The temperature rise in a material modifies its physical properties, particularly its dielectric permittivity. In many applications involving relatively high levels of power the electrical behavior of the different materials will change as they are heated by the radiation. For instance, the numerical codes that simulates the behavior of microwave heating processes in order to improve the design of the feeding antennas must take into account the load variations with temperature. The measurement of the changing dielectric characteristics of materials is of great interest for the industry. Previous works used completely filled waveguides near a shorting plate. The method supplied excellent results at low power levels, while the sample was not heated. When the power is risen, the field distribution of the TE/sub 10/ mode causes a non-uniform heating of the sample and the measured permittivity corresponds to an average value. In order to reduce this problem a method using a partially filled waveguide is presented. The reduced sample dimensions and its positioning into the waveguide assures a near homogeneous power distribution implying a uniform heating.

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