Power Measurements by Means of Thermally Excited Mechanical Vibrations

The periodic absorption of heat on the surface of a material produces a periodically varying temperature distribution in the material near the surface as well as an increase of the average temperature. The periodic component of temperature is largest on the surface and falls off with distance from the surface. This fact brings about periodic internal stresses. These stresses under proper circumstances can excite a mechanical system to oscillation. The amplitude of this oscillation is found theoretically and experimentally to be directly proportional to the power absorbed on the material surface over a wide range of power. This phenomenon was employed to construct several linear power detectors for microwave application in the X-and K-bands. By using piezoelectric and electromagnetic transducers, a voltage linearly proportional to the power was obtained.