Q7Microwave Theory And Techniques

Why microwave power (not voltage/current) is measured: at microwave frequencies, direct measurement of voltage and current waveforms is impractical (as discussed in Part A, Q.4 of this paper), so nearly all microwave power meters instead convert the incident RF power into a measurable secondary effect — almost always heat — and infer the power from that effect.

Bolometer/thermistor method: a bolometer element (a barretter, whose resistance increases with temperature, or more commonly a thermistor, whose resistance decreases with temperature) is placed in the microwave line so absorbed RF power heats it and changes its DC resistance. The bolometer forms one arm of a Wheatstone bridge; the DC bias current needed to keep the bridge balanced (and hence the bolometer at constant resistance/temperature) is measured before and after RF power is applied, and the difference in DC power required to maintain the same bolometer temperature equals the absorbed RF power (the substitution principle) — a technique capable of measuring very low power levels (microwatts to milliwatts) with good accuracy.

Thermocouple method: a thermocouple sensor mounted at the end of a matched microwave load generates a small DC thermoelectric EMF proportional to the temperature rise caused by absorbed RF power; being inherently a self-generating (no external bias needed) square-law power detector with good sensitivity and reasonable speed, thermocouple power sensors are widely used in modern power meters for the low-to-medium power range.

Calorimetric method: for higher microwave power levels (watts to kilowatts) beyond the range of bolometers or thermocouples, the RF power is dissipated into a calibrated absorbing load (often a flowing-liquid or water-load calorimeter), and the resulting temperature rise (or flow-rate/temperature-difference product for a flow calorimeter) is measured directly to calculate absorbed power via P = (mass flow rate)×(specific heat)×(temperature rise) — a slower but highly accurate technique often used for calibrating other power sensors, and the standard approach for high-power microwave sources such as radar transmitters and industrial microwave heating equipment.