Chapter 3 - UHF RFID Antennas

Figure 3.1 commercially UHF RFID tags

Figure 3.2 Electric and magnetic fields around a dipole antenna

Figure 3.3 Simple circuit model of dipole antenna near resonance

Figure 3.4 Radiation pattern around a dipole antenna

Figure 3.5 Relationship between cylindrical and ribbon dipoles

Figure 3.6 Straight vs. meandering dipole

Figure 3.7 commercial tip-loaded dipole tags

Figure 3.8 Spiral-loaded tag

Figure 3.9

Figure 3.10

Figure 3.11 Uda's circuit model of T-match

Figure 3.12 Smith chart view of impedance matching using the T-

match

Figure 3.13 Intuitive inductor-based circuit model for the T-

match

Figure 3.14 New inductor-based circuit model of the T-match dipole

Figure 3.15 Transformed circuit model of RFID tag

Figure 3.16 Meandering dipole used in the first cut

Figure 3.17 Circuit model of proposed antenna

Figure 3.18 Return loss for circuit with f0 = 897 MHz, β = 0.165

Figure 3.19 Return loss for circuit with f0 = 890 MHz, β = 0.17

Figure 3.20 Circuit predicted impedance looking into antenna

Figure 3.21 Geometry of matching circuit

Figure 3.22 Geometry of the completed antenna

Figure 3.23 Return loss of circuit and simulated antenna

Figure 3.24 Radiation pattern of designed antenna

Figure 3.25 Tag on semi-infinite dielectric

Figure 3.26 Tag operating above a ground plane, shown with image

Figure 3.27 Impedance of a near-resonant dipole above a 300 mm2

ground plane

Figure 3.28 Microstrip antenna fed by a microstrip transmission line

Figure 3.29 Transmission line model of a microstrip antenna

Figure 3.30 Traditional unbalanced way to feed the microstrip antenna

top

bottom

Figure 3.31 Narrowband microstrip antenna

Figure 3.32 Large, wideband microstrip antenna

Figure 3.33 Impedance of variable length dipole in free space (left) and 3.2 mm separation from an

infinite metal ground plane (right)

Figure 3.34 Permissible region

Figure 3.35 A prototype microstrip antenna

Figure 3.36 A combined dipole / microstrip antenna

Figure 3.37 Circuit model of combined dipole / microstrip

antenna functioning in air

Figure 3.38 Circuit model of combined dipole / microstrip antenna functioning on metal

Figure 3.39 (a) dipole / microstrip antenna in air

Figure 3.39 (b) dipole / microstrip antenna on metal

Figure 3.40 (a) Power transfer efficiency of combined dipole /

microstrip antenna in air

Figure 3.40 (b) Power transfer efficiency of combined dipole /

microstrip antenna on metal (b).