Antennas and transmitters
by Chris Woodford. Last updated: March 6, 2015.
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 Great Images in NASA.
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: 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:
- 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.
- 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).
- 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.
Artwork: How a wave travels from a transmitter to a receiver: 1) By line of sight; 2) By ground wave; 3) Via the ionosphere.
How long does an antenna have to be?
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!
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, though we won't go into that here).
The long and short of it
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.
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.
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.
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.
Photo: Right: US military telecommunications workers climb the framework of a different kind of antenna shaped like a tower. Photo by Pierre-Etienne Courtejoie courtesy of US Army.
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
(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.)
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.
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.
Who invented antennas?
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? British 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, another 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
And the rest, as they say, is history!
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.