More Musings On Energy-sucking Radio AntennasBelow are some various collected thoughts regarding the idea that a tiny antenna can draw large EM waves into itself.
SHORTCUTS
MECHANICAL ANTENNASHere's a more intuitive way to picture the "energy sucking" effect.Suppose I have a bar magnet mounted on an axle so I can flip it over endwise.
If I spin it, it flips end-over-end and produces a large oscillating b-field in
the environment. If I place a coil near it, then power the coil with AC, I can
force the magnet to rotate. Obviously I've just built a synchronous motor. (Yes,
I'll probably need to spin the magnet by hand to get it to "lock" onto the AC
fields from the coil.) In this synchronous motor, if the bearing friction is low, after the rotating
magnet locks itself onto the coil's AC magnetic field, no energy is drawn from
the coil. The magnet will synch up with the coil so as to draw zero energy. But
phase of the magnet and the AC fields are important. If I now give my spinning bar magnet a frictional load, the magnet phase will
begin to lag behind the AC field of the coil, and the magnet will start drawing
significant energy from the coil. The magnet is sucking energy out of the space
around itself, and the coil is depositing energy back into the space. (It turns
out that this phase lag in the magnetic field is the CAUSE of the energy drain.)
My synchronous motor is doing some work. Note the details of what's happening here. First the electromagnet coil
stores energy as a b-field in the surrounding region. Next, then the magnet
partially cancels that field. The magnet simultaneously feels a driving force.
It accelerates. The "cancelled" energy didn't vanish. Instead it ends up as
kinetic energy in the spinning magnet. The magnet is essentially drawing energy
out of the coil, and, if the spinning magnet was not there, the coil would lose
no energy. Now, what happens if I pull the coil to a distance from the rotating magnet?
The torque will become less, and the magnet will lose synchronization unless I
either reduce the frictional load, or MAKE THE BAR MAGNET STRONGER. Suppose I
make the magnet stronger. Now the magnet is still extracting energy from the
coil at the same rate as before, even though the distance between coil and
magnet has increased. What if I increase the distance more and more, yet make
the magnet VERY VERY strong? My synchronous motor still works fine. (Note: if we
suppose that the frictional load was small to begin with, then my magnet
wouldn't have to be THAT strong.) Instead of using a distant coil, what if I drive the magnet with a radio wave
from a distant transmitter? The spinning magnet will work as before. It will lag
behind the incoming fields, and it will continue to extract energy from the
surrounding fields and use it to heat its frictional load. If we plot the energy
flow lines (Poynting field), we'll see the radio waves in a large region being
focused onto the spinning magnet and diving into it. The magnet will still
remain locked to the "drive" fields, and it will lag behind them by 90 degrees
at most. The magnet is being spun by the radio waves. The absorbed energy ends
up as frictional Notice that the magnet can be MUCH smaller than the wavelength of the radio
waves. It's the field of the magnet that intercepts the energy, not the physical
magnet poles. Also note that the physical magnet itself is not directly
interacting with the incoming waves. Instead, the magnet's NEARFIELD B-FIELD
interacts with the radio waves, and this altered b-field then applies a force to
the magnet. The static field of the magnet absorbs energy from the radio waves,
then it delivers that energy to the magnet as a mechanical force exerted over a
distance. The nearfield b-field acts like an antenna! Since energy is absorbed
from the radio waves, the spinning magnet must be casting a large "EM shadow,"
and punching a big hole in the incoming wavetrain. The magnet might be tiny, but
its magnetic field can extend to a great distance. It's as if the rotating
magnet surrounds itself with a large black "absorber cloud" which blocks the
incoming EM waves. Obviously the magnet can only "reach out" within about 1/4
wavelength around itself. My synchronous motor has now become an
"energy-sucking" antenna. To make the picture complete, replace the spinning bar magnet with a tiny
coil and capacitor, and put a series resistor in the loop to act as a
"frictional" load. The radio transmitter could be very far away from the coil,
but if the alternating current in the resonator coil can build up and produce an
extremely strong magnetic field, this "motor" can still suck lots of energy from
the driving fields in the space around it. It's like a synchronous motor with no
moving parts. It's like a tiny boat which can erect a huge sail to catch the
wind.
PERMANENT MAGNET AS SUPERCONDUCTING ANTENNAIn order to cause a tiny antenna to intercept EM waves across a vast area, the "Q" of the antenna mus be very high. In other words, the resistance of the coil of wire must be extremely low. The natural resistivity of everyday metals severely limits how large the "virtual size" of the antenna might be. We need superconductors if you want to "grab" really huge amounts of energy. Or do we?A superconductor coil resembles a permanent magnet. The big difference is
that the current in the SC coil is available to external circuts, but the
"current" within the electron spins of a ferromagnet are not. However, there's a
trick we can play. If we SPIN a permanent magnet, or WIGGLE a permanent magnet,
it behaves like a superconducting coil for AC. It produces an intense AC
magnetic field, and if the phase of this field is correct, it can "suck energy"
from incoming EM waves. A powerful permanent magnet, if it is allowed to wiggle,
acts like a large loop antenna. To tap the received energy, simply place an
inductive pickup coil near the wiggling magnet. Obviously this will only work when the received frequency is fairly low. A
large rotating neodymium magnet can "grab energy" from 60Hz radiation, but not
from 10KHz radiation. Even so, there might be places where rotating magnets
could serve as miniature antennas. If you build a micro robot, how will you power it? With effing humongous chemical batteries? Maybe you could use solar cells (need large area), or transmit energy to an onboard inductive pickup coil (hard to wind such a thing.) A resonant pickup coil would be good, but the Q-factor needs to be high. What if you place lots of tiny magnets on lots of tiny fibers so the magnets resonant mechanically? At the resonant frequency, the array of magnets will act like a fairly large "virtual pickup coil." Wind a one-turn coil around your magnet array, and you've got a fairly high voltage AC power supply on board your robot. VARIOUS QUESTIONSHow can an electron in a conventional antenna absorb any energy from EM waves?Each electron in an antenna is far too small to interact with longwave EM fields! Right, but the *fields* of electrons perform the interaction, and the physical diameter of the particle is not very important. The electron can be infinitely small as long as its fields occupy a significant region. Incoming EM waves "collide" with the fields of the electron rather than hitting the electron itself. If the electron's fields are altered, they can drag the electron along. Antenna wires contain mobile electrons, but normally the fields of these
electrons are cancelled by the fields of the protons. To be able to interact
with EM waves, the electrons and protons must extend their fields outwards. To
do so, they must be relatively moving and/or separated from each other. In other
words, to intercept lots of EM energy, make sure your antenna creates a strong
field of its own. But this implies that, even for conventional antennas, the antenna is not
just a passive absorber. Instead it's an active, field-generating device. The
fields of the nearfield region *are* the antenna, and the electrons and
protons are not. The fields of the nearfield region *are* the antenna,
and the metal parts of the antenna are not. But if the wiggling electrons in an
antenna can generate a field in the nearfield region, and if this EM pattern can
behave as an "absorbtive surface" which in turn applies forces to the electrons
...then the fields of the nearfield region are "sucking energy" from the
surrounding space and delievering it to the thin antenna wire. Even though
conventional quarter-wave dipole is electrically long, it still needs the
"energy sucking effect" in order to present a large "absorbtive surface" which
couples it to the incoming EM waves. Researchers of 1900 were not too wrong when they laid out large copper sheets
to act as radio antennas. They wanted to provide a large-area absorber for the
incoming waves. Eventually they found that thin wires work equally well. Yet
thin wires lack the area, so how can they absorb much EM energy? Simple: it's
the wire's *fields* that act as the large-area wave absorbers. Once we
realize this, then the "energy sucking antennas" seem far less weird.
How can the magnetic field and the electrostatic field around a small
antenna absorb any EM energy, since these fields are 90 degrees out of
phase? This is really cool. As the small antenna operates, its dipole-shaped e-field
wants to sit at the crossover point of the e-field timing of the incoming wave.
That way it can best distort the incoming waves in order to suck them into the
antenna. When the antenna's e-field is in that position, the "leading" face of
the dipole e-field is oriented so as to strengthen the wave's own field, while
the "trailing" edge of the antenna's field weakens it. This bends the EM waves
inwards. As the EM wave moves along, the antenna's field cycles past its maximum
value, and when the e-field of the EM wave reaches maximum, the antenna's dipole
field is zero. The antenna's e-field lags behind the e-field of the incoming EM
wave by 90 degrees. On the other hand, the antenna's circular magnetic field works best if it
sits at the strongest part of the incoming wave. (The antenna's b-field is in
phase with the b-field of the incoming wave.) The "leading" part of the
antenna's circular b-field can strengthen the incoming b-field, while the
"trailing" part of the antenna's field can weaken it, which again bends the
energy-flow vectors inwards. By being 90deg out of phase, the fields generated by the antenna have the
ideal timing to absorb the incoming EM energy. I suppose this means that they
alternately draw their energy first from the e-field of the incoming wave, then
from its b-field. If an antenna is like a waterwheel, then this "waterwheel" has
a set of alternating buckets, one for "E", the next for "M", etc.
PHASED FIELDS THAT ABSORB... AND ALSO EMIT?When a simple coil is driven with alternating current of low frequency, the magnetic field around the coil grows and shrinks twice per cycle. If we could see the coil's flux lines, they would appear to balloon outwards into space as the current climbs towards maximum, and when the current cycled back to zero, they would seemingly be sucked back into the coil again, and deliver their energy back into it. Because the frequency is low (and the coil is small), the coil emits almost no EM radiation. All of the b-field energy that expands into the space around the coil is regained when the fields collapse again. The coil is NOT a radio transmitter.If the "energy sucking" effect is real, then this expanding/contracting field
is a key concept. OSCILLATING fields, but with NO RADIATION. The field around
the coil vibrates, but it cannot escape. It's the AC analog of the fields of a
bar magnet. Now along comes a freely-propagating EM wave. If the wave has the
same frequency as the AC in the coil, and the phase is right, then the coil
absorbs energy from the EM wave (which leaves an EM shadow behind it as the wave
continues on past.) The fields created by the coil have produced an asymmerical
effect on the fields of the EM waves. The trapped and vibrating fields have
absorbed the incoming radio waves! This is not just simple superposition.
Instead, the coil is screwing up the the b-field of the EM waves, which destroys
their ability to propagate (and so they are absorbed by the coil.) I find it fascinating that the coil's fields can disrupt the EM wave, even
though the coil itself cannot radiate. Very counterintuitive. Not like normal
superposition at all! The fields in the coil's nearfield zone behave almost like
a physical object, like a "black absorber cloud" which blocks EM waves. It's
like aiming a laser beam at another laser beam and finding that, rather than
passing through each other, the first beam swallows up the second one! (This
only works with nearfield fields though.) Right away I think: what happens at other phases besides -90 degrees? I plot
the superposed fields at a 0 degree phase lag, and also at 180 degrees. I find
that, at both these phase values, the coil's field DOES NOT interact with the
incoming EM anymore. Instead the coil's field simply expands and contracts as
usual, and the EM waves pass right by. OK, what about +90 degrees? Aha! The coil now seems to do the OPPOSITE of absorbtion. It EMITS energy
into the EM wave and amplifies it. It creates a "bright shadow." Without the incoming wave, the coil was just an inductor (no radiation.) With
the incoming wave present, suddenly the coil can transmit! Heyyyyy. If the coil
was a single atom, this would be an example of triggered fluorescence. It's
Stimulated Emission. Radio Amplification by Stimulated Emission of Radiation. A
low frequency LASER, but apparently without any Quantum Mechanics! (But then, QM
was always wave/particle in the first place, so we should not be suprised if
lasers can be viewed entirely as EM-wave beasties.) This is very weird, no? Without the incoming EM waves, the coil just sits
there oscillating, but not emitting anything. But when the EM wave arrives with
+90 phase, suddenly the coil is able to dump energy and emit genuine EM
radiation. Very screwy! Nothing at all like the radio physics I learned in
school. Weirdness lurks in the nearfield. Hmmmm. I wonder if, in a real laser, the pumped atoms are constantly
oscillating at their resonant frequency? Instead of having a static pumped-up
electron shell, do they normally have a trapped, non-radiating EM
field-oscillation? If so, then perhaps they only can "lase" when the phase of
the stimulating beam is at the proper setting. The light phase might usually be
wrong for triggering emission. However, if there is a slight phase-drift between
the oscillating atom and the stimulating beam, then eventually the phase will
line up correctly, and the atom will suddenly "lase." Maybe it emits a whole
long transient rather than a single "photon." If I illuminate a bunch of pumped
RLC resonators with an EM wave, will they emit a big pulse? Can I base a radio
transmitter on "Q-switching?" Oooooo! What if we get even smaller? Nucleii give
off EM waves when they fuse. If I illuminate a radioactive nucleus with the
right frequency, might I induce decays and affect the half-life of radioactive
materials? Would this even work with NON-radioactives?
PULSE-EATING COILSHere's a thought experiment. If we connect a coil to a capacitor, then illuminate them with EM waves at the resonant frequency, the "energy sucking" phenomenon should occur, but the AC current in the coil can only build up to a certain level. (It will be limited by coil resistance, or by radiation leakage when the fields grow extremely intense.) The resonant circuit should swallow a particular clump of EM energy, then stop absorbing. What happens if the incoming EM waves suddenly cease? If the resonant circuit can only absorb energy when it interacts with EM waves, then maybe the same is true for emission. Maybe the coil can only emit energy when there are external EM waves present. When the EM waves are switched off, the resonant circuit should not radiate, and it should keep oscillating (imagine that the coil is a superconductor.) We've managed to "fill" the coil by hitting it with a pulse of EM energy. When the waves cease, the energy remains trapped in the coil.The decay time of the coil SHOULD NOT match the rise time, since the "rising"
requires the presence of both an incoming EM wave and also the coil's own
oscillating nearfield magnetism. With the EM wave removed, the coil does not
radiate, so its oscillation does not decay. Now what would happen if we hit the
coil with a different pulse of EM waves: one where the phase is +90 degrees?
This will make the coil "fluoresce" and dump out its contents as an EM wave. I
think.
First we emit EM waves towards a distant resonator, then we jump the phase of the emitted waves by 180 degrees.See what we have here? Signal switching without any switches! If a resonant circuit is "empty", it will absorb energy and take on the phase of any wavetrain that hits it. If later pulses of EM waves are at 0 or 180 phase, the "full" resonator ignores them. And if a "full" coil is hit by a +90 wave, the coil will "lase." ( Maybe. This is only a thought experiment.) Suppose we set up a large array of RLC resonators and pump them full of energy with small oscillator circuits. Suppose all the coils are a couple of wavelengths apart so they won't interact. If a pulse of EM waves should hit this array of coils, they'll all dump their energy into the wave, and a much stronger pulse will come out the other side! This is somewhat like a phase-array antenna. However, the individual coils do nothing until an externally-applied EM wave goes past. It's more like a laser amplifier than like a conventional PA antenna.
Sucking REAL EnergyAnother thought experiment. Suppose we use a superconductor coil as our small antenna. With resistance removed, the current in the coil can rise so high that the field grows REALLY huge, and the antenna can draw energy in from 1/4 wavelength around itself. The "energy sucking" process makes the tiny coil act very large. How much wattage can we grab from a distant transmitter? If the transmitter puts out 10KW at 500KHz, it looks like this:10KW at 500KHz wavelength = 600 meters "energy sucking" virtual antenna area = 30,000 meters^2 distance to xmitter received power 1km 25 Watts 10km 250 mW 100km 2.5 mWNot so great for motors, but you could drive some headphones. Like crystal radios do! What if we lower the frequency 10 times, to 50KHz? The antenna's effective area goes up as the square of the nearfield radius, so received power goes up by a factor of 100. We can obtain the same results as with 500KHz, but our receivers can be 10x further out. 10KW at 50KHz wavelength = 6 kilometers "energy sucking" virtual antenna area = 3,000,000 meters^2 distance to xmitter received power 10km 25 Watts 100km 250 mW 1000km 2.5 mWWe can grab a quarter watt at 100KM distance from the transmitter. (Pretty impressive if the antenna is a little coil inside a desktop radio.) Lets look at something that's much more down to Earth. How about building a
tiny tabletop model? Our transmitter will be a flyback transformer running at
30KHZ, 30KV. The receiver will be an identical device. Give both transformers a
vertical antenna. How much energy can the receiver extract from the transmitter?
If the transmitter's antenna is 10pF to ground, then when charged it carries
1/2*C*V^2 Joules of energy, or 4.5mJ. The transmitter charges and discharges
this antenna 30K times per second, for a "sloshing" EM energy flow of 270 watts.
If the receiver could "suck" each 4.5mJ pulse out of the fields, it could
extract 270 watts at most (if the flyback transformer could handle the current!)
A better estimate comes from connecting the two antennas with a capacitance.
Suppose the capacitance between the antennas is 1pF. If the load resistance of
the receiver causes the resonant voltage on the receiver to rise to a value of
1.414 times less than the transmitter voltage, then we've got a simple voltage
divider. 30KV on the transmitter antenna, 21KV on the receiver. The receiver
gathers 1.7mA of high-freq current. (At such high voltages, the 1pF between the
antennas becomes a significant conductor.) The receiver ends up drawing 35
watts. Actually, if there was no load on the receiver, its voltage would rise
until it was near 30KV. Just wind a secondary on the core of the receiving
flyback and hook up a light bulb to draw the 35 watts out of the "sky". If Tesla
used a megawatt transmitter at 5KHz, he probably could light some bulbs from
100KM away. (Ideally, that gives 2500 watts received.) Suppose we transmit at
100Hz? The wavelength is 3000KM and our receiver is probably within the
nearfield region of the transmitter, so it can grab a significant portion of the
10KW. Hey, didn't Tesla believe that lower radio frequencies were better than
high ones? For resonant power transmission they are, since the nearfield zone of
a resonant receiving antenna is larger at low frequency, yet with no less power
from the transmitter, and no less power flowing past the antenna. A small
low-frequency resonator coil is "larger," so it intercepts more radiation. None of this takes the Schumann cavity into account. If our VLF radio waves
cannot escape from the atmosphere, then the inverse square law no longer
applies, and the EM waves near the receiver are much stronger. If the VLF waves
remain trapped within the atmospheric cavity, then an ideal energy-sucking
antenna could pull in the ENTIRE output from the transmitter. If you go out and invent low-noise amplifiers, this whole issue becomes
unimportant for radio receivers. If your antenna is too small, you can simply
amplify the signal. But if you want to run motors on wireless power, 1KHz radio
is far better than 1MHz. Speaking of this, what could we do with a really powerful superconducting coil at 60Hz? The wavelength is 5000KM, and the effective area of the antenna is 2e+12 square meters. Maybe that coil could "suck energy" from the entire 60Hz power grid. The device would act like a perpetual motion machine, and the clue to its operation would be found in the strong, vibrating magnetic field that surrounds it. This sounds like some famous "Free Energy devices: the Hubbard Coil and the Hendershot Device. What other "free energy" devices involve huge coils? Hey, maybe Joe Newman's energy machine is actually a "Tesla power receiver", and is accidentally tapping into the US power grid! He should try running it at 3600 RPM. Links: ENERGY-SUCKING ANTENNAS http://www.amasci.com/tesla./tescv2.html |