1.4 - Radio Waves

Radio waves and the electromagnetic spectrum

• Radio waves are part of the electromagnetic (EM) spectrum.
• They have the longest wavelengths and the lowest frequencies of all EM waves.
• Like all EM waves, they are transverse and travel at the speed of light through a vacuum.
• Their long wavelengths make them useful for carrying signals over long distances, such as radio and television broadcasts.

Radio waves are produced and received by oscillating charges

• EM waves are made of oscillating electric and magnetic fields.
• An alternating current (a.c.) is made of oscillating charges. When it flows in a circuit, the moving charges produce oscillating electric and magnetic fields - an EM wave.
• The frequency of the radio wave equals the frequency of the a.c.
• The object in which the charges oscillate to create the radio waves is called a transmitter.

Receiving radio waves

1. When a radio wave reaches a receiver, it is absorbed and transfers its energy to the electrons in the receiver, making them oscillate.
2. If the receiver is part of a complete circuit, this induces an alternating current.
3. That induced current has the same frequency as the radio wave, allowing the signal to be recovered.

Radio waves carry communication signals

• Radio waves are used mainly for communication - they carry television and radio broadcasts from one place to another.
• Radio waves have wavelengths longer than about 10 cm, and how a radio wave travels from transmitter to receiver depends on its wavelength.
• Longer-wavelength radio waves can be received over very large distances.
• The very short wavelengths used for TV and FM radio only reach a receiver in line of sight of the transmitter, because they don't bend or travel far through obstacles.