Kind of obvious but... Harry, a DC current in a coil will not emit and radio waves, I think you made a mistake.
The following is reasonably accurate however. A flat DC current creates no radiowaves at all regardless of conductor shape. An AC current in a straight wire will emit radio waves. An AC current in a coil will emit negligible radiowaves with little powrer Pulsed DC is as good as AC for creating radiowaves. On 6/10/07, Harry Veeder <[EMAIL PROTECTED]> wrote:
A DC current in a straight wire won't emit radio waves. A DC current in a coiled wire will emit radio waves, but with little power. Harry On 9/6/2007 6:14 AM, Michel Jullian wrote: >> Essentially it's a transformer primary >> winding with an open secondary winding. > > Indeed a primary with an open secondary behaves like a pure inductor, so it's > a purely reactive load, so current in it can be made to oscillate non > dissipatively (assuming resistance of the coil is negligible). In terms of > transformer it makes perfect sense. But in terms of antenna, how could the > open air coil antenna help emitting radio waves (which requires power) towards > infinity? > > Michel > ----- Original Message ----- > From: "Robin van Spaandonk" <[EMAIL PROTECTED]> > To: <[email protected]> > Sent: Saturday, June 09, 2007 4:53 AM > Subject: Re: [Vo]:Tesla Revisted > > > 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. >
