More Musings On 'Energy-sucking' Radio Antennas

Below are some various collected thoughts regarding the idea that a tiny antenna can draw large EM waves into itself.



Here'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.


In 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.


How 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 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 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, wave-emitting 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 "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 of phase?
Ah, if we actually plot the E and M fields we'll discover that IT'S BETTER if the two fields are 90deg out of phase! (The E and M fields of the incoming EM wave are in phase, of course. Only the antenna's fields are in quadrature 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.


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 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 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?


Here'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.

First the resonator absorbs energy, then it dumps it again.

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 Energy

Another 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 mW

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? 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 mW
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 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.


Created and maintained by Bill Beaty. Mail me at: .