Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light.
Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. It does not require a medium of transport. Information is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude or their frequency. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor.
The word 'radio' is used to describe this phenomenon, and television, radio, and cell phone transmissions are all classed as radio frequency emissions.
Modulation is the process where speech, music and the like are impressed onto the carrier wave of a transmission. Detection or demodulation is the reverse proces which takes place in a receiver. The speech, music etc is recovered and may then be amplified and applied to an ear piece or a loud speaker. The most common forms of modulation are Amplitude Modulation (AM) and Frequency Modulation (FM) both of which use different modulation and detection circuits. Cell phones, digital radio and digital television use more complex modulation and demodulation systems.
The simplest AM receiver is the Crystal Set or Crystal Radio. Demodulation is achieved by a semiconductor diode. One of the earliest forms of detection consisted of a thin copper wire in contact with a small chunk of Galena Rock. Adequate signals could be heard in headphones for up to 50 km with simple arrangements.
