The SWR meter or VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line. This is an item of radio equipment used to check the quality of the match between the antenna and the transmission line. The VSWR meter should be connected in the line as close as possible to the antenna. This is because all practical transmission lines have a certain amount of loss, causing the reflected power to be attenuated as it travels back along the cable, and producing an artificially low VSWR reading on the meter. If the meter is installed close to the antenna, then this problem is minimised.
Referring to the above diagram, the transmitter (TX) and antenna (ANT) terminals are a connected via an internal transmission line. This main line is electromagnetically coupled to two smaller sense lines which are connected to resistors at one end, and diode rectifiers at the other. The resistors are chosen to match the characteristic impedance of the sense lines. One sense line senses the forward wave (connected to FWD), and the other the reflected wave (connected to REV). The diodes convert these to FWD and REV DC voltages respectively, the ratio of which is used to determine the VSWR. In a passive meter, this is indicated on a non-linear meter scale. To calculate the VSWR, first calculate the reflection coefficient:
<math>\Gamma = \frac{V_{rev}}{V_{fwd}} = \sqrt{ \frac{P_{rev}}{ P_{fwd}}}</math>
Then calculate the VSWR:
<math>VSWR = \frac{1 + \Gamma}{1 - \Gamma}</math>
Note that an SWR meter does not measure the actual impedance of a load (ie the resistance and reactance), but only the mismatch ratio. To measure the actual impedance, an antenna analyzer or other similar RF measuring device is required. Note also that for accurate readings, the SWR meter must be matched to the line impedance, ie 50 or 75 ohms as applicable. To accommodate both impedances, some SWR meters have switches on the rear, to select the appropriate load resistance for the sense lines. If a mismatch exists between the transmission line and load, the line will act as an impedance transformer. In this case, the impedance seen at the input to the line will depend on its electrical length, although (for a lossless line) the VSWR will be the same at any point along the line. Mismatched transmission lines are often used for impedance transformation, especially at UHF and microwave frequencies where their dimensions can be very short. For more information on this handy technique, see smith chart. When not actually measuring VSWR, it is best to remove the ordinary type of passive SWR meter from the line. This is because the internal diodes of such meters can generate harmonics when transmitting, and intermodulation products when receiving. Because active SWR meters do not usually suffer from this effect, they can normally be left in without causing such problems.
