We report here a study of the influences of the height and diameter of applicator antennas for microwave radiothermometry on radiobrightness temperature measurement results. The electric fields of three antennas based on numerical solution of Maxwell’s equations were modeled mathematically. The depth of the area whose temperature was measured was found to depend linearly on antenna diameter. An equation estimating the potential measurement depth of the antenna depending on its diameter for breast tissues was derived. A significant reduction in the height of the antenna (by a factor of six compared with traditional antennas) was found to have virtually no effect on the measured radiobrightness temperature in the projection of a malignant tumor, which is very important when designing flexible textile antennas.