Reduction of Proximity Effects on UHF Passive RFID Systems by Using Tags With Polarization Diversity
A new technique that enables passive ultra high frequency (UHF) radio frequency identification (RFID) tags to be read when they are placed in close proximity in an array is presented. This paper demonstrates that, in a linear tag array with a tag separation of 1 cm, the interaction between the backscattered waves and incident wave causes a significant degradation in tag sensitivity. It is found that the use of tags that have polarization diversity can improve the read performance when they are placed in close proximity to one another compared with conventional linear tags. Two ways of achieving polarization diversity are studied in this paper, namely: 1) using a circularly polarized tag and 2) using a cross-polarized tag pair. Both methods show an improvement in close proximity read performance and it is experimentally demonstrated that by using cross-polarized tag pairs in an array, one achieves on average a 2.6-dB increase in read power margin for a 57-tag array with 1 cm separation compared with using conventional linearly polarized tags.
Two rectangular loops fed in series for broadband circular polarization and impedance matching
Two rectangular wire loops above an infinite ground plane are investigated for broadband circular polarization and impedance matching. Each rectangular loop is connected to each end of a short dipole fed by a coaxial cable through the ground plane. One gap on each loop is introduced to get circular polarization and the gap position controls the sense of circular polarization. The position of the gap, the length of the short dipole, the height of the loop above the ground plane and the shape of the rectangle are very important to get a good axial ratio (AR) and voltage standing-wave ratio (VSWR). The AR bandwidth (/spl les/3 dB) and the VSWR bandwidth (/spl les/2) are 18% and 22%, respectively. The measured and computed results are in good agreement.
A Ubiquitous and Interactive Zoo Guide System
We describe a new prototype for a zoo information system. The system is based on RFID and allows to retrieve information about the zoo animals in a quick and easy way. RFID tags identifying the respective animals are placed near the animal habitats. Zoo visitors are equipped with PDAs containing RFID readers and WLAN cards. The PDAs may then read the RFID tag IDs and retrieve respective HTML-documents from a zoo Web server showing information about the animals at various levels of detail and languages. Additionally, the system contains a JXTA and XML based peer-to-peer subsystem, enabling zoos to share their content with other zoos in an easy way. This way, the effort for creating multimedia content can be reduced drastically.