A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications.

Power output[]

In broadcasting and telecommunication, the part which contains the oscillator, modulator, and sometimes audio processor, is called the "exciter". Most transmitters use heterodyne principle, so they also have a frequency conversion units. Confusingly, the high-power amplifier which the exciter then feeds into is often called the "transmitter" by broadcast engineers. The final output is given as transmitter power output (TPO), although this is not what most stations are rated by.

Power supply[]

Transmitters are sometimes fed from a higher voltage level of the power supply grid than necessary in order to improve security of supply. For example, the Allouis, Konstantynow and Roumoules transmitters are fed from the high-voltage network (110 kV in Alouis and Konstantynow, 150 kV in Roumoules) even though a power supply from the medium-voltage level of the power grid (about 20 kV) would be able to deliver enough power.

Protection equipment[]

The high voltages used in high power transmitters (up to 40 kV) require extensive protection equipment. Also, transmitters are exposed to damage from lightning. Transmitters may be damaged if operated without an antenna, so protection circuits must detect the loss of the antenna and switch off the transmitter immediately. Tube-based transmitters must have power applied in the proper sequence, with the filament voltage applied before the anode voltage, otherwise the tubes can be damaged. The output stage must be monitored for standing waves, which indicate that generated power is not being radiated but instead is being reflected back into the transmitter.

Lightning protection is required between the transmitter and antenna. This consists of spark gaps and gas-filled surge arresters to limit the voltage that appears on the transmitter terminals. The control instrument that measures the voltage standing-wave ratio switches the transmitter off briefly if a higher voltage standing-wave ratio is detected after a lightning strike, as the reflections are probably due to lightning damage. If this does not succeed after several attempts, the antenna may be damaged and the transmitter should remain switched off. In some transmitting plants UV detectors are fitted in critical places, to switch off the transmitter if an arc is detected. The operating voltages, modulation factor, frequency and other transmitter parameters are monitored for protection and diagnostic purposes, and may be displayed locally and/or at a remote control room.