Hall effect wattmeters

A transmission-type wattmeter can be formed utilizing the Hall effect to give wide-range multiplication over a frequency range extending from the low audio-frequency region to the SHF band. At the low frequencies, the Hall device can take the form of a thin wafer of indium antimonide mounted in series with the center leg of a cup core magnetic circuit. An audio-frequency wattmeter employing such a structure has been analyzed, constructed and tested. The wattmeter has a range of zero to 400 milliwatts when used in a 600-ohm circuit, and displays a frequency error of less than plus and minus 3.5 per cent over a frequency range of 100 to 6000 cps for a unity power factor load. In the SHF range, a cavity and electric probe are arranged to provide excitation for the indium antimonide wafer in the manner first employed by Barlow. The cavity, about one wavelength long, is separated from the waveguide in which power flow is to be measured by a thin brass wall, and is coupled to the waveguide by a rectangular slot in the top wall of the waveguide. The semiconductor wafer is placed in the center of the cavity, at a position having a maximum in magnetic field and a minimum in electric field for a given cavity excitation value. The electric probe, which extends from the cavity to the waveguide, provides excitation current for the semiconductor element. Three tuning adjustments are required, two for the cavity and one for the probe. A wattmeter embodying this technique has been found to provide a sensitivity of one dc microvolt per milliwatt of transmitted power at a frequency of 9.40 Gc.