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Parabolic satellite dish at Canberra, Australia

Antennas and transmitters

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by Chris Woodford. Last updated: June 29, 2020.



How antennas work


Artwork showing how antennas transmit and receive radio waves


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. (One of the ones I tune into regularly, at Sutton Coldfield in England, has a mast 270.5 metres or 887ft high, which is something like 150 tall people standing on top of one another.) But you don't need anything that big on your TV or radio at home: a much smaller antenna will do the job fine.




  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 (fiber-optic cables have largely made this obsolete).
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How long does an antenna have to be?

Telecommunications workers climb up the metal framework of an antenna





The length of the antenna isn't the only thing that affects the wavelengths you're going to pick up; if it were, a radio with a fixed length of antenna would only ever be able to receive one station. The antenna feeds signals into a tuning circuit inside a radio receiver, which is designed to "latch onto" one particular frequency and ignore the rest. The very simplest receiver circuit (like the one you'll find in a crystal radio) is nothing more than a coil of wire, a diode, and a capacitor, and it feeds sounds into an earpiece. The circuit responds (technically, resonates, which means electrically oscillates) at the frequency you're tuned into and discards frequencies higher or lower than this. By adjusting the value of the capacitor, you change the resonant frequency—which tunes your radio to a different station. The antenna's job is to pick up enough energy from passing radio waves to make the circuit resonate at just the right frequency.

AM and FM antennas: the long and short of it

typical AM radio antenna

Photo: The AM "loopstick" antenna inside a typical transistor radio is very compact and highly directional. The pink-colored wire that makes up the antenna is wrapped around a thick ferrite core (the black rod). Usually, as you can see here, there are two separate antennas on the same ferrite rod: one for AM (medium-wave) and one for LW (long-wave).

Let's see how it 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, which is roughly the length of a telescopic FM radio antenna when it's fully extended.

Now for AM, the wavelengths are about 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 powerful antenna, you just don't know it's there! The AM antenna on the inside of a transistor radio works in a very different way to the FM antenna on the outside. Where an FM antenna picks up the electric part of a radio wave, an AM antenna couples with the magnetic part instead. It's a length of very thin wire (typically several tens of meters) looped anything from a few dozen to a few hundred times around a ferrite (iron-based magnetic) core, which greatly concentrates the magnetic part of the radio signals and produces ("induces") a bigger current in the wire wrapped around them. That means an antenna like this can be really tiny and still pack a punch. Without the ferrite rod, a loop antenna either needs many more turns of wire (so thousands instead of hundreds or dozens) or the loops of wire need to be a lot bigger. That's why external FM antennas for radio sets sometimes take the form of a big loop, maybe 10–20cm (4–8in) in diameter or so.

How FM stick antennas pick up electric fields while FM loop antennas pick up magnetic fields.

Artwork: Top: Electromagnetic radio waves consist of vibrating electric waves (blue) and magnetic waves (red) traveling together at the speed of light (black arrow). Bottom: Left: An FM antenna picks up the relatively short wavelength, high-frequency electric part of FM radio waves. Right: An AM ferrite loop antenna picks up and concentrates the magnetic parts of longer wavelength, lower frequency electromagnetic waves.

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. In smartphones, typically the antenna stretches around the inside of the case. Let's see how that computes: if the frequency is 800MHz, the wavelength is 37.5cm (14.8in), and half the wavelength would be 18cm (7.0in). My current LG smartphone is about 14cm (5.5in) long, so you can see we're in the right sort of ballpark.

Typical telescopic FM radio antennaTypical short cellphone antenna

Photo: 1) 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. 2) Cellphones have particularly compact antennas. Older ones (like the Motorola on the left) have stubby external antennas or ones that pull out telescopically. (The exposed part of the antenna is the bit my finger is pointing to and there's another part we can't see running around the edge of the circuit board inside the case.) Newer cellphones (like the Nokia model on the right) have longer antennas built completely inside the case.

More types of antennas




Important properties of antennas




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. But directionality isn't always a good thing. Think about your cellphone. You want it to be able to receive calls wherever it is in relation to the nearest phone mast, or pick up messages whichever way it happens to be pointing when it's lying in your bag, so a highly directional antenna isn't much good. Similarly for a GPS receiver that tells you where you are using signals from multiple space satellites. Since the signals come from different satellites, in different places in the sky, it follows that they come from different directions, so, again, a highly directional antenna wouldn't be that helpful.






Photos: More antennas: 1) The antenna that powers an RFID tag stuck inside a library book. The circuit inside it has no power source: it gets all its energy from the incoming radio waves. 2) The dipole antenna inside a PCMCIA wireless Internet Wi-Fi card. This one works with 2.4GHz radio waves of wavelength 12.5cm, which is why it only needs to be about 6cm long or so.

Who invented antennas?

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





And the rest, as they say, is history!

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Text copyright © Chris Woodford 2008, 2018. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008/2018) Antennas and transmitters. Retrieved from [Accessed (Insert date here)]

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