In reply to Harry Veeder's message of Fri, 08 Jun 2007 15:00:21 -0500: Hi, [snip] >I can't explain it with em theory, but it behaves like a simple pendulum. >Ignoring friction, once the pendulum is set in motion it will keep swinging >with the same amplitude until the pendulum is used to power a clock or some >other device.
Precisely, so if no power is drawn, then none is transmitted (theoretically). The trick is that the inductance of the transmitting coil remains high until a resonant load is attached. Since most things in the environment are out of resonance the impedance stays high, and the transmitter itself appears as a high impendence to its own power source. Essentially it's a transformer primary winding with an open secondary winding. BTW this implies that losses can be reduced even further by increasing the Q factor of both transmitter and receiver. The effect of which is to narrow the bandwidth, ensuring that even less "spurious receivers" are to be found in the environment, and consequently less loss. Of course the flip side is that it's harder to match the resonant frequency of the receiver to that of the transmitter. > >Harry > >On 8/6/2007 11:27 AM, Michel Jullian wrote: > >> Maybe it would be possible for the emitter/primary to know there is a >> receiver/secondary around drawing power from it, if none it could turn off, >> and turn on for a brief time every few seconds to check of it's needed. Maybe >> it could even modulate its output power to fit the needs? >> >> 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? They say energy is not radiated away if it's not used by a >> receiver, >> I can't really see why. I suspect that the receiver is within a wavelength of the transmitter, so that this is a near field effect, which would imply that greater distances could be achieved by using lower frequencies, though I suspect that one of the corollaries of Murphy's law says that as the frequency drops, so does the energy transfer efficiency. ;) Regards, Robin van Spaandonk The shrub is a plant.
