Testing to ensure the transmitter is correctly matched to the feedline from the antenna is needed after any change that might perturb the system. Instruments such as SWR meters, antenna analyzers, or impedance bridges are used to measure the degree of match or mismatch. In multi-frequency communication stations like amateur radio stations, and for multi-kilowatt transmitters like wide-area AM stations, the ATU is adjustable to accommodate changes in frequency, in the transmitting system, or to its environment. Narrow-band transmitters like cell phones and walkie-talkies have a built-in matching circuit, permanently set to work with the installed antenna. In transmitting systems with an antenna separated from the transmitter and connected to it by a long transmission line ( feedline), there may be another matching network (ATU) at the antenna that matches the transmission line's impedance to the antenna. Or a separate piece of equipment connected between the transmitter and the antenna. The transmatch might be a circuit incorporated into the transmitter itself, When excessive reflected power is detected, self-protection circuits in modern transmitters automatically reduce power to safe levels, and hence reduce the power of the signal leaving the antenna even more than the loss from some of the power being reflected away from the antenna.Īutomatic power reduction by safety circuits typically causes most of the power loss ( see below).īecause of this, feedline matching is a standard part of almost all radio transmitting systems. In high-power transmitters it may overheat the transmitter's output amplifier.
Without a matching system, in addition to reducing the power radiated by the antenna, the reflected (or "backlash") current can cause signal distortion and overheat transformer cores. Use with transmitters Īntenna tuners are used almost universally with solid-state transmitters. If the impedance seen by the transmitter departs from this design value due to improper tuning of the combined feedline and antenna, overheating of the transmitter's final stage, distortion, or loss of output power may occur. Transmitters are designed to feed power into a resistive load of a specific value: 50 Ω (Ohms), by modern convention. In transmitting systems with an antenna distant from the transmitter and connected to it by a transmission line ( feedline), in addition to a matching unit where the feedline connects to the transmitter, there may be a second matching network (ATU) near the antenna or incorporated into the design of the antenna, to bridge the transmission line impedance over to the antenna's feedpoint impedance.Īntenna tuners are particularly important for use with transmitters. The system transmatch may be a circuit incorporated into the transmitter itself, a separate piece of equipment connected to the feedline anywhere between the transmitter and the antenna, or a combination of several of these. To avoid possible damage resulting from applying power into a mismatched load, and 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. 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. However the impedance of any antenna normally varies, depending on the frequency and other factors, and consequently changes the signal impedance that appears at the other end of the feedline, where the line connects to the transmitter. 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. Īntenna tuners are particularly important for use with transmitters. English language technical jargon makes no distinction between the terms. 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. Its purpose is to optimize power transfer by matching the impedance of the radio to the impedance of the end of the feedline connecting the antenna to the transmitter. Front view of a modified ‘ π’ type antenna tuner, with interior partially exposed.Īn antenna tuner is an electronic device inserted into the feedline between a radio transmitter and its antenna.
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