Multifrequency microwave radiometry for non-invasive measurement of temperature in biological objects has been investigated in our laboratory. An open-ended rectangular waveguide filled with a dielectric has been used as a contact-type antenna of a radiometer operating over a 1-4GHz range. In the radiometric measurement, the radiometer measures the thermal radiation emitted by the object via the antenna as the brightness temperature. The brightness temperature is related to the physical temperatures in the object through the radiometric weighting function. By virtue of the reciprocity of antenna, the weighting function can be derived from the field distribution induced in the object by the same antenna when it is operated in the active mode. In this paper, the FD-TD method is used to analyze the problem of coupling between the rectangular waveguide antenna and a biological object. The objects studied in this paper are a homogeneous and a four-layered lossy media. Working frequency is 1.2GHz, which is the center frequency of the lowest-frequency band of our radiometer. Numerical results are presented in the form of SAR patterns. It is found that the SAR patterns tend to spread out in the lateral directions in the bolus, skin and fat layers due to the diffraction which becomes stronger at lower frequencies. Results also suggest that the lateral spreading can be controlled to a certain extent by choosing the size elf antenna flange properly.

An open-ended rectangular waveguide filled with a dielectric has been used as a contact-type antenna of microwave radiometer for non-invasive measurement of temperature in a biological object. In this application, the thermal radiation emitted by the object is measured as the brightness temperature by the instrument via the antenna. The brightness temperature is related to the physical temperatures in the object through the radiometric weighting function. By virtue of the reciprocity of antenna, the weighting function can be derived from the field distribution induced in the object by the antenna when it is operated in the active mode. In this work, we treat a problem of the rectangular waveguide antenna radiating into a four-layered medium by modal analysis. The results are first compared with those obtained by the FD-TD method to indicate that the results of the two methods are in a good agreement. The operation of an antenna used in a radiometer system in our laboratory is analyzed by this method and the weighting functions at different frequencies are computed, and the results are presented along with discussions on the results.