Multi-band operation is traditionally achieved in RFID antennas by using several resonant elements or exploiting high order harmonics. Some dual-band design solution has been recently proposed to achieve 870MHz and 2.45GHz or 2.45 GHz and 5.8 GHz compact multifunction transponders. The basic idea is to load a traditional tag antenna with parasitic tuning elements such as inner slots or tuning stub. The goal is to avoid a size increase, and the tuning elements are engineered to be embedded inside the radiating element itself. Figure 16 shows four examples of dual band tags.
Figure 16: Examples of dual-band tag designs
a) A planar antenna with shaped slots used to both achieve two working frequencies (resonances or complex impedance matching) at 868MHz and 2450 MHz, and to reduce the antenna size at the smaller frequency
b) Microstrip Sierpisky Gasket printed antenna optimized for dual band operations at 2.45 GHz and 5.8 GHz. A microstrip line feeds the first iteration of the Sierpinsky fractal metallization and the ground plane under the radiating element is removed. The metallic triangle’s size fixes the fundamental mode while the inner triangular slot the second resonance.
c) Slot-loaded dipole, for operations at 870MHz and 2450 MHz. A coupled slot is here introduced within the dipole conductor whose global length fixes the lower working frequency. The slot acts as a sort of current trap which breaks off the dipole into a smaller length conductor working at the higher frequency. Additional slots, or any other lumped and distributed impedance, can be introduced to further tune the impedance.
d) PIFA with open tuning stub. By changing the width and the length of the stub, the antenna impedance can be matched to the microchip at two different frequencies (870 MHz, 915 MHZ) at same time. |
