Neat find, Desmostylus. The only problem is, it’s not exactly portable, what with the deep earth ground requirement.
Tell it to someone who cares.
Well, send me $15 and I’ll tell you what I really think.
Besides opting for low frequencies (like AM band,) there’s another technique. The guy in Chicago with the RF-run motors used it:
The Effective Area of an antenna is independent of the width of the antenna wire… but it’s also independent of antenna length! The incoming radio waves “see” all antennas as fuzzy disks of half-wavelength diameter.
If so, then why does a long AM radio antenna intercept far more energy than a short one? It’s because the longer antenna couples better to the waves. The antenna absorbs RF energy because the incoming waves induce a current in the antenna wire which creates a magnetic field. The waves also induce a voltage between the ends of the antenna wire which creates an e-field. These fields become strong if the antenna is at a resonant length (such as half-wavelength long.) These fields superpose on the incoming waves to produce an interference pattern, and some of the RF energy cancels out and goes missing. The missing energy is exactly the energy absorbed by the antenna. This is how all antennas function. They absorb RF energy because the antennas’ own EM field cancels out some of the radio waves’ EM field.
Here’s the kicker. To absorb radio waves, you don’t necessarily need a half-wave antenna. All you really need is a pattern of alternating E and B fields in a particular phase relationship with the incoming radio waves. A half-wave antenna produces such fields via resonance, and hence acts as a good absorber. But a small coil or a pair of capacitor plates can also produce such fields if it is driven artificially. Like “antisound” systems which absorb sound waves through cancellation, we can build an “anti-EM” system which absorbs RF energy through cancellation.
The upshot: if you attach a high-Q resonator to a fairly small antenna, the small antenna will produce the strong AC fields of a long antenna, and the small antenna absorbs incoming energy in the way that a long antenna does. In theory we can force a tiny antenna to behave as if it’s between 1/4 and 1/2 wavelength in size. A tiny coil can absorb RF energy from hundreds of meters around itself.
Semi-ridiculous implications: if we have a superconducting, infinite-Q circuit available, we can hook it to a superconducting loop antenna, tune it to the same frequency as a nearby AM radio tower, and the tiny antenna will absorb RF energy as if it were the size of that AM radio tower. The fields surrounding the small antenna simply grow larger until they’re similar in strength and extent to the fields around the transmission tower.
Or we could tune the infinite-Q superconducting resonator to 60Hz, then produce immense EM fields which absorb an enormous amount of power from the continental power grid. The technique is known and it’s illegal. It’s called “resonant power theft.”
Less-ridiculous implications: putting a high-Q resonator on your small loop antenna (or short whip antenna) will allow it to absorb far more energy than it otherwise would. Ham radio hobbyists use this effect on their mobile rigs. You don’t need a quarter-wave tower to communicate on the 160 meter band, you just need a whip antenna with a carefully tuned high-Q resonator connected in series. Because of the resonant circuit, the voltage/current of that antenna (and its EM field strengths) will be vastly stronger than those of a simple whip antenna. With a resonator added, both your transmitter and your receiver will “think” that the antenna is far larger than its physical length.
And finally… the above is why an atom of 0.1 nanometer diameter can efficiently absorb light waves of 500 nanometer wavelength. Atoms are “small” antennas which are thousands of times shorter than the waves which they emit and absorb, yet they behave as efficient antennas because they essentially possess sharply tuned internal resonators. At their resonant frequency, atoms “seem” to be a half wavelength in diameter. Papers I found about this effect: Short resonant antenna: Article abstracts
Although this device uses very little power, it’s antenna has a pretty large effect on the rf energy in it’s effective area, doesn’t it? So will it “suck down” the available energy for other users? Will a listener need to crank up the volume to get the same level as before Trigonal Planar’s leds were turned on?
Also, will the extra load be detectable at the source (transmitter)?
The resonant antenna? Yes and no. It should cast an “RF shadow” far downrange. But for devices which are very close to this antenna, they can couple to the antenna’s local RF field and steal power. Here’s a product based on the “resonant antenna enlargement effect.” These things draw in energy and surround themselves with an intense RF field. Then any AM radio placed near the device will have improved reception. They’re entirely passive, they’re not really “amplifiers.” Think of them as giant atoms which draw in longwave “light.” Of course you have to tune them to the frequency you want to receive.
Some AM radios can demonstrate this effect. Turn on two radios and tune them both to a very weak station. Now turn one of them off, then slightly change the tuning of the unpowered radio. The sound level of the active radio will change! Rotate the unpowered radio to maximize the effect. This might not work with tiny pocket-size AM radios, but if you use an old tube-type set as the unpowered radio, its large internal loop antenna should produce an effect which small nearby radios can sense. No doubt this is what inspired the “Selectatenna” type of products.
Not if the LED device is outside of the transmitter’s “nearfield” zone (if it’s more than a wavelength or so away.) Once the waves have departed from the transmitter, they have no further effects unless the receiver should somehow reflect some significant waves back to the transmitter. But if the LED device was closer than a half-wavelength, then its energy drain would show up as an impedance change at the transmitter output.
A big “resonant theft” device is detectable. I heard a story long ago about the giant BBC transmitters in the UK which send shortwave programs world wide. Some distant monitoring stations detected a constant drop in signal strength. So the BBC sent out people to map the antenna’s pattern, and they found a large hole it it. They tracked it down to a farmer. He apparently discovered that one of his barbed wire fences produced sparks, so he wired an electric heater to it, and was grabbing large RF wattage to heat one of his outbuildings. Apparently the fence was the proper length to act as a good longwire antenna, and he must have been sitting in a major lobe of the transmitter pattern. The story says that since there was no law against what the farmer was doing, the BBC cut a deal with him to run electric power lines out to his shed, in exchange for him not making big holes in the BBC shortwave antenna pattern. An urban legend, but the physics sounds somewhat correct, so maybe it’s true. (Maybe the farmer was a ham, and knew enough to add a tunable matching network in order to get maximum shed-heating effect.)
Thanks, bbeaty I read a spy story a long time ago in which the good guys were able to locate a spy by rotating their mobile transmitter antenna till the signal peaked, and then they measured the peak to get distance. A reverse RDF, I guess.
It worked well in the story.
I think the Brits use this process to sense the presence of radios in private homes. (And then they can cross-check to see if the owners are paying their radio license fees.) Either that, or they look for emissions from the radios’ “IF” section. But the first method would still work even when the radio was turned off.
The British TV detector vans and such work by picking up the IF oscillator’s radiation.
The position is determined using triangulation with a loop antenna, but it’s the signal null in line with the loop that they look for, not the signal peak. That’s just because for most antennas, the off-axis null is much more sharply defined than the on-axis peak.