A Wide-band Dual Circular Polarization UHF RFID Reader Antenna Based on Miniaturized Branch Line Coupler

In this paper, a dual circularly polarized (CP) antenna with broad bandwidth for ultra high frequency (UHF) radio frequency identification (RFID) reader application is presented. The proposed antenna consists of two layers of FR4 substrates with thickness of 1.6mm and one layer of air. The air layer can be utilized to achieve higher gain and broader bandwidth. The rectangular radiation patch is situated on the top surface of the upper substrate. To decrease the overall size of the antenna, a miniaturized branch-line coupler based on capacitance loading technology is used as the feeding network. And the miniaturized branch-line coupler is located on the bottom side of the lower substrate. One port of the coupler is used as input, one port is connected to a 50 ohm load, and other two ports are the output, which have a fixed 90 degree phase difference. To realize circular polarization radiation, the two output ports are placed orthogonal. The circular polarization of different rotation (left-hand circular polarization (LHCP) or right-hand circular polarization (RHCP)) can be realized when the input port and the load port are exchanged. The ground plane with a pair of bended slot is on the top side of the lower substrate. Energy is coupled to the radiation patch though the slot on the ground. To enhance the directionality of the radiation pattern, the slot has a length of 0.42*lambda(0), where lambda(0) is the wavelength of 0.9 GHz in free space. When one port is excited, the proposed antenna obtains a -10 dB bandwidth of 190 MHz (0.81-1.0 GHz) and 3-dB AR bandwidth of 160 MHz (0.85-1.01 GHz). The peak gain is 7.4 dBi at the frequency of 0.9 GHz. In the global UHF RFID band (0.84-0.96 GHz), the gain is greater than 3 dBi. Thus the proposed antenna can be well used in long distance read RFID system and can be used in different areas, such as America, Japan, Europe and so on.