New proposals for the measurement of microwave power by radiation pressure

Instruments for the measurement of high-frequency power by radiation pressure have excited growing interest in recent years. Forces arising in this way may be interpreted in terms of a rate of change of electromagnetic momentum associated with a gradient of energy in the field. Both linear and angular-momentum forces of this kind have been applied to the operation of wattmeters, and the present paper calls attention to some distinctive features of these two arrangements when used in different parts of the frequency spectrum. Proposals are made for two new types of instrument, both based on linear forces and operating in the 8 mm waveband. One of these is a multiple-cavity reflection-type wattmeter, and the other is a design which depends upon a gradient of stored energy in a solid dielectric suspended in the high-frequency field. The possibilities of applying these instruments to determine power in imperfectly matched waveguide circuits and of making absolute measurements by these techniques are briefly discussed.