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oups.com...
Why calibration is required for VSWR meter...

The meter simply measures the ratio of the power travelling along a feeder
line to an antenna against the reflected power back from the antenna and
feeder due to a mismatch of impedence.

The VSWR meter does not strictly need to be calibrated. Simple meters tend
to be frequency sensitive and are provided with a control to adjust the
meter to read full scale for forward power. Operating a switch or reversing
the connections allows reverse power to be assessed.

A VSWR meter will also only read accurately if the impedence of the antenna
and feeder system is the same as the design impedence of the metrer, usually
around 50 ohms.

More complex measurement circuitry and sensing devices can be used to
compensate for frequency sensitivity and the meter calibrated against known
loads in a factory. The important thing is to know how much reflected power
is in the system because this power largely ends up being dissipated as heat
in the transmitter final stages.