Platinum Metals Rev., 1964, 8, (1), 15
A High Temperature Waveguide Termination
Platinum as Both Guide and Heating Element
In measuring the output from various noise sources by means of a radiometer, a standard noise source is required to calibrate the instrument. This generally takes the form of a hot load, i.e. a termination of known temperature connected to the radiometer by a suitably matched waveguide. For most microwave receivers, a load immersed in boiling water can be used.
Because of the high noise level of microwave receivers in the short millimetric band, a relatively high output is also necessary from the standard noise source if accurate calibration of the radiometer and hence the receiver is to be achieved. This makes it necessary to operate the hot load at an extremely high temperature. In a recent paper, Q. V. Davis of the Royal Radar Establishment, Great Malvern, describes (J. Sci. Instr ., 1963, 40, (11), 524) a hot load that has been developed for operation at 1250°C. This load comprises a one inch long, conically tapered rod of pyrophyllite operating at this temperature in a heated circular waveguide.
In the construction of this waveguide assembly platinum sheet 0.005 inch thick is formed around a mandrel to make the round waveguide 0.143 inch in diameter, the ends of the sheet being extended parallel to each other to copper blocks. The platinum is resistance heated by a current of 500 amp passed through the copper blocks. The whole assembly is enclosed in laminated asbestos to reduce heat losses.