On Sat, 27 Oct 2007, John Berry wrote: > According to Mr. Beaty the reason energy sucking antennas work is not > because the field of the transmitter is actually sucked into the receiving > element, but because the field of the receiving element enhances the voltage > induced.
Probably my article wasn't clear enough. The field of the receiving element CANCELS OUT a large region of radio waves. To do this it has to be at the same frequency, and have the right phase and amplitude. The transmitter is sending out EM waves, and the small receiving antenna is punching a huge "shadow" in those waves. The missing energy is the absorbed energy. In order for a tiny antenna to create a huge downstream shadow, that receiving antenna must send out fairly intense waves of its own. Or in other words, this is an interference pattern, where some incoming plane waves from the distant transmitter are combined with some sphere-waves coming from the receiving antenna. It's counter-intuitive, in part because the cancellation is mostly happening a quarter wavelength away from the receiver (which might be tens or hundreds of yards away.) Right at the receiver are some very intense EM fields. Or another way to understand the process is to look at diagrams of energy-flow of EM waves (called Poynting vector diagrams.) see the GIF diagrams near the bottom of: http://amasci.com/tesla/nearfld1.html The synchronized fields of the reciever cause the plane-waves from the transmitter to deflect inwards as if they've passed through a lens. > I have a problem with this however, it doesn't IMO make any sense. The fact > that the receiving element might now be created a sizable net field does not > impact on the effect of the transmitters field on the particles (electrons) > in the receiver since as I believe is generally accepted electric and > magnetic fields superimpose and it is not the effect of field against field Superposition can give a net energy loss (meaning, an energy absorption.) Now if you just add two plane-wave patterns together, the superposition does cancel out waves in one place, but it makes them twice as strong in another place. It only moves EM energy around. If you instead superpose a spherical wave with a plane wave, or superpose two spherical waves, then, depending on the phase relationship you can create a net loss of energy. So, if you put two radio transmitters near each other, and adjust them for 180deg relative phase, the waves in the distance all cancel out and vanish, and instead you've created something like a transformer, or a pair of capacitor plates. The two "transmitters" can then exchange EM energy without broadcasting anything. > but rather of (transmitter) field against matter. (whether or not the > antenna is creating a net field should not matter) Wire antennas are far too narrow to interact with longwave EM. We might expect that a piece of wire could absorb light waves, or maybe microwaves of millimeter wavelength. But how could it absorb AM broacast band 600 meters long? Simple: the antenna resonantes with the incoming waves, broadcasts it's own EM field, and the receiver's EM field cancels out the incoming waves from the distant transmitter. Also note that this isn't a personal crackpot theory of mine. I figured it all out while trying to explain to children how receiving antennas work, but then RF engineers tell me "what's the big deal, that's just how receiving antennas actually work, don't people already know that?" And then while looking carefully, I do find homework problems in physics books which deal with similar concepts. However, I've never found a physics or engineering book which teaches these concepts directly. Instead they just teach the math, and the "antenna aperture" concept. > Also if the mere presence of a field means that the receiver is more > 'sensitized' due to it's larger field then it need not be at any frequency. Unfortunately the absorption process requires that the receiver emit a signal that's just the right frequency, the right phase, and even the right amplitude. Resonant antennas (halfwave, or loopstick) do this naturally. (((((((((((((((((( ( ( ( ( (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
