An antenna analyzer or in British aerial analyser (also known as a noise bridge, RX bridge, SWR analyzer, or RF analyzer) is a device used for measuring the input impedance of antenna systems in radio electronics applications. In radio communications systems, including amateur radio, an antenna analyzer is a common tool used for fine tuning antenna and feedline performance, as well as troubleshooting them. Antenna bridges have long been used in the broadcast industry to tune antennas. A bridge is available which measures complex impedance while the transmitter is operating, practically a necessity when tuning multi-tower antenna systems. In more recent times the direct-reading network analyzers have become more common. Types of analysers There are several different instruments of varying complexity and accuracy for testing antennas and their feed lines. All can also be used to measure other electrical circuits and components (at least, in principle). The simplest is an SWR meter, which only indicates the degree of mismatch; the actual mismatched impedance must be inferred by measuring several nearby frequencies and performing a few simple calculations. The SWR meter requires a transmitter or signal generator to provide a few watts power test signal. An antenna bridge is able to measure at low power, but also requires a supplied test signal; depending on the bridge circuit, it can be used to measure both reactance and resistance by reading values marked on knobs that have been adjusted for a match. The noise bridge and network analyzers both supply their own very low-power test signals; both are able to measure both resistance and reactance, either by calculation or by reading knobs adjusted for a match. Modern analyzers directly display resistance and reactance, with the calculations done internally by a microprocessor. Antenna bridge A bridge circuit has two legs which are frequency-dependent complex-valued impedances. One leg is a circuit in the analyzer with calibrated components whose combined impedance can be read on a scale. The other leg is the unknown – either an antenna or a reactive component. To measure impedance, the bridge is adjusted, so that the two legs have the same impedance. When the two impedances are the same, the bridge is balanced. Using this circuit it is possible to either measure the impedance of the antenna connected between ANT and GND, or it is possible to adjust an antenna, until it has the same impedance as the network on the