377 ohm per sq. space cloth, absoption, reflection and transmission
Randy M. Dumse
would happen to an incident plane wave of electromagnetic radiation? Would
the wave be fully absorbed? Would any parts be reflected, or transmitted. If
no, why is a reflector at 1/4 wavelength behind such a cloth in a Salisbury
screen necessary to make an effective radar absorbing material? (Per
http://www.randf.com/rf_screen.html)
Does the quality of the space cloth have anything to do with the absorption
vs. reflection and transmission? Would a perfectly made 377 ohm per sq.
cloth have perfect absorption, or do other effects prevent it from perfect
performance?
Since the space cloth is a good absorber, does that mean its index of
refraction has a high part imaginary component? Could a pure imaginary
component cause a reflection from the cloth (per Feynman lectures II Ch
33-5)?
Thank you in advance.
--
Randy M. Dumse
and...@attglobal.net
>
> Given a large plane of a space cloth measuring 377 ohms per square
ohms per square what? What is your question supposed to be asking
about physics?
John Anderson
Randy M. Dumse
> > Given a large plane of a space cloth measuring 377 ohms per square
>
> ohms per square what? What is your question supposed to be asking
> about physics?
Amazingly as it might first appear, the answer is, "ohms per square
anything". 377 ohms per square inch, per square centimeter, etc. etc.
To understand this consider the square inch as a starting point. If have 4
square inch "patches" each measuring 377 ohms per square inch, and arrange
them in a square of 4 square inches, and measure the resistance from corner
to corner, the result will be, 377 ohms. If you think of the problem as a
resistor network, and wire two 377 ohm resistors in parallel (377/2 ohms)
then connect that two in series with two more 377 ohm resistors in parallel
(377/2 ohms), you wind up with a final resistance (377/2 ohms + 377/2 ohms)
of 377 ohms, even though it is a 2 inch square. Neat, huh?
What is this asking about physics? The impedance of empty space is 377 ohms.
The value (to three places) is the ratio of the E to H fields
(mu0/epsilon0)^1/2. So a resistive surface measuring 377 ohms per square
matches empty space, and is a very lossy surface. But beyond that, I don't
remember the details. My question is can it theoretically absorb everything,
or is inherently limited from full absorption.
--
Randy M. Dumse
Bilge
Re: 377 ohm per sq. space cloth, absoption, reflection and transmission
>to usenet:
><and...@attglobal.net> wrote in message news:3BCFB8...@attglobal.net...
>> > Given a large plane of a space cloth measuring 377 ohms per square
>>
>> ohms per square what? What is your question supposed to be asking
>> about physics?
>
>Amazingly as it might first appear, the answer is, "ohms per square
>anything". 377 ohms per square inch, per square centimeter, etc. etc.
>
>To understand this consider the square inch as a starting point. If have 4
>square inch "patches" each measuring 377 ohms per square inch, and arrange
>them in a square of 4 square inches, and measure the resistance from corner
>to corner, the result will be, 377 ohms. If you think of the problem as a
>resistor network, and wire two 377 ohm resistors in parallel (377/2 ohms)
>then connect that two in series with two more 377 ohm resistors in parallel
>(377/2 ohms), you wind up with a final resistance (377/2 ohms + 377/2 ohms)
>of 377 ohms, even though it is a 2 inch square. Neat, huh?
>
>What is this asking about physics? The impedance of empty space is 377 ohms.
>The value (to three places) is the ratio of the E to H fields
>(mu0/epsilon0)^1/2. So a resistive surface measuring 377 ohms per square
>matches empty space, and is a very lossy surface. But beyond that, I don't
>remember the details. My question is can it theoretically absorb everything,
>or is inherently limited from full absorption.
Randy M. Dumse
news:slrn9t1li...@radioactivex.lebesque-al.net...
> What's the AC impedance of your material?
Hi Bilge. The impedance here is not a measured quantity, but the impedance
of free space from theory. The impedance of free space is the square root of
the ratio of magnetic permeability of free space to the vacuum dielectric
permeability as given in the formula Z0 = (mu0/epsilon0)^1/2. (You might
recognize these better if written as mu-naught and epsilon-naught. You'll
find them in college level book on EM.)
> Theoretically, you have a 377 ohm resistor at DC.
I think the resistance is considered a pure resistance at all frequencies.
In technical application, such as the Salisbury screen, the bands are
microwave from 1 to 18 GHz. But there is also mention of applications RF
from 30 MHz to 1000 GHz. You can see http://www.eccosorb.com/notes.asp for
these notes and more:
"Another means of matching the impedance of free-space is to lump the loss
in a resistive sheet with resistivity equal to 377 ohms per square. One can
then construct an absorber known as the Salisbury Screen. This absorber can
be understood best when viewed on the basis of transmission line theory. It
may be recalled that a simple transmission line termination can be effected
by placing a pure resistance across a shorted transmission line at a point a
quarter wave in front of the short. There is complete absorption at that
frequency when this resistance is equal to the characteristic impedance of
the line. The free-space analogy involves placing a resistive sheet a
quarter-wave length in front of a conductive surface. Here the resistivity
of the sheet must equal 377 ohms per square to be equal to the impedance of
free space."
From this site, I picked up information today suggesting there is no front
face reflection from a 377-ohm material. The low the dielectric constant the
more transmissive, and the higher the more is reflected. Hence a very lossy
medium will act as a reflector. This tracks with what Feynman said in the
reference about a good absorber being a good reflector (Feynman lectures II
Ch 33-5). Apparently the 377-ohm material is not as good an absorber as I
had first thought, so there is some of the source of my confusion on the
subject.
--
Randy M. Dumse
Bilge
>news:slrn9t1li...@radioactivex.lebesque-al.net...
>> What's the AC impedance of your material?
>
>Hi Bilge. The impedance here is not a measured quantity, but the impedance
>of free space from theory.
It doesn't matter. And, it's a measured quantity. Why do you think
antennas are designed with matching in mind? It's a reactance. Do you
know the difference in a reactance and resistance?
> The impedance of free space is the square root of
>the ratio of magnetic permeability of free space to the vacuum dielectric
>permeability as given in the formula Z0 = (mu0/epsilon0)^1/2. (You might
>recognize these better if written as mu-naught and epsilon-naught.
I know exactly what it is. Would you like it in terms of
something more fundamental:
Z = 4 pi\alpha\hbar/e^2
where alpha = e-m coupling constant, e = electron charge.
> You'll find them in college level book on EM.)
I wrote a 350 page solutions manual to a college level book on E&M.
Does that count?
>> Theoretically, you have a 377 ohm resistor at DC.
>
>I think the resistance is considered a pure resistance at all frequencies.
No kidding. What's the AC impedance? That is geometry dependent.
The impedance is the TOTAL impedance, so the geometry matters for the
reactive part, since for a reactance, geometry is the ONLY thing that
matters.
[...]
>Ch 33-5). Apparently the 377-ohm material is not as good an absorber as I
>had first thought, so there is some of the source of my confusion on the
>subject.
>
On your part, perhaps. Listen again: You have a bad conductor. That's
all. There is nothing magic about it. See Jackson, the section on admittance.
By the way, see how well it absorbs a 5 MeV gamma ray.
Randy M. Dumse
> It doesn't matter. And, it's a measured quantity.
Well, in this case, the impedance of a black hole horizon is the ultimate
application. There are no test cases at hand to take measurements. I am
trying to decide if treating the horizon as if it were a space cloth gives
any insight into an infalling em-wave's experience at the horizon, whether
the horizon itself might trap some of the em, reflect any, or (the
possibility I'm most curious about currently) divert the energy into surface
currents (~analogous to creeping waves), or, if the horizon just transmits
all of incident em-waves right through as empty space, per accepted theory.
I want to see if I can find any connections to cases where smaller black
holes would scatter longer wave radiation (as I know they do from
Chandrasekhar's and Futterman, Handler, Matzner's books).
But I don't want to come right out and admit that's the comparison I'm
making, because then I will get an automatic reference to theory, "the black
hole horizon does not reflect or absorb, only transmits like empty space" as
demonstrated by the whole communities response to Bekenstein's suggestion,
black hole horizons ought to have thermodynamic radiation when it was
"clear" they had no such thing. (After all, that was exactly the response I
got 10 months ago when I first started participating in this newsgroup.)
> Why do you think antennas are designed with matching in mind?
Oh..., less heat in the finals from SWR inducing parasitic excursions over
ratings leading to premature failure? That and matching impedances reduce
reflections and improve overall efficiency and power deliver into the
medium.
> It's a reactance. Do you
> know the difference in a reactance and resistance?
Oh... grumble, grumble, think, think, well not right off the top of my head.
The same reason a 8 ohm speaker doesn't measure 8 ohms on a VOM. Hum...
that's because there's a coil (inductor) in the speaker which takes up
energy until saturation, and returns that energy to the circuit as current
drops. Let's see. The moving voice coil and the permanent magnet complicate
things quit a bit here... but that's beside the point. Okay. Resistance is
associated with loss, and reactance is associated with an imaginary
component which stores and returns AC energy. That's about as much as I can
pull up from memory.
> I know exactly what it is. ...
> I wrote a 350 page solutions manual to a college level book on E&M.
> Does that count?
Oh yeah! It counts. Very much so. Good to know. Sorry if I sounded like I
was talking down to you. In truth, I just wasn't sure where to talk, so I
tried to take the middle road.
In return I'll mention, I got my novice class ham radio license at age 9 and
my advanced class ticket at 11. I finally made peace with impedances,
inductance and reluctance, et al, in college, which had always puzzled me
from my self studies, and haven't used it since. (I went off an was a naval
officer for a few years, then became a successful entrepreneur, until I
retired.) I've forgotten more than I remember. (Sadly, most of that is the
ability to do the math.)
> >> Theoretically, you have a 377 ohm resistor at DC.
> >
> >I think the resistance is considered a pure resistance at all
frequencies.
>
> No kidding. What's the AC impedance?
I don't know, what is the AC impedance of a black hole horizon?
Best I can answer is 377 ohms per square. DC resistance would be entirely a
different reading depending on where on the horizon you "stuck" the probes.
For instance, in "The Membrane Paradigm" I see a calculation of 30 ohms pole
to pole. But I haven't comprehended the whole of the text, so I can't give
you much more detail than that.
> That is geometry dependent.
> The impedance is the TOTAL impedance, so the geometry matters for the
> reactive part, since for a reactance, geometry is the ONLY thing that
> matters.
Geometry? As in angle of incidence et al? Well, strangely enough, if we look
at a Schw. BH, and consider it as an optic, so we are only dealing with
shell observers, all incoming rays look like they normal to the horizon at
the limit of the horizon.
> See Jackson, the section on admittance.
Looking it over now.
> By the way, see how well it absorbs a 5 MeV gamma ray.
Well, assume I hadn't brought up black hole horizons, and please tell me
what happens with a 5 MeV gamma ray, cause that's the kind of difference
between the theoretically ideal situation and the real world one. If a black
hole horizon can't take a 5 MeV gamma ray, I'm dying to hear the story.
--
Randy M. Dumse
Caution: Objects in mirror are more confused than they appear.
Randy M. Dumse
news:imnA7.88269$aW5.1...@dfw-read.news.verio.net...
> Bilge <ro...@radioactivex.lebesque-al.net> wrote in message
> news:slrn9t2hm...@radioactivex.lebesque-al.net...
> > know the difference in a reactance and resistance?
>
> Oh... grumble, grumble, think, think, well not right off the top of my
head.
Woke up this morning realizing, since reactance is measured in ohms,
reactance is the AC version of resistance. Impedance equals resistance +
reactance. But I am confused about how resistance and reactance relate. A
pure resistor (no inductance or capacitance) resists both DC and AC without
regard to frequency (DC being the lowest limit special case of AC). So I
guess in this case (space cloth or black hole horizon) there is no reactance
(infinite ohms for this component), perhaps? Hope you can shed some light on
this Bilge (or anyone else willing to help.)
Bilge
(By the way. I'm familiar with the units ohms/sq, as I once checked
into some conductive Kapton, where upon I was informed by the people at
DuPont (iirc), how ohms/sq did not mean per square meter,cm, or any other
length, which I also found rather clever).
>Randy M. Dumse <r...@newmicros.com> wrote in message
>news:imnA7.88269$aW5.1...@dfw-read.news.verio.net...
>> Bilge <ro...@radioactivex.lebesque-al.net> wrote in message
>> news:slrn9t2hm...@radioactivex.lebesque-al.net...
>> > It's a reactance. Do you
>> > know the difference in a reactance and resistance?
>>
>> Oh... grumble, grumble, think, think, well not right off the top of my
>head.
>
>Woke up this morning realizing, since reactance is measured in ohms,
>reactance is the AC version of resistance. Impedance equals resistance +
>reactance. But I am confused about how resistance and reactance relate. A
Reactance and resistance relate as follows: Draw a circuit:
V C
--------||---+ Capacitor Inductor Resistor
|
) L Z 1/jwC jwL R
)
)
| j == sqrt(-1) [note: engineers use j = - i,
-----VVVVVv--+ backwards from
physicists]
GND R
The voltage drop is V - I/jwc - IjwL - IR = 0 , I == current
Note, that the DC impedance is infinite, since capacitors don't
pass DC and the high frequency limit (f->oo), is also infinite,
despite an inductor being a loop of wire in many many cases.
Now solve just like 3 resistances in series, using Z^2 = |ZZ*|
Z = [R + j(wl - 1/wc]]
In short, you have a complex plot for impedance (Check to see if I
should have L on the positive and C on the negative imaginary axis,
but this won't affect the argument here).
wL Im Z
| | So, We have an impedance where R is along the
| +---Re Z real axis and by definition C,L are opposite
| signs on an imaginary axis, where the convention
+--------------- R is to take the frequency w, as positive. Now,
| any total, frequency dependent impedance is
| given by writing the vector sum in polar form,
| as a total Z x a phase angle, A series RC
| circuit, for example:
1/wC
Z = R + 1/jwC Z* = R - 1/jwC
ZZ* = R^2 + 1/(wC)^2 = [(wRC)^2 + 1]/(wC)^2
Z = sqrt([wRC]^2 + 1)/wC Otherwise known as a highpass filter
Z is the hypotenuse, and the phase
+---> R angle is the angle the of the resultant
| total impedance vector with the real
|1/wC axis: tan(\phi) = 1/wRC, in this case.
V
So, basically, reactances are impedances
along the Im axis and do not dissipate
energy.
Inductors and capacitors are nature's way of storing energy in B and
E fields. Resistors are nature's way of turning some organized E&M
disturbance into random thermal motion of the constituents upon which
the radiation is incident.
>pure resistor (no inductance or capacitance) resists both DC and AC without
>regard to frequency (DC being the lowest limit special case of AC). So I
That's correct, but impedance refers to both the real and imaginary
parts.
>guess in this case (space cloth or black hole horizon) there is no reactance
>(infinite ohms for this component), perhaps?
But, there HAS to be a reactance of some sort (the exception being if
wL = 1/wC in which case, you have a tuner and Z=0 only at 1 frequency).
> Hope you can shed some light on
>this Bilge (or anyone else willing to help.)
Let's model your idea of free space as a lossless transmission line.
After all, waves propagate in transmission lines, too.
L L L L
o---UUUUU---+---UUUUU---+---UUUUU---+---UUUUU---+......
| | | |
------- ------- ------- -------
------- C ------- C ------- C ------- C
| | | |
| | | |
___ ___ ___ ___
\ / \ / \ / \ / GND
' ' ' '
What's Z? Well, it's Z = jwL + [ 1/jwc || the rest]
But, the rest is just Z, so you get a finite quadratic:
Z = jwL + Z/(1+jwcZ)
So, your transmission line has Z independent of length. I claim that
if you sit down and work this out, you will get Z = [L/C]^1/2.
Now, it's convenient that resistive films come in unit of ohms/sq.
That is just a geometric convenience. Let's look at a capacitor in the
same sort of way. In CGS units, a parallel plate capacitor has a capac-
itance of C = A/d. (yes, capacitance in given in cm).
A square of material cannot be a pure resistance. It can come close,
but the fact that it is maade of atoms and the like, means if you apply
a potential accross it, it will have some capacitance, inductance and
resistance.
What you want to do, if you are interested in absorption and scattering,
is to construct something like a black disk and choose a phenomenological
potential like V + iW.
-----> \ and study the diffraction pattern. Look under
---> | \ things like optical model, or effective range
---> | + or phase shift analysis.
---> | /
-----> /
Randy M. Dumse
> But, there HAS to be a reactance of some sort (the exception being if
> wL = 1/wC in which case, you have a tuner and Z=0 only at 1 frequency).
The "non-frequency dependent" nature of cloth and horizon makes me think
there is no reactance at issue here. Let me speculate on this idea a bit.
Going back to the space cloth, there is loss at all frequencies. Only when
you use a Salisbury Screen with a reflector (separated by 1/4 wave length)
does frequency come in to the equation. The original incident wave becomes
the resultant reflected wave and the transmitted wave at the cloth. But
since this is a resistive material, there is also an absorbed component. So,
back to the title of this thread, we have three components absorption,
reflection and transmission.
Going forward to the black horizon, there is a loss at all frequencies as
well. Accepted theory says the incident wave becomes the transmitted wave,
i.e. the horizon is empty space and has no local properties (all reactance
with no resistance). Under those premises there is no absorption of energy
at the horizon, and there is no reflected wave. Yet the Membrane Paradigm
clearly suggests there is a resistance at the horizon which obeys ohm's
law, and observation of matter jets from black hole candidates give physical
evidence of the correctness of the Membrane Paradigm. Therefore, there must
be reflection and absorption at the horizon. This is a new physical
prediction, as best I know. I think the idea of reflection from black hole
horizons to date has been more about superradiance of scattered
gravitational waves.
I've been thinking of reflection from black holes for years. The thought
first came to me when I was thinking of quantum foam. I was thinking how
smooth the surface of a black hole is down to the planckain level. It
occurred to me it would be the smoothest surface of matter possible, and
being so smooth would make an excellent mirror. A black mirror. A mirror
which reflected any incoming light, but did not let it leave. This
resistance/reactance approach is the first heuristic argument I've seen for
reflection from the surface. In the case of a black horizon, reflection and
absorption represent the same thing, roughly described as photons balanced
on the horizon, neither quite escaping or falling in. These photons are both
reflected (reversed in direction of travel and phase) and absorbed (trapped
on the surface).
That was interesting. A new physical prediction.
Now, moving on, I am back at the original motivation for considering the
space cloth. If a space cloth were perfect, would there be any transmitted
wave? Or would all incident EM energy be absorbed? This is what most
interests/puzzles me. For if a space cloth were a perfect absorber, then not
just some radiation would be reflected/trapped on the horizon, but all of it
would be. Why might this be significant? Because then we'd have a new method
of describing transfer of momentum for a black hole. The act of an incoming
photon transferring its momentum to the horizon by reflection can cause the
black hole to recoil. Also, the act of horizon deformation and horizon
acceleration are linked, so the impulse imparted by the incoming photon can
then be linked to the black hole recoil through the horizon deformation.
> A square of material cannot be a pure resistance. It can come close,
> but the fact that it is maade of atoms and the like, means if you apply
> a potential accross it, it will have some capacitance, inductance and
> resistance.
Then this is why you mentioned the 5 MeV ray? If the space cloth was made of
atoms and the like, it would have some capacitance, inductance and
resistance, meaning when we got to the atomic levels, the quantum structure
of the material would show up and it would not appear as a continuous
material with the 377 ohms per sq. This is interesting, because a black
horizon wouldn't have this problem. Not at least at the atomic scale. On the
other hand, at the planckian scale, it would again break down and not be
such a consistent surface. This gives me other ideas to ponder at the loop
gravity level.
> What you want to do, if you are interested in absorption and
scattering,
> is to construct something like a black disk and choose a phenomenological
> potential like V + iW.
>
>
> -----> \ and study the diffraction pattern. Look under
> ---> | \ things like optical model, or effective range
> ---> | + or phase shift analysis.
> ---> | /
> -----> /
I agree. I've tried to seek this reflection/absorption phenomena in a
number of ways. It was thinking of it in this way that the previous physical
prediction, that the horizon would act like a large magnifying glass for
objects on it came out. One method I haven't been able to get very far with
is the idea of index of refraction at the horizon. "If light moves faster
than light inside the horizon, what would the index of refraction of the
horizon be?" I find the principle of least time more appropriate. If
infalling radiation reflects and stops on the horizon, it has taken the path
of least time. Tau ticks less than once for the rest of infinity by measure
of the rest of the universe there.
--
Randy M. Dumse
Bilge
>transmission to usenet:
>news:slrn9t9v9...@radioactivex.lebesque-al.net...
>> But, there HAS to be a reactance of some sort (the exception being if
>> wL = 1/wC in which case, you have a tuner and Z=0 only at 1 frequency).
>
>The "non-frequency dependent" nature of cloth and horizon makes me think
>there is no reactance at issue here. Let me speculate on this idea a bit.
>
>Going back to the space cloth, there is loss at all frequencies. Only when
try wrapping a radio with your "cloth" and see how many stations go away.
I'll guess, zero.
>you use a Salisbury Screen with a reflector (separated by 1/4 wave length)
>does frequency come in to the equation. The original incident wave becomes
>the resultant reflected wave and the transmitted wave at the cloth. But
>since this is a resistive material, there is also an absorbed component. So,
>back to the title of this thread, we have three components absorption,
>reflection and transmission.
like V+iW.
>
>Going forward to the black horizon, there is a loss at all frequencies as
with wavelengths longer than than their scharzchild radius.
[...] >
>I've been thinking of reflection from black holes for years. The thought
Then you should have run across terms like "perfect blackbody".
>first came to me when I was thinking of quantum foam. I was thinking how
>smooth the surface of a black hole is down to the planckain level. It
Look up "bekenstein number"
[...]
>
>Now, moving on, I am back at the original motivation for considering the
>space cloth. If a space cloth were perfect, would there be any transmitted
>wave? Or would all incident EM energy be absorbed? This is what most
>interests/puzzles me. For if a space cloth were a perfect absorber, then not
It's not. Perfect absorbers are also perfect radiators.
[...]
>Then this is why you mentioned the 5 MeV ray? If the space cloth was made of
>atoms and the like, it would have some capacitance, inductance and
No. The reason I mentioned a 5 MeV gamma ray is that a 5 MeV gamma
ray won't even notice the cloth is there unless it's inches thick and
won't stop one unless it's probably several meters thick.
>resistance, meaning when we got to the atomic levels, the quantum structure
>of the material would show up and it would not appear as a continuous
>material with the 377 ohms per sq. This is interesting, because a black
The "quantum structure" will show up long before you get near
1 keV let alone an MeV. As in eV's.
[...]
>I agree. I've tried to seek this reflection/absorption phenomena in a
>number of ways. It was thinking of it in this way that the previous physical
If you want to study thin films, use old-fashioned E&M. If you want
to study black holes, use general relativity.
Jim Carr
... sci.physics added with followups there ...
In article <WQBA7.88313$aW5.1...@dfw-read.news.verio.net>
"Randy M. Dumse" <r...@newmicros.com> writes:
>
>Woke up this morning realizing, since reactance is measured in ohms,
>reactance is the AC version of resistance.
Not quite, since there is also resistance in AC. Reactance is one
of two ways to "impede" the flow of an AC current, the one that
relies on how a circuit element reacts to an attempt to change
the instantaneous value of the current.
>Impedance equals resistance + reactance.
Only if you use complex numbers.
>But I am confused about how resistance and reactance relate.
The same way that 1 and i=sqrt{-1} relate.
--
James Carr <j...@scri.fsu.edu> http://www.scri.fsu.edu/~jac/
SirCam Warning: read http://www.cert.org/advisories/CA-2001-22.html
e-mail info: new...@fbi.gov pyr...@ftc.gov enfor...@sec.gov
Old Man
I believe that this wavelength cut-off is a result of Optics (or EM + GTR)
and not a result of GTR alone. According to the (exact) Schwarzschild
solution for null geodesics (photons) with c = G =1, energy E, orbital
angular momentum L, black hole mass m,
(1/2) E^2 = (1/2) (dr / dt)^2 + Veff
where the effective potential is given by
Veff = (L^2 / 2r^2) (1 - (2m / r))
This yields a potential maximum at r = 3m wherein unstable circular photon
orbits occur. Photons are absorbed for L < E*m*sqrt(27) and are deflected
and escape for L > E*m*sqrt(27). This absorption cut-off appears to be
independent of initial photon energy and is dependent only upon the initial
photon impact parameter. However, from EM we know that, for a plane EM
wave incident upon a spherical absorber, the interference pattern depends
upon wavelength and absorber radius.
[Old Man]