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Parabolic satellite dish at Canberra, Australia

Antennas and transmitters

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by Chris Woodford. Last updated: March 27, 2018.

Imagine holding out your hand and catching words, pictures, and information 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 system. Antennas like this are sometimes called 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 space and back. Antennas and transmitters are the key to virtually all forms of modern telecommunication. Let's take a closer look at what they are and how they work!

Photo: The enormous 70m (230ft) Canberra deep dish satellite antenna in Australia. Photo by courtesy of NASA on the Commons.

How antennas work

Suppose you're the boss of a radio station and you want to transmit your programs to the wider world. How do you go about it? You use microphones to capture the sounds of people's voices and turn them into electrical energy. You take that electricity and, loosely speaking, make it flow along a tall metal antenna (boosting it in power many times so it will travel just as far as you need into the world). As the electrons (tiny particles inside atoms) in the electric current wiggle 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 home a few miles away? The radio waves you sent flow through the metal antenna and cause electrons to wiggle back and forth. That generates an electric current—a signal that the electronic components inside my radio turn back into sound I can hear.

Artwork showing how antennas transmit and receive radio waves

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

Transmitter and receiver antennas are often very similar in design. For example, 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 transmit and receive all pass through a single satellite dish—a special kind of antenna shaped like a bowl (and technically known as 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 have to send powerful signals over long distances. But you don't need anything that big on your TV or radio at home: a much smaller antenna will do the job fine.

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

How waves can travel from a transmitter to a receiver either by line of sight, through a ground wave, or by bouncing off the ionosphere.

Artwork: How a wave travels from a transmitter to a receiver: 1) By line of sight; 2) 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 old-fashioned long-distance telephone networks, microwaves were used to carry calls this way between very high communications towers.
  2. They can speed round the Earth's curvature in what's known as a ground wave. AM (medium-wave) radio tends to travel this way for short-to-moderate distances. This explains why we can hear radio signals beyond the horizon (when the transmitter and receiver are not within sight of each other).
  3. They can shoot up to the sky, bounce off the ionosphere (an electrically charged part of Earth's upper atmosphere), and come back down to the ground again. This effect works best at night, 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 absorbed 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?

Typical telescopic FM radio antenna

Photo: This telescopic FM radio antenna pulls out to a length of about 1–2m (3–6ft or so), which is roughly half the length of the radio waves it's trying to capture.

The simplest antenna is a single piece of metal wire attached to a radio. The first radio I ever built, when I was 11 or 12, was a crystal set with a long loop of copper wire acting as the antenna. I ran the antenna right the way around my bedroom ceiling, so it must have been about 20–30 meters (60–100 ft) long in all!

Telecommunications workers climb up the metal framework of an antenna

Photo: Antennas that use 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 see here is just the tower that holds the antenna high in the air. Photo by Pierre-Etienne Courtejoie courtesy of US Army.

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

Why do you need two antennas in a radio? The signals on these different 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)—so they vibrate about a hundred times faster. Since all radio waves travel at the same speed (the speed of light, 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 need two antennas because a single antenna can't pick up such a hugely different range of wavelengths. It's the wavelength (or frequency, if you prefer) of the radio waves you're trying to detect that determines the length of the antenna you need to use. Broadly speaking, the length of the antenna has to be about half the wavelength of the radio waves you're trying to receive (it's also possible to make antennas that are a quarter of the wavelength, compact miniaturized antennas that are about a tenth the wavelength, and membrane antennas that are even smaller, though we won't go into that here).

The long and short of it

typical AM radio antenna

Photo: The AM antenna inside a typical transistor radio. Note how the pink-colored wire that makes up the antenna is wrapped around a thick ferrite core (the black rod).

Let's see how that works for FM. If I try to listen to a typical radio broadcast on an FM frequency of 100 MHz (100,000,000 Hz), the waves carrying my program are about 3m (10ft) long. So the ideal antenna is about 1.5m (4ft) or so long. A shorter antenna will still pick up signals, but a longer one will be more effective. That's why you often have to pull out your antenna on a radio: folded in, it's not long enough to resonate (electrically oscillate) with the radio waves you're trying to capture.

Typical short cellphone antenna

Photo: Cellphones have particularly compact antennas. Older ones (like the Motorola on the left) have stubby antennas or ones that pull out telescopically. (The antenna is the bit my finger is pointing to.) Newer cellphones (like the Nokia model on the right) have longer antennas built completely inside the case.

Now for AM, the waves are 100 times greater, so how come you don't need an antenna that's 300m (0.2 miles) long to pick them up? Well you do need a big antenna, you just don't know it's there! The AM antenna inside a transistor radio is a huge length of thin wire wrapped around a ferrite (iron-based magnetic) core, which greatly boosts the incoming signals, and that means it can be much smaller and more compact but still pick up the signals you need.

So far so good, but what about cellphones? How come they need only short and stubby antennas like the one in this photo? Cellphones use radio waves too, also traveling at the speed of light, and with a typical frequency of 800 MHz (roughly ten times greater than FM radio). That means their wavelength is about 10 times shorter than FM radio, so they need an antenna roughly one tenth the size.

Types of antennas

The simplest radio antennas are just long straight rods. Many indoor TV antennas take the form of a dipole: a metal rod split into two pieces and folded horizontally so it looks a bit like a person standing straight up with their arms stretched out horizontally. More sophisticated outdoor TV aerials have a number of these dipoles arranged along a central supporting rod. Other designs include circular loops of wire and, of course, parabolic satellite dishes. Why so many different designs? Obviously, the waves arriving at an antenna from a transmitter are exactly the same, no matter what shape and size the antenna happens to be. A different pattern of dipoles will help to concentrate the signal so it's easier to detect. That effect can be increased even more by adding unconnected, "dummy" dipoles, known as directors and reflectors, which bounce more of the signal over to the actual, receiving dipoles. This is equivalent to boosting the signal—and being able to pick up a weaker signal than a simpler antenna.

Line artwork: four common types of antenna compared: dipole, folded-dipole, reflector, and Yagi.

Artwork: Four common types of antenna (red) and the places where they pick up best (orange): A basic dipole, a folded dipole, a dipole and reflector, and a Yagi. A basic or folded dipole antenna picks up equally well in front of or behind its poles, but poorly at each end. An antenna with a reflector picks up much better on one side than the other, because the reflecting element (the red, dipole-like bar on the left) bounces more signal over to the folded dipole on the right. The Yagi exaggerates this effect even more, picking up a very strong signal on one side and almost no signal anywhere else. It consists of multiple dipoles, reflectors, and directors.

Three features of antennas are particularly important, namely their directionality, gain, and bandwidth. Dipoles are very directional: they pick up incoming radio waves traveling at right angles to them. That's why a TV antenna has to be properly mounted on your home, and facing the correct way, if you're going to get a clear picture. The telescopic antenna on an FM radio is less obviously directional, especially if the signal is strong: if you have it pointed straight upward, it will capture good signals from virtually any direction. The ferrite AM antenna inside a radio is much more directional. Listening to AM, you'll find you need to swivel your radio around until it picks up a really strong signal. (Once you've found the best signal, try turning your radio through exactly 90 degrees and notice how the signal often falls off almost to nothing.) Although highly directional antennas may seem like a pain, when they're properly aligned, they help to reduce interference from unwanted stations or signals close to the one you're trying to detect.

The gain of an antenna is a very technical measurement but, broadly speaking, boils down to the amount by which it boosts the signal. TVs will often pick up a poor, ghostly signal even without an antenna plugged in. That's because the metal case and other components act as a basic antenna, not focused in any particular direction, and pick up some kind of signal by default. Add a proper directional antenna and you'll gain a much better signal. Gain is measured in decibels (dB), and (as a broad rule of thumb) the bigger the gain the better your reception. In the case of TVs, you get much more gain from a complex outdoor antenna (one with, say, 10–12 dipoles in a parallel "array") than from a simple dipole. All outdoor antennas work better than indoor ones, and window and set-mounted antennas have higher gain and work better than built-in ones.

An antenna's bandwidth is the range of frequencies (or wavelengths, if you prefer) over which it works effectively. The broader the bandwidth, the greater the range of different radio waves you can pick up. That's helpful for something like television, where you might need to pick up many different channels, but much less useful for telephone, cellphone, or satellite communications where all you're interested in is a very specific radio wave transmission on a fairly narrow frequency band.

An anti-theft RFID tag built into a book labelGold-colored antenna inside a PCMCIA laptop Wi-Fi card

Photos: More antennas: 1) The antenna that powers an RFID tag stuck inside a library book. 2) The antenna inside a PCMCIA wireless Internet Wi-Fi card.

Who invented antennas?

Transmitter and receiver diagram from Oliver Lodge's 1888 electric telegraphy patent

Artwork: Oliver Lodge's illustration of sending radio waves through space from a transmitter (red) to a receiver (blue) some distance away, taken from his 1898 patent US 609,154: Electric Telegraphy. Courtesy of US Patent and Trademark Office.

There's no easy answer to that question because radio evolved into a useful technology through the second half of the 19th century thanks to the work of quite a few different people—both theoretical scientists and practical experimenters.

Who were these pioneers? Scottish physicist James Clerk Maxwell figured out a theory of radio around 1864, and Heinrich Hertz proved that radio waves really did exist about 20 years later (they were called Hertzian waves in his honor for some time afterward). Several years later, at a meeting in Oxford, England on August 14, 1894, English physicist, Oliver Lodge, demonstrated how radio waves could be used for signalling from one room to another in what he later described (in his 1932 autobiography) as "a very infantile kind of radio-telegraphy." Lodge filed a US patent for "electric telegraphy" on February 1, 1898, describing apparatus for "an operator, by means of what is now known as 'Hertzian-wave telegraphy' to transmit messages across space to any one or more of a number of different individuals in various localities..." Unknown to Lodge at that stage, Guglielmo Marconi was carrying out his own experiments in Italy around the same time—and ultimately proved the better showman: many people think of him as the "inventor of radio" to this day whereas, in truth, he was only one of a group of forward-thinking people who helped turned the science of electromagnetic waves into a practical, world-changing technology.

None of the original radio experiments used transmitters or receivers that we would instantly recognize today. Hertz and Lodge, for example, used a piece of equipment called a spark-gap oscillator: a couple of zinc balls attached to short lengths of copper wire with an air gap in between them. Lodge and Marconi both used Branly coherers (glass tubes packed with metal filings) for detecting the waves they'd transmitted and received, though Marconi found them "too erratic and unreliable" and eventually designed his own detector. Armed with this new equipment, he carried out systematic experiments into how the height of an antenna affected the distance over which he could transmit a signal.

And the rest, as they say, is history!

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Text copyright © Chris Woodford 2008, 2016. All rights reserved. Full copyright notice and terms of use.

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