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Antenna AnalyzerAn antenna analyzer (or noise bridge, or RX bridge) is a Wheatstone bridge where the legs are frequency-dependent complex-valued impedances. The east and west corners of the bridge are driven with a white noise source, and the north and south corners are connected to the input of a radio receiver which is tuned to the desired frequency range. Because receivers are designed to be very sensitive, the amplitude of the white noise is kept very low. The bridge is balanced when the complex ratio of the northwest impedance to the southwest impedance equals the complex ratio of the northeast impedance to the southeast impedance. These ratios will generally vary with frequency so this balance may occur only in a fairly narrow frequency range. In general practice, one impedance will be the antenna, and the other three will be chosen so that the bridge is balanced when the antenna's impedance is 50 ohms with no imaginary component. When the antenna is balanced, the white noise heard in the receiver will reach a null (minimum). Tuning the antenna for the desired frequency then becomes a simple matter of making adjustments while listening to the receiver. Unlike an SWR meter which measures the standing wave ratio in the cable feeding the antenna, a complex-impedance antenna analyzer requires no transmitted power applied to the antenna. The SWR meter works by minimizing the reflected wave from the antenna back to the transmitter, which is minimal when the antenna is balanced. A badly mistuned antenna can produce enough reflected power to damage the transmitter. Since an RX bridge can be used without transmitting, there is no risk of transmitter damage.
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