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Ask Dr. SETI ®

Chapter 6: Technology

How Do Parabolic Dish Antennas Work?

Dear Dr. SETI:
Can you tell me how parabolic dish antennas work?

ES (California)

The Doctor Responds:
They work pretty well, actually.

OK, I'll get serious. First, imagine a naked flashlight bulb, burning in space, sending out its light into the black void. Its beam pattern is isotropic; omnidirectional. It radiates equally poorly in all directions. You can see its illumination from across a darkened room, but you can't read by it. It throws too little light upon the page.

But a practical flashlight contains more than a naked bulb. It also has a reflector behind the bulb: a parabolic mirror to direct the beam of light. Behind the mirror, it is dark. Off to the sides, it is dark. But all the bulb's radiation has to go somewhere. Where it goes is out, in the direction that the mirror focuses it. Instead of a dim glow in all directions, the flashlight gives us a bright spot, aimed just one way. You can read by that beam, because it concentrates otherwise wasted energy in a desired direction. That's how parabolic mirrors focus energy.

We're talking here about transmitting, but mirrors work equally well in both directions, so you can also use them to receive. Imagine your eye trying to collect dim light from a distant fire. You can just barely perceive the flicker. Now, face away from the fire. In front of you, place a large parabolic mirror, facing back toward you, and toward the dim glow of the fire behind you. Looking into the mirror, you see bright light. That's because the large mirror intercepts far more photons (massless particles of pure light energy) than your small eye can. And it focuses all those photons to a single spot. If you put your eye at that spot (the mirror's focal point), you can gather up all those photons, and see not a flicker but a blaze. This is exactly how Newtonian optical telescopes work.

So much for flashlights and optical telescopes. Now, how about radio telescopes? Same thing, except they "look" not at visible photons, but at radio photons, particles of invisible light perhaps 100,000 times bigger than the ones your eyes can see. So, to work as well with radio waves as your flashlight does with light waves, the parabolic mirror has to be that much bigger than the one in your flashlight. If it is large enough, a radio parabolic antenna will turn a flicker into a flame, one bright enough to be seen by your radio eye (a microwave receiver), clear across the cosmos.

Without a parabolic reflector, the best microwave receiver can't see very far, because it gathers few photons, and any distant signal burns too dimly. But put your microwave receiver at the focal point of a large parabolic mirror, and suddenly the photons you gather are magnified a million-fold, and a weak echo becomes a roar.

You do the same thing with sound. Across a crowded room, two people are conversing. You'd like to listen in, but their sound waves are not strong enough to vibrate your eardrum, or to stimulate your hammer, anvil, and stirrup. So, you cup your hands behind your ear, in a parabolic shape. You've just built a bigger antenna, and (with luck and a bit of concentration), you can now eavesdrop on unsuspecting strangers.

That's exactly what radio astronomers do, only it is not sound, but rather invisible light, by which we eavesdrop on unsuspecting stars.

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