Radio is the wireless transmission of signals through free space by electromagnetic radiation of a frequency significantly below that of visible light, in the radio frequency range, from about 30 kHz to 300 GHz. These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.
Information, such as sound, is carried by systematically changing some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.
The etymology of “radio” or “radiotelegraphy” reveals that it was called “wireless telegraphy”, which was shortened to “wireless” in Britain. The prefix radio- in the sense of wireless transmission, was first recorded in the word radioconductor, a description provided by the French physicist Édouard Branly in 1897. It is based on the verb to radiate .
The word “radio” also appears in a 1907 article by Lee De Forest. It was adopted by the United States Navy in 1912, to distinguish radio from several other wireless communication technologies, such as the photophone. The term became common by the time of the first commercial broadcasts in the United States in the 1920s. The term was adopted by other languages in Europe and Asia. British Commonwealth countries continued to commonly use the term “wireless” until the mid-20th century, though the magazine of the BBC in the UK has been called Radio Times ever since it was first published in the early 1920s.
In recent years the more general term “wireless” has gained renewed popularity through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network, Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS. Today, the term “radio” specifies the actual type of transceiver device or chip, whereas “wireless” refers to the lack of physical connections; one talks about radio transceivers, but another talks about wireless devices and wireless sensor networks.
Radio systems used for communications will have the following elements. With more than 100 years of development, each process is implemented by a wide range of methods, specialized for different communications purposes.
Transmitter and modulation
Each system contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space.
Amplitude modulation of a carrier wave works by varying the strength of the transmitted signal in proportion to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. It
was the method used for the first audio radio transmissions, and remains in use today. “AM” is often used to refer to the mediumwave broadcast band .
Frequency modulation varies the frequency of the carrier. The instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier’s frequency among a set of discrete values, a technique known as frequency-shift keying.
FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech . Normal TV sound is also broadcast using FM.
Angle modulation alters the instantaneous phase of the carrier wave to transmit a signal. It is another term for Phase modulation.