayne - new thread
 

In article , rickman writes:
On 8/16/2015 11:08 AM, bilou wrote:
There is also the regularly reapearing projects of transmitting
power from a geostationnary solar panel to ground by microwave.
I have great difficulties to understand how it could be efficient.

I'm lost. What is the problem? If you are worried about the losses in
transmitting power to the ground, I think you are missing the point.
The only thing that matters is the amount of power delivered, vs. the
cost of the system. In other words, you are comparing apples to
oranges. You may have losses in the transmission of power, but all
power systems have losses. The important part is the system, not one
isolated portion of it. The point of the orbital PV cells is to greatly
increase their power output. As long as those gains can make up for
other system losses *and* the increased cost of the system, the system
costs less per watt.

I don't know what to say about that comment.

Yes, electric power generation has its inefficiencies, although I
understand that at this point it is possible to get 50% and even
somewhat better with combined cycle gas turbine generation. And
there are a lot of what seem to be poorly documented losses in
long distance power distribution. But no one says to ignore the
losses.

I do understand the idea of measuring the entire system and not
individual pieces, and the fact that power received by an orbital
array but not delivered to the target is unimportant.

But everything in orbit is incredibly expensive. First, you have
to get it there. Then you have to have built it to be so reliable
that it will not break down. Except that it will. And then you
have to be prepared to spend more money to send humans up to repair
it.

In that sense, I guess, efficiency is the least of the designer's
worries. And all those other problems, plus the safety issue
mentioned later in your post, are the reason it's never been done.

Still, I would be curious as to whether that 50% rule - if it is
accurate at all (does a laser beam hitting a black body have to
reflect back 50%?) - applies to a microwave downlink.

George

Antenna re-radiation Was: ping Wayne - new thread
 

On 8/16/2015 6:21 PM, George Cornelius wrote:

Still, I would be curious as to whether that 50% rule - if it is
accurate at all (does a laser beam hitting a black body have to
reflect back 50%?) - applies to a microwave downlink.

What is the 50% rule?

--

Rick

Antenna re-radiation Was: ping Wayne - new thread
 

In message , George Cornelius
writes
Still, I would be curious as to whether that 50% rule - if it is
accurate at all (does a laser beam hitting a black body have to
reflect back 50%?) - applies to a microwave downlink.

If it's a true black body it's all absorbed , and either conducted away
as heat or re-radiated at the temperature of the black body at
different wavelengths

A better laser analogy is a set of lenses and a focussed fibre-optic
between them. It's possible to arrange this to be nearly 100% efficient.

Brian

--
Brian Howie

Antenna re-radiation Was: ping Wayne - new thread
 

On 8/18/2015 12:15 PM, Brian Howie wrote:
In message , George Cornelius
writes
Still, I would be curious as to whether that 50% rule - if it is
accurate at all (does a laser beam hitting a black body have to
reflect back 50%?) - applies to a microwave downlink.

If it's a true black body it's all absorbed , and either conducted away
as heat or re-radiated at the temperature of the black body at
different wavelengths

A better laser analogy is a set of lenses and a focussed fibre-optic
between them. It's possible to arrange this to be nearly 100% efficient.

Brian


It is my understanding that interstellar space (between galaxies) is
about as good a black body as can be obtained. If a source is directed
into that space, I very much doubt a return could be detected by mortals.

Antenna re-radiation Was: ping Wayne - new thread
 

In message , John S
writes
On 8/18/2015 12:15 PM, Brian Howie wrote:
In message , George Cornelius
writes
Still, I would be curious as to whether that 50% rule - if it is
accurate at all (does a laser beam hitting a black body have to
reflect back 50%?) - applies to a microwave downlink.

If it's a true black body it's all absorbed , and either conducted away
as heat or re-radiated at the temperature of the black body at
different wavelengths

A better laser analogy is a set of lenses and a focussed fibre-optic
between them. It's possible to arrange this to be nearly 100% efficient.

Brian


It is my understanding that interstellar space (between galaxies) is
about as good a black body as can be obtained. If a source is directed
into that space, I very much doubt a return could be detected by mortals.

Yes it's an infinite heat-sink.

Brian
--
Brian Howie

Antenna re-radiation Was: ping Wayne - new thread
 

On Fri, 21 Aug 2015 08:52:36 +0100, Brian Howie wrote:
In message , John S writes:

It is my understanding that interstellar space (between galaxies) is
about as good a black body as can be obtained. If a source is directed
into that space, I very much doubt a return could be detected by mortals.

Yes it's an infinite heat-sink.

More properly like an infinitely long piece of coax.

Jonesy
--
Marvin L Jones | W3DHJ | W3DHJ | http://W3DHJ.net/
Pueblo, Colorado | @ | Jonesy | __
38.238N 104.547W | jonz.net | DM78rf | 73 SK