Abstract: In order to meet the requirements of high gain, directionality and miniaturization of the antenna unit in the microwave imaging system for stroke detection, a planar monopole antenna with a size of 0.28λ×0.28λ×0.17λ (λ denotes the free space wavelength at lower frequency)is designed. The antenna is composed of a gradual radiating element and a slotted ground plate. The radiating element is composed of a combination of a semi-ellipse and a stepped rectangle, and the slotted structure of the ground plate is similar to the structure of the radiating element. The perfect electric conductor (PEC) is used as the reflector of the antenna to enhance the directional radiation characteristics of the antenna. The results show that the working frequency band of S₁₁ <-10 dB is 1.15-3.05 GHz, and the maximum gain reaches 7.58 dB, the radiation pattern of the antenna shows the characteristics of directional radiation in the whole working frequency band. The antenna is processed and tested, and the measured results are in good agreement with the simulation results. The antenna can be used in microwave detection of stroke and other fields.