Thanks Bill and Michael and Robin for your explanations regarding the ratio of the emitting coil size wrt the wavelength making the device an inefficient radio emitter.
I found the following documents on the web with the help of Google Scholar and its "web search" feature: - the arxiv preprint of their 2006 theoretical paper "Efficient wireless non-radiative mid-range energy transfer": http://arxiv.org/ftp/physics/papers/0611/0611063.pdf - a WIPO patent (WO/2007/008646) WIRELESS NON-RADIATIVE ENERGY TRANSFER http://www.wipo.int/patentscopedb/en/wads2.jsp?IA=US2006026480&ID=id00000004722606&VOL=67&DOC=000582&WEEK=03/2007&WO=07/008646&TYPE=A2&PAGE=0&DOC_TYPE=PCT (where Soljacic is curiously spelled "Solajacic") ...which you EM wizards might be kind enough to comment for the rest of us. BTW in my quick glance at the patent I saw a mention of RFID, isn't this indeed how RFID stickers are powered? Michel ----- Original Message ----- From: "William Beaty" <[EMAIL PROTECTED]> To: <vortex-l@eskimo.com> Sent: Saturday, June 09, 2007 11:09 PM Subject: Re: [Vo]:Tesla Revisted > 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 >
