The 42CD is a wattmeter from Boonton. A Wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power; other types are required for radio frequency measurements.
The Boonton 42CD RF Microwattmeter measures the range 100 nW to 100 mW f.s.; uses 41-5 series power sensors (sold separately). The frequency range is 0.2 MHz to 18 GHz depending on the particular sensor used.