Summary
International Symposium on Antennas and Propagation
2010
Session Number:4FC4
Session:
Number:4FC4-1
Circularly Polarized Coplanar Crossed Dipole Antenna for RFID Handheld Reader
Yong-Hao Huang, Hua-Ming Chen, Zhong-Zhe Yang, Yang-Kai Wang, Yi-Fang Lin,
pp.-
Publication Date:2010/11/23
Online ISSN:2188-5079
Summary:
Radio frequency identification (RFID) system in the ultra-high frequency (UHF) band has been receiving much interest in several service industries, purchasing and distribution logistics, manufacturing companies and goods flow system [1]. The RFID system generally consists of the reader and the tag, and the UHF RFID system operates at the bands of North America (902-928 MHz), Taiwan (920-928 MHz) and Europe (865-867 MHz). The reader with an antenna sends a RF signal to a tag and receives a backscattered signal from the tag. The reader antenna is one of the important components in RFID system and has been designed with CP operation, because circularly polarized antennas can increase orientation diversity at the cost of polarization mismatch losses between the reader and the tag antenna. Moreover, a CP antenna with a low profile, small size, and light weight is needed in RFID handheld reader. A typical technique for generating CP radiation is to excite two orthogonal degenerate resonant modes on the radiator with a 900 phase difference. Single-fed circularly polarized annularring, square and circular patch antennas with symmetrical or asymmetrical perturbation elements are reported [2]. Using perturbation cuts or strips to suitably differentiate the two orthogonal modes at resonant frequency, the antenna can easily radiate CP wave. However, those axial ratio (AR) bandwidths are usually narrow, which does not meet the bandwidth requirements in modern communication systems. For the design of the low profile patch antennas for the RFID reader applications, circular polarization can be generated from the slot-coupled radiation elements [3] or by employing a Wilkinson power divider feeding network [4]. Those feeding mechanisms are attractive for the broadband CP antenna designs for portable RFID applications, but those size are relative large for the requirement of handheld device. To improve the operating bandwidth and not increase the antenna size, using the planar crossed dipole antenna is a possible method. In [5, 6], it was shown that single-fed orthogonally crossed dipoles connected in parallel could generate CP radiation. If the lengths of the two dipoles were chosen such that the real parts of the input admittances are equal and the angles of the input admittances differ by 900 that the radiated wave in a normal direction will be circularly polarized. According to these conditions, several crossed dipole antennas were developed to improve the bandwidth and AR bandwidth [7, 8]. In this Letter, a small and single-layer coplanar crossed dipole antenna with meander arms has been proposed to obtain CP radiation. The proposed antenna uses two unequal length dipoles connected in parallel to a coaxial cable feed and requires no matching network. The antenna operates at its half wavelength of orthogonal modes for the UHF band, and the two resonant modes for CP radiation are generated by the unequal length of dipole arms. Details of the antenna design and the obtained experimental results are presented and discussed.