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How long does an antenna have to be?

Published:2022/1/11 16:13:18 Visits:
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.

AM and FM antennas: the long and short of it

et'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.

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.


Picture show: 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

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