Antenna tuner

An antenna tuner is a passive electronic device inserted between a radio transmitter and its antenna. Its purpose is to optimize power transfer by matching the impedance of the radio to the signal impedance on the feedline to the antenna.

Various alternate names are used for this device; English language technical jargon makes no distinction between the terms:^[1][a]

In all cases, the word "antenna" can be replaced by "feedline"; likewise, the words "couple", "match", and "tune" each can replace the others without changing the meaning.

The most efficient place to put a matching network is as close to the antenna as feasible. In one regard tuner location is very flexible: A tuner can match up the radio and the antenna from any spot along the feedline between them, but its placement is complicated by the consequences that the chosen spot has for lost transmit power. Putting just one line tuner near the transmitter and far from the antenna leads to worse power-loss, if the feedline in use is low-impedance coaxial cable that's currently standard; flexible tuner placement is practical only when the matchbox and the antenna are connected by high impedance feedline that is no longer popular.

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: Essentially all radio transmitters built after the 1950s are designed for 50 Ω (Ohm) output.^[2][b] However the impedance of any antenna normally varies, depending on the frequency and other factors, and consequently changes the signal impedance seen at the opposite end of the feedline, where it connects 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 either the amplifier, or the cores of transformers along the line, or both.^[c]

To avoid possible damage resulting from applying power into a mismatched load, and to prevent self-protection circuits in the amplifier from cutting back the power output, matching networks are a standard part of almost all radio transmitting systems.^[d] The system transmatch may be a circuit incorporated into the transmitter itself, a separate piece of equipment connected to the feed line 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 transmission line (feedline), in addition to a line matching unit where the feedline connects to the transmitter, there may be a second matching network (transmatch / ATU / tuning unit) to bridge the transmission line's characteristic impedance over to the antenna's feedpoint impedance. That impedance match can either be accomplished by a separate tuning unit mounted near the antenna, or by a relatively short section of a different cable spliced into the main feedline or a dead-end line section branching off from the main feedline, or as an set of extra metal segments integrated into the antenna feedpoint itself.

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