Figure 7.197 Simulated and measured VSWR responses [95].

The slot lines 1 and 2 form an asymmetric coplanar waveguide (CPW). The loop acts as a matching stub for transmission from the CPW to the slot line 1 as was shown in the CPW-fed TSA design [96]. Figure 7.197 shows both simulated and measured VSWR characteristics, illustrating a wide frequency operation covering 2.82 to 10.6 GHz for VSWR of less than 2. In the figure, the simulated VSWR response of a TSA only is also shown along with geometry of the corresponding TSA as an inset.

It is noted that the VSWR response of the TSA takes on values higher than 2 over all frequencies except around 5 GHz, suggesting existence of uncompensated excess electric energy before combining the ER source with the MR source. As a consequence of combination of two sources, the excess energy of the ER source is compensated by the excess energy of the MR source, resulting in lowering the VSWR significantly. This implies, in other words, self-resonance in an antenna system, in which the time averaged electric and magnetic energies in the vicinity of the antenna system are balanced.

7.2.4.3Composite structure

7.2.4.3.1 Slot-monopole hybrid antenna

A coplanar waveguide (CPW)-fed inductive slot antenna combined with a monopole antenna, featuring a dual-band operation is introduced in [97]. It is a common understanding that CPW antennas can provide relatively wide bandwidth, be easily integrated with surface-mount devices, and be designed to show dual-band operation [98, 99]. Figure 7.198 illustrates the antenna system, a CPW-fed inductive slot combined with a bifurcated L-shaped monopole, showing a dual-band operation as is depicted in Figure 7.199, which gives return losses of antennas, including those of a bifurcated L-shaped monopole (Figure 7.200) and a CPW-fed inductive slot (Figure 7.201) for a comparison. The antenna dimensions are given in each figure. The antenna is fabricated on the dielectric substrate having εr = 4.4 and thickness of 1.6 mm. The CPW line has a strip width of 4 mm and a gap width of 0.4 mm, corresponding to a 50

Figure 7.198 Dual-band CPW-fed slot and L-shaped monopole antenna ([97], copyright C 2008 IEEE).

0

Frequency (MHz)

Figure 7.199 Measured and simulated return-loss characteristics ([97], copyright C 2008 IEEE).

characteristic impedance. The stand-alone CPW-fed slot is designed for the higher band 5.4 GHz operation, while the bifurcated L-shaped monopole is for the lower band

2.4GHz operation. Other types of monopole having bifurcated I-shape or F-shape are designed and studied. Gain of the antenna is measured as 1.51 dB at 2.45 GHz and 4.91 at

5.2GHz, and the relative bandwidth of the antenna is 11.4% at 5.3 GHz and 7.8% at

2.5GHz, while the stand-alone CPW-fed slot antenna is 10.5%.

Figure 7.201 CPW-fed inductive slot antenna ([97]).

7.2.4.3.2 Spiral-slots loaded with inductive element

A miniaturized cavity-backed slot loop low-profile antenna was introduced in [100] (Figure 7.202). It is referred to as CBCSLA (Cavity-Backed Composite Slot Loop Antenna). The antenna is essentially a small magnetic loop, radiating similarly to that of a small electric dipole; that is, omnidirectional pattern in the horizontal plane with vertical polarization. The geometry is designed to be small and low height, having the diameter as small as λ/10 and the height less than λ/100. The design concept is to modify a simple slot loop; firstly by embedding it in a shallow cavity, secondly meandering it to reduce the size, and thirdly sectionalizing the structure into six λ/2 slots around the circle to accomplish resonance and achieve sufficient input impedance matching. The process of topology change is shown in Figure 7.203(a), (b), and (c). In the figure (c),