Question

what essential contruction design is needed to recieve an incomming horizontal EM wave if the reciever is built for interpreting changes in the electric field? what if it was built to recieve the magnetic field?

Answer 1

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, partly electric and partly magnetic, 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. (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.

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:

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 (fiber-optic cables have largely made this obsolete).
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.

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.

How long does an antenna have to be?

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 size and type of the antenna you need to use. Broadly speaking, the length of a simple (rod-type) 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 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.

The long and short of it

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.

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.

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 I expect there's another part we can't see inside the case itself.) Newer cellphones (like the Nokia model on the right) have longer antennas built completely inside the case.

More 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 antennas 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.

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.

Important properties of antennas

Three features of antennas are particularly important, namely their directionality, gain, and bandwidth.

Directionality

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. 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.

Gain

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.

Bandwidth

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.