Abstract :
The calibration of microwave radiometers is normally achieved by replacing the antenna with a cooled reference termination and then applying corrections for ohmic and reflective losses of the antenna, and for the nonideality of the reference termination. The uncertainty in this correction is the dominating factor in the precision of many high-quality radiometers, and for some applications it is unacceptably large. An alternative is to point the antenna at a target of known temperature. To the extent that this temperature is known and the target is reflectionless and encompasses the full view of the antenna, the calibration is ex´act and no further corrections are required. A target suitable for high-precision radiometry has been developed that has an accurately known radiometric temperature, a very low reflection coefficient, and whose geometry is well suited to the calibration of horn antennas. The essence of the temperature reference is described. A piece of porous microwave absorber having a convoluted surface for a low reflection coefficient is fitted with a cap of nonporous plastic foam whose mating surface is the inverse of that of the absorber. The microwave-absorbing material is then soaked with the chosen cryogen, the nonporous cap forcing the liquid to conform to the shape of said absorbing material. This procedure 1) ensures that the temperature at which the microwaves are absorbed (and therefore thermally emitted) is exactly that of the cryogen, and 2) avoids the reflection that would be produced by the dielectric discontinuity of a plane surface of cryogen.
