Figure 8.116 Circular loop antenna with extended line fed by (a) internal line and (b) external line ([103], copyright C 2006 IEEE).

8.3.2.4Loop antenna

A small circular loop antenna (CLA) operating at 911.25 MHz is designed for application to an RFID tag [103]. A one-wavelength and a half-wavelength CLA with a short stub are fabricated as shown in Figure 8.116, where (a) depicts an antenna with an internal stub, while (b) illustrates an antenna with an outer stub. The antenna dimensions are given in the figure. The stub increases the loop length so that the current path is increased and the diameter of the loop is reduced. The ratios of size reduction compared with a general CLA are 83% and 92%, respectively, for the one-wavelength CLA and the half-wavelength CLA. The return loss, −10 dB bandwidth, and gain are −11.9 dB, 12 MHz (1.3%), and 1.18 dBi, for the one wavelength CLA and −16.5 dB, 48 MHz (5%) and −0.58 dBi, respectively.

Another example is a square loop, but a part of the element is taken out, leaving three corners, as shown in Figure 8.117(a) [104]. Two elements are used to form a quasi-Yagi type antenna, with one element as a driver and another as a reflector, in order to produce a directional pattern as depicted in Figure 8.117(b), and consequent higher gain. The antenna is designed to install in a hand-held RFID reader as illustrated in Figure 8.117(c) for its operation in the near-field environment. The total dimensions of the antenna are 115 × 115 × 13.8 mm for operation of 433.92 MHz. The 10-dB bandwidth is 2.31 MHz (0.53%), covering the required bandwidth of 433.67–434.17 MHz for the ISO/IEC standards. When the reader is held by an operator, the radiation pattern deviates slightly

from the desired direction, the orientation of the reader, because the operators typically hold the reader at a slant. Then, the gain is 2.5 dBi at angle θ = 20◦ (azimuth) at ϕ = 0◦ (elevation), while 2.1 dBi at θ = 0◦.

The antenna radiates circular polarization and the bandwidth in terms of both

impedance and for axial ratio less than 3, is 0.58% (433.23–435.7 MHz) and 1.01% (430.5–434.9 MHz) at θ = 0◦ and 20◦, respectively.

Figure 8.A4 Current and voltage at the feed point of a folded dipole (a) original mode, (b) and (b ) A mode, and (c) and (c ) B mode [27b].

using Za and Zb, impedances of A mode and B mode, respectively, seen at the terminals b–d,

When the length of antenna L is a half wavelength, that is, one side is a quarter wavelength, Za becomes infinite, then I1 = I2 = I, and (I1 + I2)/2 = I. This means that the current on the outer conductor of the coax (I1 – I2) disappears, and the same current I flows in and out from the coax, implying that a balun is not required. This is the