pi network

An antenna tuner (and any of the names in the list below) is a device that is inserted between a radio transmitter and its antenna; when properly adjusted (tuned) it improves power transfer by matching the impedance of the radio to the impedance of the end of the feedline connecting the antenna to the transmitter.
Various alternate names are used for this device: antenna matching unit, impedance matching unit, matchbox, matching network, transmatch, antenna match, antenna tuning unit (ATU), antenna coupler, feedline coupler. English language technical jargon makes no distinction between the terms.Antenna tuners are particularly important for use with transmitters. Transmitters are typically designed to feed power into a reactance-free, resistive load of a specific value: Radio transmitters built after the 1950s are almost all designed for 50 Ω (Ohm) cabling. However the impedance of any antenna normally varies, depending on frequency and other factors, and consequently changes the impedance appearing at the other end of the feedline, connected to the transmitter. In addition to reducing the power radiated by the antenna, an impedance mismatch can distort the signal, and in high power transmitters may overheat the amplifier.To avoid possible damage resulting from applying power into a mismatched load, ATUs are a standard part of almost all radio transmitting systems. If the transmitter detects a load impedance that departs from the designed load, safety circuits in modern transmitters automatically cut back the power output to protect the equipment from the consequences of the impedance mismatch.
The system ATU may be a circuit incorporated into the transmitter itself, or a separate piece of equipment connected anywhere between the transmitter and the antenna, or a combination of several of these. In transmitting systems with an antenna distant from the transmitter and connected to it by a long transmission line (feedline), in addition to an ATU where the feedline connects to the transmitter there may be a second matching network (or ATU) near the antenna, or incorporated into the design of the antenna, to match the transmission line's impedance to the antenna's.

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