A Signal That Travels From One Antennae to Another Antennae

Imagine holding out your hand and catching words, pictures, and data passing by. That's more or less what an antenna (sometimes called an aerial) does: it's the metal rod or dish that catches radio waves and turns them into electrical signals feeding into something like a radio or television or a telephone organization. Antennas like this are sometimes chosen receivers. A transmitter is a different kind of antenna that does the opposite job to a receiver: it turns electrical signals into radio waves so they can travel sometimes thousands of kilometers around the Earth or even into infinite and back. Antennas and transmitters are the key to virtually all forms of modern telecommunication. Permit'due south take a closer look at what they are and how they work!

How antennas work

Suppose you're the boss of a radio station and y'all want to transmit your programs to the wider world. How exercise you lot go near information technology? You utilize microphones to capture the sounds of people'southward voices and turn them into electrical energy. You have that electricity and, loosely speaking, make it flow along a tall metallic antenna (boosting information technology in power many times so it will travel just as far equally you lot need into the globe). Every bit the electrons (tiny particles inside atoms) in the electrical current jerk back and forth along the antenna, they create invisible electromagnetic radiation in the form of radio waves. These waves travel out at the speed of light, taking your radio program with them. What happens when I turn on my radio in my abode a few miles abroad? The radio waves you sent menstruum through the metal antenna and cause electrons to jerk back and forth. That generates an current—a signal that the electronic components inside my radio turn back into sound I can hear.

How a transmitter sends radio waves to a receiver. 1) Electricity flowing into the transmitter antenna makes electrons vibrate up and downwards it, producing radio waves. 2) The radio waves travel through the air at the speed of calorie-free. 3) When the waves make it at the receiver antenna, they make electrons vibrate within it. This produces an electric current that recreates the original signal.

Transmitter and receiver antennas are often very similar in design. For instance, if you're using something like a satellite phone that can send and receive a video-telephone call to any other place on Earth using space satellites, the signals you lot transmit and receive all pass through a single satellite dish—a special kind of antenna shaped like a bowl (and technically known equally a parabolic reflector, because the dish curves in the shape of a graph called a parabola). Often, though, transmitters and receivers look very different. TV or radio broadcasting antennas are huge masts sometimes stretching hundreds of meters/feet into the air, because they accept to ship powerful signals over long distances. But you don't demand anything that big on your Tv or radio at home: a much smaller antenna will do the task fine.

Waves don't always zap through the air from transmitter to receiver. Depending on what kinds (frequencies) of waves nosotros want to send, how far we want to send them, and when nosotros desire to practice it, there are really three different ways in which the waves can travel:

Artwork: How a wave travels from a transmitter to a receiver: 1) Past line of sight; two) By ground wave; 3) Via the ionosphere.

1. As we've already seen, they can shoot by what's called "line of sight", in a straight line—just like a beam of light. In erstwhile-fashioned long-altitude telephone networks, microwaves were used to carry calls this way between very high communications towers.
2. They can speed circular the Earth's curvature in what's known as a basis wave. AM (medium-wave) radio tends to travel this mode for brusque-to-moderate distances. This explains why nosotros can hear radio signals beyond the horizon (when the transmitter and receiver are not within sight of each other).
3. They can shoot upwardly to the heaven, bounce off the ionosphere (an electrically charged part of Earth's upper temper), and come dorsum down to the basis once more. This effect works best at nighttime, which explains why distant (foreign) AM radio stations are much easier to pick up in the evenings. During the daytime, waves shooting off to the sky are captivated by lower layers of the ionosphere. At night, that doesn't happen. Instead, higher layers of the ionosphere catch the radio waves and fling them back to Earth—giving us a very effective "sky mirror" that can help to carry radio waves over very long distances.

How long does an antenna have to be?

Photograph: This scope FM radio antenna pulls out to a length of about ane–2m (3–6ft or so), which is roughly half the length of the radio waves it's trying to capture.

The simplest antenna is a unmarried piece of metal wire fastened to a radio. The commencement radio I e'er built, when I was 11 or 12, was a crystal set with a long loop of copper wire interim as the antenna. I ran the antenna right the style effectually my sleeping accommodation ceiling, and then it must have been about 20–30 meters (60–100 ft) long in all!

Photo: Antennas that utilize line-of-sight communication need to be mounted on high towers, like this. You can see the thin dipoles of the antenna sticking out of the top, but most of what you run across here is just the tower that holds the antenna high in the air. Photo by Pierre-Etienne Courtejoie courtesy of U.s. Ground forces.

Almost modernistic transistor radios accept at least two antennas. I of them is a long, shiny telescopic rod that pulls out from the case and swivels around for picking upwardly FM (frequency modulation) signals. The other is an antenna within the case, usually fixed to the main circuit board, and it picks up AM (amplitude modulation) signals. (If you're not sure about the departure between FM and AM, refer to our radio article.)

Why practise you lot need two antennas in a radio? The signals on these unlike wave bands are carried by radio waves of different frequency and wavelength. Typical AM radio signals have a frequency of 1000 kHz (kilohertz), while typical FM signals are about 100 MHz (megahertz)—then they vibrate about a hundred times faster. Since all radio waves travel at the aforementioned speed (the speed of calorie-free, which is 300,000 km/s or 186,000 miles per second), AM signals have wavelengths about a hundred times bigger than FM signals. You demand ii antennas because a single antenna can't pick upward such a hugely different range of wavelengths. Information technology's the wavelength (or frequency, if you prefer) of the radio waves you lot're trying to detect that determines the length of the antenna yous need to use. Broadly speaking, the length of the antenna has to be well-nigh half the wavelength of the radio waves y'all're trying to receive (information technology's also possible to make antennas that are a quarter of the wavelength, though we won't become into that here).

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