COMPUTATION OF THE FIELD DISTRIBUTION GENERATED BY A RECTANGULAR APERTURE IN A 4-LAYERED LOSSY DIELECTRIC MEDIUM BY MODAL-ANALYSIS

An open-ended rectangular waveguide filled with a dielectric has been used as a contact-type antenna of microwave radiometer for non-invasiv e measurement of temperature in a biological object. In this applicati on, the thermal radiation emitted by the object is measured as the bri ghtness temperature by the instrument via the antenna. The brightness temperature is related to the physical temperatures in the object thro ugh the radiometric weighting function. By virtue of the reciprocity u f antenna, the weighting function can be derived from the field distri bution 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 wave guide 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 agreeme nt. The operation of an antenna used in a radiometer system in our lab oratory is analyzed by this method and the weighting functions at diff erent frequencies are computed, and the results are presented along wi th discussions on the results.