Transmitter
From Network Dictionary Wiki
A device that generates radio waves and sends them to the antenna.
A transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio frequency (AF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and a radio receiver, with the combined unit referred to as a transceiver.
More generally and in communications and information processing, a "transmitter" is any object (source) which sends information to an observer (receiver). When used in this more general sense, vocal cords may also be considered an example of a "transmitter".
In industrial process control a "transmitter" is any device which converts measurements from a sensor into a signal to be received, usually sent via wires, by some display or control device located a distance away. Typically in process control applications the "transmitter" will output a 4-20 mA current loop or digital protocol to represent a measured variable within a range. For example, a pressure transmitter might use 4 ma as a representation for 50 psig of pressure and 20 ma as 1000 psig of pressure and any value in between proportionately ranged between 50 and 1000 psig. Older technology transmitters used pneumatic pressure typically ranged between 3 to 15 psig (20 to 100 kPa) to represent a process variable.
Skin effect and waveguides A varying magnetic field will generate an electric field in a conductor. Conversely, a varying electric field in a conductor will generate a magnetic field. At high frequencies, inside a conductor this reciprocal effect creates essentially a "dead zone" in the center of the conductor, which substantially reduces the effective cross-sectional area of the conductor. In other words, if a cable is one inch (2.5 cm) in diameter, half an inch in the center may carry essentially no signal. This phenomenon is known as the skin effect. At microwave frequencies, the effect is so severe that there is no point in having a center, so cables are replaced with hollow waveguides, which can be thought of as metal pipes.
Cooling of final stages Low-power transmitters do not require special cooling equipment. Modern transmitters can be incredibly efficient, with efficiencies exceeding 98 percent. However, a broadcast transmitter with a megawatt power stage transferring 98% of that into the antenna can also be viewed as a 20 kilowatt electric heater.
For medium-power transmitters, up to a few hundred watts, air cooling with fans is used. At power levels over a few kilowatts, the output stage is cooled by a forced liquid cooling system analogous to an automobile cooling system. Since the coolant directly touches the high-voltage anodes of the tubes, only distilled, deionised water or a special dielectric coolant can be used in the cooling circuit. This high-purity coolant is in turn cooled by a heat exchanger, where the second cooling circuit can use water of ordinary quality because it is not in contact with energized parts. Very-high-power tubes of small physical size may use evaporative cooling by water in contact with the anode. The production of steam allows a high heat flow in a small space.