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A polarisation diversity antenna for an ultra-high-frequency RFID tag is presented. The antenna consists of two dual-planar inverted-F antennas arranged in a cross-configuration, and mountable on a metallic object. The antenna can work together with a linearly polarised reader antenna that has an arbitrary polarisation direction. The proposed tag antenna has a nearly uniform read range between 9.1 and 10.2 m, regardless of the polarisation direction of the reader's transmit field.

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rameters unchanged. The impedance characteristics of the antenna for different values of a (i.e., X f ) are shown in Figure 2(a).The input resistance component of the resonant antenna R a has a simple dependence on the distance D. In the orthogonally proximity-coupled structure, the coupled resistance of the radiating patch per unit length is almost uniform along the feedline, and, therefore, the total coupled resistance can be easily controlled by simply varying the distance D while keeping the feedline dimensions unchanged. As the distance D goes up, the input resistance R a increases and, therefore, the diameter of the ␣-shaped impedance locus increases. The impedance characteristics of the antenna for different values of D are shown in Figure 2(b).As illustrated in Figure 2, the proposed antenna presents a simple and easy way to match its impedance to an arbitrary tag chip impedance. In the prototype antenna the distance D and the self-reactance of the feedline X f were tuned to maximize the 3-dB return loss bandwidth with some margin for fabrication tolerances. The measured impedance locus is shown in Figure 2(a), which agrees well with the simulated one. The measurement was carried out with the antenna placed at the center of a 400 ϫ 400 mm 2 metal plate. Figure 3 shows the simulated and measured return losses of the antenna with respect to Z c . The measured 3-dB return loss bandwidth is 44 MHz which is a little wider than the simulated value of 35 MHz. From the result, we can see that the prototype antenna totally covers the 26 MHz bandwidth requirement in North America. Figure 4 shows the simulated radiation efficiency of the antenna considering the ohmic and dielectric losses, and an efficiency of 28 -66% is obtained in the 902-928 MHz band. The simulated directivity of the antenna on an infinite metal surface is about 6 dBi. Figure 5 presents the measured co-and cross-polarization radiation patterns of the antenna placed at the center of the 400 ϫ 400 mm 2 metal plate. The antenna shows good crosspolarization performance, with the cross-polarization level of Ϫ20 dB. CONCLUSIONA low-cost, wideband patch antenna using an orthogonally proximity-coupled feed has been proposed for RFID tags on metallic surfaces. The antenna can be directly matched to an arbitrary complex impedance of a tag chip and has wideband characteristics. The prototype of the antenna was fabricated and measured, and 44 MHz bandwidth at 3-dB return loss has been achieved. The antenna also shows good cross-polarization performance. STOPBAND-IMPROVED DUAL-MODE BANDPASS FILTER USING SIDE-SLIT PATCH RESONATOR