Re: teeny atoms absorb huge EM waves

[William Beaty <billb@ESKIMO.COM>]

On Sun, 1 Aug 1999, John Denker wrote:

> At 02:15 PM 7/29/99 -0700, William Beaty wrote:
> >  Modern radio
> > receivers would not employ this effect, since their antennas are decoupled
> > from any resonant circuits by the input amplifier stage.  (We want our
> > antennas to be relatively broad-band, not sharply tuned.)
>
> Radio receivers wouldn't benefit.  They care about signal-to-noise ratio,
> not absolute signal energy.  (To say it another way: nowadays the noise
> temperature of the RF preamp is really, really low.)  A tuned antenna would
> resonate with noise just as well as signal.  Receivers can cut down the
> noise bandwidth electronically just as well as they could with a resonator.

Yep, the original article was on VLF/ELF research, where the signals are
low, the bandwidth small, and receivers must use long integration times in
order to get the received energy up above the noise energy of the input
stage.  Increasing the received energy would be useful in this situation.
Whenever it's inconvenient to add front-end amplifiers to an RF receiver,
and where the antenna is much smaller than one wavelength, we could
increase the "effective size"  of the antenna by adding a resonant
circuit.  I've been told that common AM radios use antenna-tuning.  This
clears up a question I've always had about AM radios: how can they get
away with such a small antenna?  Do they simply have immense front-end
gains?  Maybe not.  If their ferrite loop antenna is tuned to the received
frequency, then it will create its own EM field, and take advantage of the
same "energy sucking" effect that atoms use to grab light waves.  A
portable AM radio is like a "giant atom".


> > How would I perform calculations on this system to show that extra energy
> > would flow into an oscillating antenna?  Use a numerical simulation of the
> > near-field space around a short dipole antenna?  (Gah!)
>
> Read up on
>   * Optical theorem
>   * Born approximation
>   * Hugyhens' construction.  I saw a manuscript that David A. B. Miller
> wrote a few years ago on this, showing that the usual hand-wavy version of
> H.C. could be made quite rigorous.  I don't know if/where that got
> published.  If you can't find it let me know and I'll bug DABM for it.

That reference sounds like it would be a good place to start.

Below is a very crude, 1-dimensional model of a real-world receiving
antenna with and without a resonator.

Suppose I transmit a VLF signal at 1KHZ with a transmitting antenna at 10
megavolts and 100km distance from the receiver.  If my receiving antenna
is a plate suspended 1m from a ground-plane, then we form a capacitive
voltage divider as shown below.  If the receiver antenna's capacitance is
10pF, and the capacitance between that antenna and the transmitter is
1/10,000 times smaller ( 1m / 100Km )  then the signal on the receiver is
100 volts, but with a very large impedance.  I'll put a load resistor on
the receiving antenna that matches the divider's series capacitance (so
that I can draw significant energy from it.)  The divider's capacitance is
dominated by the 10pF between antenna and ground.  This gives a
1/(2*PI*F*C) = 16 megohm load resistor, and it drags the received voltage
down from 100V to 70V.  The energy received by this antenna is 300
microwatts.


  __________  -->
 | 10 MVolt |_______
 | @ 1KHz   |       |
 |__________|       |
    |            ___|___     Capacitance from transmitter to receiver
   _|_                       ( very small, say 1/10,000 pF )
  ////           _______
                    |
                    |
     receiving      |______________    <--- 70.7V @ 1KHz
     antenna        |              |
   (metal plate) ___|___           \
                          10pF     /  16 Megohm
                 _______           \
                    |              /
                    |______________|
                   _|_
                  ////


Now lets add a tuned circuit:

  __________  -->
 | 10 MVolt |_______
 | @ 1KHz   |       |
 |__________|       |
    |            ___|___     Capacitance from transmitter to receiver
   _|_                       ( very small, say 1/10,000 pF )
  ////           _______
                    |
                    |
                    |_____________
                    |            |
    antenna         |             \_
   (metal plate) ___|___          (_)
                          10pF    (_) Coil
                 _______          (_)
                    |             (_)
                    |             /
                    |____________|
                    |
                   _|_   1KHz resonance
                  ////


At resonance the 10pF capacitance vanishes, since an ideal parallel-
resonant circuit looks like an infinite resistor.  The capacitive
voltage-divider is no longer there.  How high will the antenna's voltage
rise?  To ten megavolts!  (But only if we stay with this crude 1-D model.)
If the coil's resistance is very small (Q is incredibly high) then the
voltage on the tuned circuit will rise until it reaches the same voltage
relative to ground as the distant transmitter.

However, voltage is not power, and it might take months to build up that
much voltage across an ideal resonator.  Let's put a resistor across the
tuned circuit so we create a flow of real energy and drag the voltage down
to .707 of the unloaded voltage.  The resistance should equal the
impedance of the series capacitance ( 10 ^ -16 uF) or 1600 giga-ohms.
Power intercepted by the previous receiver was 300 microwatts.  In this
receiver it has risen to 30 watts, or ten thousand times higher than the
earlier circuit which lacked a resonator.

Now for my dirty secret.  The original paper was:

  J. Sutton and C. Spaniol, "An Active Antenna for ELF Magnetic Fields",
  PROCEEDINGS OF THE 1990 INTERNATIONAL TESLA SYMPOSIUM.

It was inspired by N. Tesla's scheme for transmitting significant
electrical energy without wires.  Throughout his writings, Tesla harps on
the fact that his small resonant receivers "draw energy" from incoming EM
waves.  Now I'm finally starting to see what Tesla was talking about.  The
"absorbtion radius" of antennas which are far smaller than a wavelength
can be greatly increased by connecting them to a high-Q resonant
circuit.

Tesla's wireless-power idea sounds crazy, yet apparently it employs the
same physics whereby tiny atoms can absorb/radiate EM waves of wavelengths
thousands of times larger than the diameter of the atom.

Here's another way to look at it.  If a ground-referenced antenna wire
intercepts a particular displacement-current from an EM wave, it will
develop a particular voltage relative to ground, and if V and I are in
phase (resistor load), then the total absorbed power is V*I.  If we were
to artificially impress a large AC voltage on the antenna with the same
phase as before, and if the same displacement-current is still
intercepted, then V*I is greatly increased because V is greatly increased.
A resonator stores the received energy and uses it to create a huge AC
voltage on the antenna wire, and therefor to "funnel" or "suck" energy out
of the EM wave.

Here's another reference, and a portion of the intro paragraph:

  H. Paul and R. Fischer  "Light Absorbtion by a dipole", SOV. PHYS. USP.,
  26(10)  Oct. 1983  pp 923-926

  In the so-called semiclassical radiation theory the atoms are described
  quantum mechanically, while the radiation field is considered as a
  classical quantity.  Such a treatment appears to be justified in case of
  strong fields, as they are, in particular, generated by lasers.  (In
  fact, this procedure has proved to be very successful in Lamb's famous
  gas laser theory(1).)  Specifically, in the process of light absorption
  by a two-level atom in the physical picture provided by the
  semiclassical theory is as follows: The field induces and oscillating
  electric dipole moment, in the sense of a quantum mechanical expectation
  value, on the atom, and the total energy flow into the atom is given by
  the work done by the field on that dipole.  Note that in this model
  absorption appears as a continuous process.  This description of
  light absortion is in close correspondence to classical electrodynamics,
  the main difference, however, being that the amplituide of the induced
  dipole moment, contrary to that of a harmonic oscillator, can grow, with
  time, up to a maximum value only (given by the transistion matrix
  element for the electric dipole operator), irrespective of how intense
  the incident field might be.  Clearly, this feature reflects the
  saturation effect present in a two-level system.

  When calculating the energy flow into the atom, along the lines
  mentioned, one arrives at the result that its maximum value
  (corresponding to the maximum value of the induced dipole moment) is
  larger, by orders of magnitude, than the energy flow in the
  (undisturbed)incident field throught the geometric atomic cross section.
  (A typical example is presented in Sec. III.)  From this, one must
  conclude that an atom has the ability to "suck up" energy from a spatial
  region that is by far larger than its own volume.  One might put the
  question as to the underlying specific physical mechanism.  Acutally, an
  answer is readily given in the framework of classical electrodynamics.
  An oscillating dipole generates a wave, in any case, the difference
  between absorbtion and emission, as a net result, being brought about
  only by the different phase relations between the incident and the
  emitted wave.  Specifically, in the absorptive case this phase relation
  gives rise to the effect that the lines of energy flow in the total
  field are "bent" in a rather large neighborhood of the atom such as to
  direct the energy flow into the atom. It is the aim of the present
  paper to give a detailed picture, based on a numerical study, of this
  bending phenomenon which has been discussed qualitatively already by
  Fleming(2).


So, apparently black holes aren't the only thing in physics that "suck"!

:)


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