You will see that to produce the piggy in a certain direction, kids use the incoming rays of the sun only as an electron exciter: an exciter that will give the electrons in the mirror the phasing necessary to make the piggy go into a certain direction. If they could somehow excite the electrons with the proper phases, they would not need the incoming light; they could make the piggy dash around in the dark.
But they can't, and neither can adults, at least not with light. However, they can scale things up and use light with a roughly 100,000 times longer wavelength, which is much the same thing as light, except that it does not excite (i.e. wiggle electrons in) the cones of our retinas and that it goes by the name of radio waves.
Now let us go back to the figures and let the circles stand for neither bottles nor electrons, but little T-shaped radio antennas ("dipoles" in radio jargon). If we plastered them all over a rectangle without connecting them to anything, they would indeed act as a mirror for incoming radio waves, at least for those with the right wavelength. They would reradiate the incoming wave just like the electrons of a mirror.
But who needs an incoming wave any more? This time we can feed the output of a radio transmitter into them, and it is a very easy matter to give each antenna a prescribed phase. The result is an antenna array that produces a radio beam. For reasons of secondary importance, it is easiest to produce a beam in the direction perpendicular to the rectangular array, just as if it had been excited by an incoming wave in that same direction.
Most readers will have seen this type of array near airports or on photographs of WWII radars. Sometimes the whole array is mounted on a rotating base, so that the radar beam revolves much like the light beam on the roof of a police car as it (the array) scans the sky for its targets.
But the beam produced by such an array is not as sharp (parallel, undiffused) as the light beam producing the piggy.
Why not? Because the energy of any electromagnetic beam
¾light or radio¾is squashed the closer round the "piggy angle" the larger the array or mirror producing it; but "large" means measured in wavelengths, not absolute lengths. A 4-inch hand mirror is about a million wavelengths long; the 30-foot radar antenna at the airport only a paltry 10 or 20.It does not give much of a piggy.
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Vol. 14, No. 7
Newsletter: Access to Energy Newsletter Archive Volume: Issues Issue/No.: Vol. 14, No. 7 Date: November 30, 2004 08:46 AM Title: Commissar Cuomo
Copyright © 2004 - Access to Energy Newsletter Archive
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