First, thermistors have a limited dynamic range, typically about 25 dBm, a relatively narrow range by the standards of today's RF technology. To cover a range of power levels, you'll need to use several sensors set for different power levels. Second, because thermistors rely on temperature changes, they exhibit a relatively slow response rate that may prove unacceptable in some applications. Since their introduction in the mid 1970s, thermocouple sensors have found wide use for sensing RF and microwave power. Engineers generally attribute their popularity to two characteristics.
Thermocouples operate over a wider power range than thermistors, and they are rugged.
A thermocouple produces a small voltage that depends on the potential difference between two dissimilar metals that experience a temperature differences. The temperature difference occurs along the entire length of the metals, not just at the junction. In a closed circuit, current generated by the two voltages represents temperature sensed at the measuring junction. A voltmeter in the circuit measures the net thermoelectric voltage. In an RF sensor, the energy absorbed by the sensor produces a current that an instrument can translate into an RF energy level.