This paper presents a modified design of directional monopole antenna with parabolic-shaped ground plane. To increase the directivity, axis of parabola in the ground plane is rotated 45 degrees (in comparison with the previous antenna) to extend throughout the direction of the substrate's diagonal. Consequently, vertex of the parabola is placed at the optimum point in the corner of the substrate. The aim of this attempt is to design an extended and symmetrical ground plane around the patch, with more clarity, to maximize its capability as a reflector. Directivity is further improved by inserting parabolic-shaped slots at the corners of the ground plane. Simulation and measurements show that the proposed antenna has stable directional radiation pattern and higher gain compared to the previous directional monopole antennas. Impedance bandwidth of the antenna covers the frequency range of 4-9 GHz. Measured HPBW is among the degrees 54-22 between 4 and 9 GHz. Gain and HPBW of the antenna are improved 1.3-3.1 dB and 5-15 degrees, respectively among the bandwidth in comparison with previous antenna. Results confirm the good characteristics of the antenna for use in microwave imaging, where high resolution is required.
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