Transmitter

Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not usually called transmitters, even though they often have similar circuits.

The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones, walkie-talkies, and garage door openers.

[3][2] An alternating current flowing back and forth in an antenna will create an oscillating magnetic field around the conductor.

Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.

A body called the International Telecommunication Union (ITU) allocates the frequency bands in the radio spectrum to various classes of users.

In some classes, each transmitter is given a unique call sign consisting of a string of letters and numbers which must be used as an identifier in transmissions.

Exceptions to the above regulations allow the unlicensed use of low-power short-range transmitters in consumer products such as cell phones, cordless telephones, wireless microphones, walkie-talkies, Wi-Fi and Bluetooth devices, garage door openers, and baby monitors.

Spark transmitters could not transmit audio (sound) and instead transmitted information by radiotelegraphy: the operator tapped on a telegraph key which turned the transmitter on-and-off to produce radio wave pulses spelling out text messages in telegraphic code, usually Morse code.

The pulses were audible as beeps in the receiver's earphones, which were translated back to text by an operator who knew Morse code.

Their energy was spread over a broad band of frequencies, creating radio noise which interfered with other transmitters.

Practical frequency modulation (FM) transmission was invented by Edwin Armstrong in 1933, who showed that it was less vulnerable to noise and static than AM.

The development of radar during World War II motivated the evolution of high frequency transmitters in the UHF and microwave ranges, using new active devices such as the magnetron, klystron, and traveling wave tube.

The invention of the transistor allowed the development in the 1960s of small portable transmitters such as wireless microphones, garage door openers and walkie-talkies.

Commercial FM broadcasting transmitter at radio station WDET-FM , Wayne State University, Detroit, US. It broadcasts at 101.9 MHz with a radiated power of 48 kW .
A radio transmitter is usually part of a radio communication system which uses electromagnetic waves ( radio waves ) to transport information (in this case sound) over a distance.
Animation of a half-wave dipole antenna transmitting radio waves , showing the electric field lines. The antenna in the center is two vertical metal rods, with an alternating current applied at its center from a radio transmitter (not shown). The voltage charges the two sides of the antenna alternately positive (+) and negative (−) . Loops of electric field (black lines) leave the antenna and travel away at the speed of light ; these are the radio waves. This animation shows the action slowed enormously
Hertz discovering radio waves in 1887 with his first primitive radio transmitter (background).