Microwave power meter is an instrument which measures the electrical power at microwave frequencies. Usually a microwave power meter will consist of a measuring head which contains the actual power sensing element, connected via a cable to the meter proper, which displays the power reading. The head may be referred to as a power sensor or mount. Different power sensors can be used for different frequencies or power levels. Historically the means of operation in most power sensor and meter combinations was that the sensor would convert the microwave power into an analogue voltage which would be read by the meter and converted into a power reading. Several modern power sensor heads contain electronics to create a digital output and can be plugged via USB into a PC which acts as the power meter.

Microwave power meters have a wide bandwidth—they are not frequency-selective. To measure the power of a specific frequency component in the presence of other signals at different frequencies a spectrum analyzer or measuring receiver is needed. Power meters generally report the power in dBm (decibels relative to 1 milliwatt), dBW (decibels relative to 1 watt) or watts.

Microwave RF Power Sensor Technologies

There are a variety of different technologies which have been used as the microwave power sensing element. Each has advantages and disadvantages.

  • Thermal Power Sensor – Thermocouple power sensors make up the majority of the thermal power sensors sold at present. They are generally reasonably linear and have a reasonably fast response time and dynamic range. The microwave power is absorbed in a load whose temperature rise is measured by the thermocouple. Thermocouple sensors often require a reference DC or microwave power source for calibration before measuring; this can be built into the power meter. Thermistor-based power sensors are generally only used in situations where their excellent linearity is important, as they are both much slower and have a smaller dynamic range than either thermocouple or diode-based sensors. Other thermal sensing technologies include microwave calorimeters and bolometers and quasi-optic pulsed microwave sensors.
  • Diode-based Microwave RF Power Sensor – Many microwave power heads use one or more diodes to rectify the incident microwave power, and have extremely fast response. The diode would generally be used in its square-law region and hence give an output voltage proportional to the incident RF power. In order to extend their dynamic range beyond the square-law region, linearity correction circuits or multiple diode stacks are used. Despite this, diode sensors generally have poor linearity and can be inaccurate when measuring modulated signals; like thermocouple sensors, they often require a reference source.
  • Other Power Sensor based on Field Strength – these include torque-vane, electron-beam, MEMS, Hall effect and atomic fountain based sensors.