tailieunhanh - Radio Frequency Identification Fundamentals and Applications, Design Methods and Solutions Part 3

Tham khảo tài liệu 'radio frequency identification fundamentals and applications, design methods and solutions part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 42 Radio Frequency Identification Fundamentals and Applications Design Methods and Solutions Fig. 4. Backscattered field as a result of changing the load impedance L 6cm a 1mm . Equation 9 assumes a polarization match between the antenna used by the reader and the antenna on the tag and that the tag is in the far-field of the reader. Several comments can be made about 9 . First the power at the tag depends very much on Gtg the gain of the spacefilling antenna on the RFID tag . A larger gain would mean more power for the passive IC on the tag. Second using a longer wavelength would improve the power at the tag. Third a good match between the passive IC and antenna is essential for improving power delivery to the passive IC on the tag. Fourth the power available to the tag reduces by the distance squared as the tag and reader antenna are moved apart. An alternate expression for 9 can also be derived. Solving for R in 9 results in the following expression Braaten et al. 2008 Rao et al. 2005 R ê J 4 yj QGrdG gPrd Ptg 10 Next if the threshold power required to activate the passive IC and communicate with the reader is denoted as Pth then a maximum read range rmax can be derived from 10 with a small substitution __ Ằ qG-rtfGtgPrd max - 4w 11 Ptd Equation 11 is a very useful expression for predicting the max read range of a passive RFID tag. Normally Pth of the passive IC used on the tag is known while Prd and Grd are fixed. This leaves the two variables Gtg and q available for a designer. Typically a tag is Design of Space-Filling Antennas for Passive UHF RFID Tags 43 designed to make the most of rmax. One method of maximizing rmax is to design a tag with a good value of q and a sizeable Gtg. In summary 11 effectively shows which antenna properties on the RFID tag will determine the max read range of the tag. In the next section the concept of effective apertures is presented and the maximum power available for the passive IC on the RFID tag is discussed. It will be .