Limitations
An SWR meter does not measure the actual impedance of a load (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. For accurate readings, the SWR meter must be matched to the line impedance, usually 50 or 75 ohms. To accommodate multiple impedances, some SWR meters have switches on the rear, to select the resistance appropriate for the sense lines.
An SWR meter should be connected to the line as close as possible to the antenna: All practical transmission lines have a certain amount of loss, which causes the reflected wave to be attenuated as it travels back along the line. Thus, the SWR is highest closest to the load, and only improves as the distance from the load increases.
The internal diodes of an SWR meter may generate objectionable harmonics when transmitting and produce intermodulation products during reception, if it is left in the system after measurement. Active SWR meters do not usually suffer from this effect, and may be left in without causing such problems.
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