RF power measurements at low levels are very useful, to evaluate operation of a system
in all areas. Internal low-level stages may be measured directly, while high powers may
be sampled, and radiated energy detected with a small antenna. While transmitting, both
forward and reflected power on a transmission line may be monitored through a
directional coupler.
At microwave frequencies, power measurements are often made with laboratory
instruments using a bolometer or thermocouple. The instruments usually require AC
mains power, and are fragile and expensive to repair if abused. For portable use, diode
detectors are used for relative power indication. Microwave diodes are non-linear,
temperature sensitive, vary from unit to unit, and have limited dynamic range and
calibration repeatability