Y

Lb

Wb

Wc X

Lc

Y

L

Z

h X

Figure 8.109 Geometry of an IFA top loaded with Hilbert trace [97].

Figure 8.110 A folded patch with H-shaped slot loaded with a chip ([99], copyright C 2010 IEEE).

by appropriately designing the size and the shape of the ground plane. With this antenna configuration, antenna performances can be improved, while achieving miniaturization of the antenna.

8.3.2.3Slot type antennas

A slot embedded folded rectangular patch is designed to apply to a wearable UHF RFID sensor tag [99]. Layout of the tag is illustrated in Figure 8.110, where dimensional parameters are also provided. The antenna is essentially a series-fed L-shaped patch and radiates from the open edge of the patch and the slot embedded on the planar element. A sensor chip is located in the middle of the gap between the two slots. Two prototype

Table 8.8 Parameters of two tags [mm] ([99], copyright C 2010 IEEE)

8 cm

Figure 8.111 Fabricated antenna TAG-1(left) and TAG-2 (right) ([99], copyright C 2010 IEEE).

RFID tags TAG-1 and TAG-2 are fabricated (Figure 8.111) and the parameters are given in Table 8.8. A mechanical motion-vibration sensor is integrated into the antenna structure. This kind of sensor is useful for detection of motion in medical applications, to assist diagnosis of some neurological diseases, involving compulsory arm movements in domestic healthcare, for example to track the behavior of aged persons [99]. The maximum reading range of TAG-1 is 1.5 m by using a short-range reader that emits not more than 0.5 W EIRP at the tag’s microchip power of −30 dBm, while 2.1 m for TAG-2. When a long-range reader (emitting up to 3.2 W EIRP) is used, it increases to 4 m for the TAG-1 and 5.5 m for the TAG-2.

The antenna has a size comparable to a credit card and can be applied to any part of the body. Improvement of the tag is concerned with realization of flexible conformal tags by applying textile technology.

A compact antenna using series OCSRR (Open Complementary Split Ring Resonator) is designed for the passive RFID tags [100]. The OCSRR unit is composed of a circular slot and a conductor loop inserted inside the slot as shown in Figure 8.112(a). Figure 8.112(b) gives the equivalent circuit of the OCSRR unit. The inductance LOCSRR

COCSRR

Figure 8.112 (a) An OCSRR unit and its dimensional parameters and (b) the equivalent circuit ([100], copyright C 2010 IEEE).

Figure 8.113 Proposed UHF RFID antenna comprised of series connected OCSRR units ([100], copyright C 2010 IEEE).

is created by the conducting loop between the two inner and outer ring slots and the capacitance COCSRR is between the inner conducting disk and the surrounding outer conducting planes. By connecting OCSRR units in series, and adjusting the size, input resistance and bandwidth can be increased, which is advantageous for matching of small tag antennas that generally have small resistance and high reactance components. Particularly with several units, the inductance becomes four times that of the conventional CSRR and makes for a feasible power harvesting circuit in the passive IC on the RFID tag. The antenna is comprised of four units of OCSRR as shown in Figure 8.113.

The antenna operates at 920 MHz and obtains a maximum reading range of 5.48 m with dimensions of 0.036λ0 × 0.17λ0 (λ0 is the free space wavelength at 920 MHz).

Other examples of RFID tags using slots are a thin L-shaped slot dipole loaded with a meander line at the end of the dipole as shown in Figure 8.114 [101], and a patch antenna on which circular slots are embedded to produce circular polarization as shown in Figure 8.115 [102].