3-47Voltage Standing-Wave RatioThe ratio of maximum voltage to minimum voltage on a line is called the VOLTAGE STANDING-WAVE RATIO (vswr). Therefore:The vertical lines in the formula indicate that the enclosed quantities are absolute and that the twovalues are taken without regard to polarity. Depending on the nature of the standing waves, the numericalvalue of vswr ranges from a value of 1 (Z_{L} = Z_{0}, no standing waves) to an infinite value for theoreticallycomplete reflection. Since there is always a small loss on a line, the minimum voltage is never zero andthe vswr is always some finite value. However, if the vswr is to be a useful quantity, the power lossesalong the line must be small in comparison to the transmitted power.Power Standing-Wave RatioThe square of the voltage standing-wave ratio is called the POWER STANDING-WAVE RATIO(pswr). Therefore:This ratio is useful because the instruments used to detect standing waves react to the square of thevoltage. Since power is proportional to the square of the voltage, the ratio of the square of the maximumand minimum voltages is called the power standing-wave ratio. In a sense, the name is misleadingbecause the power along a transmission line does not vary.Current Standing-Wave RatioThe ratio of maximum to minimum current along a transmission line is called CURRENTSTANDING-WAVE RATIO (iswr). Therefore:This ratio is the same as that for voltages. It can be used where measurements are made with loopsthat sample the magnetic field along a line. It gives the same results as vswr measurements.Q28.At what point on an open-circuited rf line do voltage peaks occur?Q29.What is the square of the voltage standing-wave ratio called?Q30.What does vswr measure?