More Musings On 'Energy-sucking' Radio Antennas
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
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
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
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
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
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
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
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
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! It cannot act like a quarter-wave dipole! Right, but the *fields* of electrons perform the interaction, and the physical diameter of the particle is not very important for the photo-absorption process. The electron can be infinitely small as long as its fields occupy a significant region. It still casts a large EM shadow. Incoming EM waves "collide" with the fields of the electron rather than hitting the electron's surface 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
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 doesn't remain neutral, but instead 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,
The fields of the nearfield region *are* the antenna, and the
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 "absorptive 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
"absorptive surface" which couples it to the incoming EM waves.
Wires are thin, after all. How can they cast a large shadow.
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. Like solar cells, but for VLF radiation.
Eventually they found that thin wires work
equally well as wide sheets. 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
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
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
the "leading" face of the dipole e-field is oriented so as to strengthen
own field, while the "trailing" edge of the antenna's field weakens it.
the EM waves inwards.
As the EM wave moves along, the antenna's field cycles past its maximum
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
in phase with the b-field of the incoming wave.) The "leading" part of
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
alternately draw their energy first from the e-field of the incoming wave,
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
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
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 absorption. 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
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
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
Not so great for motors, but you could drive some headphones. Like
crystal radios do! What if we lower the frequency 10 times, to 50KHz?
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
We 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
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
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
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
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.