A radio station launches a radio wave by moving electric charges rhythmically up and down their antenna. As this electric charge accelerates back and forth, it produces a changing electric field—a structure in space that pushes on electric charges—and a changing magnetic field—a structure in space that pushes on magnetic poles. Because the electric field changes with time, it creates the magnetic field and because the magnetic field changes with time, it creates the electric field. The two travel off across space as a pair, endlessly recreating one another in an electromagnetic wave that will continue to the ends of the universe. However, when this wave encounters the antenna of your radio, its electric field begins to push electric charges up and down on that antenna. Your radio senses this motion of electric charges and thus detects the passing radio wave.

## I’ve heard the reason an antenna, such as the one on your car, is so long is bec…

#### I’ve heard the reason an antenna, such as the one on your car, is so long is because it needs to be large enough for the long radio waves to pass into it. Is this true? Why are antennas for radio stations so tall and slender? — LW, Blacksburg, VA

A vertical pole radio antenna receives a radio wave by allowing that wave to push electric charges up and down the antenna. The radio senses this moving charge and is thus aware of the passing radio wave. The ideal length of a vertical receiving antenna is a quarter of the wavelength of the radio wave it’s trying to receive—in which case, charge that the radio wave’s electric field pushes up and down the antenna has just enough time to reach the end of the antenna before it has to reverse directions.

The waves used for standard AM radio transmissions have very long wavelengths—typically 300 meters—so that they require vertical pole antennas that are about 75 meters long for optimal reception. An antenna of that length is also optimal for radio transmission, which is why the antennas of AM radio stations are so long and slender. However, because such long antennas are inconvenient for most AM receivers, most AM receivers use small magnetic antennas. A magnetic antenna is a device containing an iron-like material called ferrite that draws in magnetic flux lines like a sponge. A coil of wire is wound around this ferrite so that as the magnetic flux lines of a passing radio wave enter the ferrite, they induces electric currents into the coil of wire. This coil then acts as the antenna.

But the waves used in FM radio transmission have much shorter wavelengths—typically 3 meters—so that antennas of about 75 centimeters are all that’s needed. The vertical pole radio antenna on your car is designed to receive these FM waves. The antennas of FM radio stations are also rather short, but they are usually mounted high up on a pole so that the whole structure looks like an AM radio antenna. However, if you look near the top of an FM radio tower, you’ll see the actual FM antenna as a much smaller structure.

## How does a radio receive transmissions from one station and not another, and how…

#### How does a radio receive transmissions from one station and not another, and how does it turn them into audible waves? — T, Chester, VT

Once charge is moving strongly through the resonant circuit in your radio, the radio can monitor various features of that moving charge. If the station is using the AM or amplitude modulation technique to represent sound, your radio studies the amount of charge moving back and forth through the resonant circuit. When that flow of charge—that current—is strong, it moves the speaker cone toward you and produces a compression of the air. When that current is weak, it moves the speaker cone away from you and produces a rarefaction of the air. These changes in air density and pressure reproduce the sound that the station is transmitting.

If the station is using the FM or frequency modulation technique to represent sound, your radio studies the frequency at which charge moves back and forth in the resonant circuit. Very small changes in this frequency, caused by frequency changes in the radio wave itself, are used to control the speaker cone in your radio. When the frequency is raised slightly above normal, your radio moves the speaker cone toward you and produces a compression of the air. When the frequency is lowered slightly below normal, your radio moves the speaker cone away from you and produces a rarefaction of the air. Again, these changes in air density and pressure produce sound.

## Is there a device that would provide a variable output of radiated energy in the…

#### Is there a device that would provide a variable output of radiated energy in the infrared that would be obtainable to experiment with? — NAT, Marion, SC

You can produce a broad range of infrared lights with a heat lamp. A heat lamp looks very dim because most of the thermal radiation it emits is in the infrared portion of the electromagnetic spectrum. Just attach the heat lamp to a normal light dimmer and you’ll be able to vary its infrared output over a wide range of intensities. Its frequency range will also shift farther away from the visible as you lower its temperature by turning down the dimmer. If it produces more visible light than you want, you can put a filter in front of it that absorbs visible light while permitting infrared light to pass. Such filters are certainly available from filter companies such as Hoya or Corning but cheaper versions (perhaps even plastic filters) may be found through scientific supply companies.

## Is there a homing device small enough to fit onto or inside a pc laptop? How doe…

#### Is there a homing device small enough to fit onto or inside a pc laptop? How does a homing device work?

There are homing devices small enough to fit on bugs, so there should be no problem fitting one on or into a laptop. A homing device is simply a radio transmitter and, while it has recently become possible to build a homing device that actually knows where it is and can tell you via its transmission, the techniques involved in locating most normal homing devices are those of trying to find the source of a radio transmission. Using directional receiving antennas and studying the transmission from several locations, you can figure out where the transmission is coming from.

## How does a crystal radio work?

#### How does a crystal radio work?

A crystal radio uses a crystal diode to detect tiny fluctuating currents in its antenna system. When a radio wave passes across an antenna, the wave’s electric field pushes electric charges up and down the antenna. The crystal diode acts as a one-way gate that allows some of this moving charge to flow onto another wire and then prevents it from returning to the antenna. Since the charge can’t return to the antenna, it flows elsewhere—passing through a sensitive earphone and creating sound. An AM radio station encodes sound as changes in the intensity (or amplitude) of the radio wave. As the radio wave’s intensity fluctuates, the amount of electric charge flowing through the earpiece of the crystal radio also fluctuates and you hear sound.

## How do remote garage door openers work?

#### How do remote garage door openers work? — JD, Greenville, SC

The communication from the remote to the opener is done with radio waves. When you push the button on the remote, it produces a brief burst of radio waves at a specific frequency and with a selected pattern of pulses. A radio receiver in the opener is continuously looking for a transmission at that same frequency and with that same pattern of pulses. While other garage door openers may use radio waves of the same frequency, it’s extremely unlikely that they will make use of the same pattern of pulses. This pattern of pulses is the security code that prevents unauthorized opening of your garage door. These security codes have grown longer and more sophisticated over the years. Early garage door openers had no security code at all and could be opened by almost any radio transmission at the right frequency. You could drive around neighborhoods with a remote and open garage doors right and left. But now the security codes are complicated enough that opening someone else’s garage door is almost impossible.