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Why does the size and shape of a transmitting dish aerial dictate the use of microwaves instead of long-wave radio signals?
Wiki User
∙ 11y ago
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.
Wiki User
∙ 11y ago
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Why are car radio antenna's vertical?
It's called polarization and it relates to the orientation of the transmitting antenna. If the transmitting antenna is vertical, the majority of the transmitted energy is vertically polarized so it stands to reason that to pick up the most signal the receiving antenna should be the same way. In a moving auto though more is now understood about the propagation path the signal takes before it actually gets to the moving radio. The signal isn't line of sight but takes a lot of reflective paths before it actually gets to the antenna. The signals are reflected so many times that polarization is altered and can be any direction by the time it gets to the car receiver. Radio stations get around this anomaly by circularly polarizing the signal at the transmitting antenna. Many car antennas are now embedded in the glass windows instead of wire whips formally mounted to the fenders.
Did the Australians play the theme to Hawaii 5 0 instead of the American National Anthem during the moon landing?
No. This was just a tongue-in-cheek inclusion in that great movie "The Dish", which told the story of how the radio telescope at Parkes played a vital role in transmitting the moon landing of 1969. There is no truth to it. Australians like to poke fun at themselves in movies.
What can you use instead of LimeWire?
Ares
Does beyonce have another sister instead of solange?
No
When dressing up like Sandy from Grease can you wear pink shoes instead of red?
Yes. Pink shoes instead of red would be fine.
Why are there microwaves instead of radio waves in a mobile device?
your mum :D
What people used instead of microwaves in 1800?
Wood stoves. (They burned wood or coal to get hot).
Why are microwaves efficient?
They create heat inside the object being cooked, instead of wasting it outside.
What did they used to use in the 1800's instead of microwaves?
NO! there wasnt much of elecrticity yet.
Why can't metal objects be inside a microwave oven when this is working?
They reflect the microwaves instead of absorbing them.
Why does the glass panel on a microwave contain a metal mesh?
It has a metal mesh to reflect the microwaves back into the microwave oven instead of out.
Why are microwave doors lined with a metal net?
The metal net reflects microwaves that hit the door back to the food instead of through the glass and into you.
What can you rub over a persons body where a tic is in the skin to make them get out?
If you remove the tick that way, you increase the risk of the tick transmitting disease. Use tweezers instead.
A microwave oven is 70 percent efficient if it is supplied with 4000 jouls of electrical energy how much heat is used to actually cooked the food?
A microwave oven does not use heat to cook food like a conventional oven, instead it uses microwaves to cook the food. The microwaves produce the heat inside the food, thereby cooking the food from the inside out.
Why would a mapmaker use active remote sensing instead of passive remote sensing?
Active remote sensing uses its own electromagnetic radiation and use microwaves. It maps areas hard to map.
Could infrared light be used between earth and satellites instead of microwaves?
Not really. You'd lose much more energy during the passage through the atmosphere with IR than with microwaves to make IR a really bad choice for Earth-Orbit communication. Besides, building IR receivers is far more expensive than building microwave receivers.
Waveguide of a microwave?
A waveguide, is a hollow rectangular tube, designed to channel microwaves from the Magnetron to the outside world. Rather than use a wire, this is found to be the best way to transfer microwaves at 3 cm to 10cm wavelength. (Some 10cm systems use large coax instead) They are normally found on radar systems, to transfer the signal to and from the rotating scanner to the transceiver unit. Modern radars keep the length of the waveguide to a minimum, by housing the transceiver in the motor unit of the scanner, instead of a separate room.
