Contents I. Introduction
In the past few years, the microstrip patch antenna is widely researched in modern communication and radar system because it can be easily integrated with many circuits such as filters, amplifiers, oscillators, and mixers [1]–[3]. Despite these attractive features, the inherent disadvantage of the microstrip antenna is its narrow bandwidth. Many techniques are utilized to broaden the bandwidth of microstrip antennas. In [4], a microstrip U-slot rectangular structure is introduced and a wide −10dB impedance bandwidth of 18% ranging from 5.65 GHz to 6.78 GHz can be achieved by adjusting the size of the U-slot. However, a thick substrate is adopted, which will increase the weight of the antenna. In [5], the dual-beam E-shaped structure, which operates at the higher order TM02 mode instead of the dominant TM01 mode, is proposed. In addition, the −10dB impedance bandwidth reaches 11.8% at the center frequency of 5.5 GHz. The probe feed is adopted in this paper, which will bring the difficulties in forming the array. In [6], two-layer L-probe wideband patch antenna is presented and the −10dB impedance bandwidth is 36% at 4.75 GHz when the dielectric constant is 1.0. Differential CPW-Fed Tapered-slot structure is proposed. It has lower cross polarization due to its differential structure. It is found that - 10dB impedance bandwidth is from 2.78 GHz to 12.3 GHz, along with a sharp notch band from 5.2 GHz to 6.0 GHz. Obviously, the multilayer structure is adopted, which will bring the difficulties in design and manufacture [7]. Similarly, incorporating coplanar parasitic elements, stacked structures, special feed mechanisms [8]–[10] are proposed to broaden the bandwidth.
