On 11/2/2014 6:06 AM, Lostgallifreyan wrote:
Jeff wrote in :

...or looking at it another way the dissipation in the radiation
resistance is not in the form of heat it is the power radiated into space.


Well, I did say I didn't know the terminology. On the other hand, I'm not
talking about antenna's radiation resistance. The only thing I'm sure of here
is that some body, at some temperature, can not emit heat faster than some
rate, and that superconductors in space will warm up too fast to stay
superconducting without support to cool them.

What is going to warm them up? The point of using them for the antenna
is because they have no resistance which means the signal is not turned
into heat.

This discussion looked like it had strayed some way from the earlier talk of
antennas and radiation resistance.

No, the topic was antenna radiating all the power fed to them. The
other two things that happen to the power is to be reflected back to the
source or dissipated as heat. Superconductors eliminate the heat
dissipation.

--

Rick

rickman wrote in :

No, the topic was antenna radiating all the power fed to them.

Specifically, doing it efficiently. Just being hard to match.

Never mind the other bits, beginning to look like old ground already today.
What I might be missing about my comment on some body at some temperature
being limited in its rate of dissipation might be flawed anyway. Never mind
the risk of confusion between an antenna's radiation resistance and what I'm
trying to get at, there's another angle to this...

Am I wrong? Could it be that just as an antenna is efficient regardless of
size, IF you can feed it all the energy you're trying to transmit, is it also
true that regardless of size, that hot body will also equally transmit all
its heat? In other words, is the 'limit' analogous to matching, as in getting
the heat from the bulk volume out to its surface?

I'm hoping that answer(s) to this one might help solve a heap of confusion
for me..

On 11/2/2014 5:49 PM, Lostgallifreyan wrote:
rickman wrote in :

No, the topic was antenna radiating all the power fed to them.

Specifically, doing it efficiently. Just being hard to match.

Never mind the other bits, beginning to look like old ground already today.
What I might be missing about my comment on some body at some temperature
being limited in its rate of dissipation might be flawed anyway. Never mind
the risk of confusion between an antenna's radiation resistance and what I'm
trying to get at, there's another angle to this...

Am I wrong? Could it be that just as an antenna is efficient regardless of
size, IF you can feed it all the energy you're trying to transmit, is it also
true that regardless of size, that hot body will also equally transmit all
its heat? In other words, is the 'limit' analogous to matching, as in getting
the heat from the bulk volume out to its surface?

I'm hoping that answer(s) to this one might help solve a heap of confusion
for me..

Hmmm... All things emit energy according to their temperature and their
surface emissivity. All things also absorb energy according to their
surface emissivity. Both processes are going on at all times. So an
object loses or gains heat depending on its temperature and the
temperature of the environment. That delta temperature sets the rate
along with the surface emissivity.

In space with the environment near absolute zero (ignoring radiation
from the sun and other nearby objects) any object's radiation of heat
will be near it's maximum potential and limited only by its absolute
temperature. So yes, an object will lose heat according to it's
temperature and that will be less at lower temperatures. But that
doesn't mean a super conductor will warm up unless there is something
heating it.

--

Rick

rickman wrote in :

Hmmm... All things emit energy according to their temperature and their
surface emissivity. All things also absorb energy according to their
surface emissivity. Both processes are going on at all times. So an
object loses or gains heat depending on its temperature and the
temperature of the environment. That delta temperature sets the rate
along with the surface emissivity.


Ok, that works for me. I guess the rate of change is exponential just as
energy loss in a fading note from a stretched string, roughly reaching
equilibrium when it can't lose more energy to ambient conditions.

About warming of superconductors out there, I may be wildly underestimating
the effect of a difference of 77K. What's I'd thought of was that if a
supeconductor can only operate at a very low temperature, its thermal
emission will be low; perhaps so low that it might take very little input
(from whatever, I know not what, and especially so if its emissivity is high
making absorbtion easy) to balance that and stop it staying cold enough. My
difficulty comes from not being sure whether a difference of 77K means the
same thing at cryogenic temperatures as it does around room temperature,
because it's not an infinite continuum of temperature. I was thinking that
because it is so cold, that small amounts of heat lost from other equipment,
might find their way to a superconductor and cause bother in the absence of
forced cooling. I can't really imagine any use of superconductors in space
that would not include the risk of local heat sources.

On 11/3/2014 4:19 AM, Lostgallifreyan wrote:
rickman wrote in :

Hmmm... All things emit energy according to their temperature and their
surface emissivity. All things also absorb energy according to their
surface emissivity. Both processes are going on at all times. So an
object loses or gains heat depending on its temperature and the
temperature of the environment. That delta temperature sets the rate
along with the surface emissivity.


Ok, that works for me. I guess the rate of change is exponential just as
energy loss in a fading note from a stretched string, roughly reaching
equilibrium when it can't lose more energy to ambient conditions.

Equilibrium is when the temperatures are equal. Of course this is a bit
of a cyclic definition because of how we define temperature. Still,
that is the point, equilibrium means equal heat exchange in both
directions.


About warming of superconductors out there, I may be wildly underestimating
the effect of a difference of 77K. What's I'd thought of was that if a
supeconductor can only operate at a very low temperature, its thermal
emission will be low; perhaps so low that it might take very little input
(from whatever, I know not what, and especially so if its emissivity is high
making absorbtion easy) to balance that and stop it staying cold enough. My
difficulty comes from not being sure whether a difference of 77K means the
same thing at cryogenic temperatures as it does around room temperature,
because it's not an infinite continuum of temperature.

Trouble is you don't really think like a scientist or engineer.
Temperature *is* a continuous function and each degree is the same. If
you want to understand it, look at the math. There are no step
functions in the equations for heat exchange. Remember what I wrote,
"delta temperature" determines the rate of heat exchange. Nowhere did I
say depending on if you are in "cryogenic" ranges. The equations don't
know what we consider "cryogenic". It takes the same amount of heat to
raise a substance 1 degree at 77 °K as it does at room temperature.
Also remember that I only picked 77 °K as a convenience (boiling point
of N2) as we know there are a number of superconductors with their
transition temperature well above that. The key is "well above".


I was thinking that
because it is so cold, that small amounts of heat lost from other equipment,
might find their way to a superconductor and cause bother in the absence of
forced cooling. I can't really imagine any use of superconductors in space
that would not include the risk of local heat sources.

An antenna is also subject to EMC. It is not uncommon to mount them
clear of the rest of the craft. It's easier to insulate them from heat
sources than it is to isolate them from EMC from the rest of the craft.

--

Rick

rickman wrote in :

Trouble is you don't really think like a scientist or engineer.


I'm neither. I managed to build a phase modulation synthesiser despite that.
I get by. And I usually do that by being willing to go beyond my 'comfort
zone'. How many other people who are not engineers or scientists do you see
posting around here?

"Lostgallifreyan" wrote in message
. ..
How many other people who are not engineers or scientists do you see
posting around here?

In discussions about short antennae, quite a few from Yankland.

rickman wrote in :

It takes the same amount of heat to
raise a substance 1 degree at 77 °K as it does at room temperature.

Ok, but when I read (or hear on BBC radio science programs) that it takes FAR
more effort (energy) to pump from 2K to 1K than it does from 300K to 299K,
what am I supposed to make of that given what you just said?

On 2014-11-03 17:06:02 +0000, Lostgallifreyan said:

rickman wrote in :

It takes the same amount of heat to
raise a substance 1 degree at 77 °K as it does at room temperature.

Ok, but when I read (or hear on BBC radio science programs) that it takes FAR
more effort (energy) to pump from 2K to 1K than it does from 300K to 299K,
what am I supposed to make of that given what you just said?

That's energy to keep all the heat from the surrounding environment
out. In a system completely separated from hot material or radiation,
such as space, the energy is exactly the same, because of the way
temperature is defined.

--

Percy Picacity

On 11/3/2014 12:06 PM, Lostgallifreyan wrote:
rickman wrote in :

It takes the same amount of heat to
raise a substance 1 degree at 77 °K as it does at room temperature.

Ok, but when I read (or hear on BBC radio science programs) that it takes FAR
more effort (energy) to pump from 2K to 1K than it does from 300K to 299K,
what am I supposed to make of that given what you just said?

Ok, I'll grant that few who have not had thermodynamics really
understand heat. Thermo was not an easy part of the curriculum in
school. The reason why cooling something gets harder as it approaches
absolute zero is because the heat flow is proportional to the difference
in temperature. Even if your pump is perfect and acts as if you put the
thing being cooled in contact with a heat sink at 0 °K, the rate of heat
flow decreases as that temperature delta diminishes.

The reality is that thinking 77 °K is especially cold is a bit of an
exaggeration. Yes, it is cold by human experience, but in the world of
cryogenics it is just a step stool to board the rocket. Thinking that
any little heating effect would warm a high temperature superconductor
is thinking with your feelings and not your brain. Not that we don't
all do that. But you need more experience with this stuff to let your
instinct guide you.

--

Rick