On Fri, 8 Jun 2007, Michel Jullian wrote: > On the "how it works" side, has anybody understood the difference > between this MHz "resonant magnetic coupling" device and a radio emitter > with a tuned receiver?
Any EM antenna behaves as a hole in an opaque plate. A coil 10cm in diameter behaves as a "hole" which is 500 times smaller than the wavelength of radiation trying to pass through the hole. Such a coil makes a terrible antenna. If the coils are far smaller than one wavelength of 6MHz, then they behave as an air-core transformer. If the coils are around one wavelength diameter (like a 1/4-wave antenna,) then they behave as loop antennas for a transmitter-receiver pair. In this case the many-cm coil does not behave as an antenna. It behaves as a transformer primary, and any wireless device must contain the transformer secondary. Try this: connect a coil to the input of a portable battery-powered audio amp, turn it on, then walk around your house listening for 60Hz hum. You'll discover many regions of high AC field surrounding clock motors, fluorescent ballasts, lamp cords, etc. But this is not EM radiation. In order to form a quarter-wave antenna at 60Hz, a transformer coil would have to be 1250 kilometers across. In general, small coils don't emit significant EM waves. The question then becomes: what does "small" mean, and which emission is "significant." In EM theory, a radio antennas is much like a hole in an opaque plate. If this hole is far smaller than one wavelength, then very little radiation can pass through. And if a coil or capacitor is far smaller than one wavelength of the operating frequency, then very little EM radiation will escape from the device, and we don't call it by the name "antenna." At 6MHz, one wavelength is 3e8/6e6 = 50 meters. A quarter-wave antenna would be 12.5 meters across. A 10cm coil is too small to behave as an antenna. If it was an aperture in an opaque plate, it would be 500 times smaller than one wavelength. > They say energy is not radiated away if it's not > used by a receiver, I can't really see why. When AC coils are in operation, first the magnetic field expands into the space surrounding the coil. Then the field collapses again, and the energy is returned to the circuit before the waveform reverses polarity and the process repeats again. AC coils sequentially emit magnetic energy and then suck it back in again. (((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website billb at amasci com http://amasci.com EE/programmer/sci-exhibits amateur science, hobby projects, sci fair Seattle, WA 425-222-5066 unusual phenomena, tesla coils, weird sci
