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Equation 38-5 can be used to convert these forward and reflected power
measurements to VSWR.

Equation 38-5. SWR from forward and reflected power
We could have used any 1 to 10 ratio and obtained the same answer. For example,
if the forward power was 1 watt and the reflected power 100mW, the ratio between
forward and reflected power is still 10:1, so the SWR is 1.9:1.

VSWR METER

Radio stations usually have an instrument which measures the amount of reflected
voltage relative to the amount of forward voltage on a transmission line. Such a
device is called a VSWR meter. The forward voltage is first 'SET' to full scale and
then a reading of the reflected voltage is taken. The scale on the metre is calibrated
to read VSWR directly. This type of VSWR meter is, in fact, measuring the coefficient
of reflection, which is just the ratio of the forward and reflected voltage. Mid-scale on
these types of VSWR meters corresponds to a coefficient of reflection of 0.5
(VSWR=3) and full scale is 1.0 (VSWR = infinite). There is a simple mathematical
relationship between VSWR and the coefficient of reflection (ρ - Rho), so the
manufacturer (or builder) can calibrate the scale in terms of VSWR:

Equation 38-6. SWR from coefficient of reflection
If you want to calibrate your VSWR meter in terms of coefficient of reflection, then
just put a linear scale on the meter face from “0” to “1”. Quarter scale would be p=
0.25, third scale p = 0.33, half scale p = 0.5, etc.

A popular type of VSWR meter is the cross-needle power meter. It is popular
because it has no adjustments. The meter has two pointer needles. One moves left
to right to measure forward power and the other right to left to measure reflected
power. It has a scale for forward and reflected power. Where the needles cross the
VSWR is read from a third scale.
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