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The purpose of the shape and dimensions of any antenna is to prevent power

from being radiated in directions that aren't useful to the communications, and

concentrate the power in the direction toward the intended receiver.

In order to accomplish that, all kinds of antennas are built out of an incredible

array of pieces of wire or metal. which are all in the general ballpark of the size

of a half wavelength of the power to be radiated. Nothing much shorter than that

has much effect on how the power is focused, and on where it goes.

Commercial AM radio broadcast operates with wavelengths between roughly 175 to

550 meters (575 to 1,800 feet). When you look at an AM broadcast tower, you don't

see any antenna mounted on it. It just looks like a bare, naked tower, and you wonder

where the antenna is. The answer: The tower itself is the AM antenna, because the

wavelength is hundreds of feet, and the antenna must have similar physical dimensions.

When you go higher in frequency (shorter in wavelength), practical antennas can be

much smaller ... like the skinny rods or tubes on a TV antenna.

"Dishes" are wonderful antennas. They can focus all of the transmitted power into

a beam that's only a few degrees wide! The larger the dish is, the smaller the

beam. But what's a 'large' dish and what's a small one ? In order to be effective,

the dish must be large compared to the wavelength. If the dish is less than maybe

10 wavelengths across, then forget it. It's no help.

Dishes are great for intercity microwave in, say, the 6 GHz band, where the wavelength

is maybe 5 centimeters ... around 2 inches ... and you use a dish with a diameter of

6-ft, 8-ft, or 10-ft. That gives us plenty of bang for the buck, and that's how we build

bullet-proof communication over 20 miles with 1/2 watt of microwave power !

There's no theoretical reason that you couldn't use a dish to transmit music and news

at, say, 850 on your AM dial. The wavelength there is about 1,158 feet, so a minimal

dish of 10 wavelengths' diameter would be 2.2 miles across. Expensive, perhaps, but

you could do it if you really felt like it, I suppose.

Next time you're Surfing the web, search "Arecibo", and have a look at the photos of

the National Astronomy and Ionosphere Center, the observatory for radio astronomy

located near that town in Puerto Rico. The dish for the radio telescope there is the

largest dish antenna in the world. It's built into a natural valley, and it's 305 meters

(1,000 feet, 0.1895 mile) in diameter.

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12y ago

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