Frequency (MHz)

Figure 7.180 Return loss of DEA and SEA2 [87].

7.2.4Increase of excitation mode

By increasing the number of excitation modes (for instance, addition of a TM mode to a TE mode) enhancement or improvement of antenna performances such as gain, efficiency, bandwidth, and radiation pattern, can be expected. Composing an antenna with both TE and TM modes is one of the simplest ways. Examples are combination of a dipole with a loop, and a monopole with that of a dual slot that constitutes a selfcomplementary structure. Other types are a combination of an inductive element with a capacitive element in an antenna system that makes a conjugate structure. With the conjugate structure, the self-resonance condition can easily be achieved, even though the antenna has very small dimensions. A combination of an electric source with a magnetic source will bring out a conjugate structure as well as a complementary structure. A composite antenna system constituted with different types of antennas having different excitation modes is also used to create a conjugate antenna structure.

These means facilitate enhancement of the bandwidth, addition of functions such as multiband and multiple polarization, and so forth, in small-antenna design.

7.2.4.1Self-complementary structure

The self-complementary structure can be implemented by combining two antennas, having complementary properties of each other. There are two types; one type has rotationally symmetric structure while another type has axially symmetric structure. In a planar structure, for example, a rotationally symmetric type is constituted from one arbitrary generating structure by rotating it 180 degrees with respect to the feed point. An example was shown previously in Figure 6.35. In contrast, an axially symmetric type is fabricated by combining an arbitrary structure on a half-infinite space with a structure complementary to it on the other half-infinite space, with axial symmetry to each other (Figure 6.35). In a practical fabrication, a half structure divided by the axis of symmetry is formed on a half-infinite PEC (perfect electric conductor) plate, while another half is formed with the same structure, but as spaces on another half-infinite PEC plate. Figure 6.34 illustrated this example, showing a monopole combined with a complementary slot on the ground plane.

Figure 7.182 (a) VSWR characteristics of L-shaped self-complementary antenna and (b) efficiency of L-shaped self-complementary antenna ([88], copyright C 2002 IEEE).

Figure 7.183 Self-complementary H-shaped antenna ([91], copyright C 2007 IEEE).

as needed to maintain the complementary condition [89]. However, with this size of antenna, efficiency was observed as high as 70% almost over the frequency range of 2 GHz to 10 GHz (Figure 7.182(b)). The load resistance may be omitted to improve efficiency, if some amount of sacrifice in the bandwidth is allowed. If a smaller ground plane is used, as in practical applications to small mobile terminals, the bandwidth will become narrower, but still wide enough for practical applications.

7.2.4.1.2 H-shaped quasi-self-complementary antenna

The antenna geometry is illustrated in Figure 7.183, in which a short-circuited microstrip line used for feeding the antenna and dimensions of antenna with the scale are shown [91]. A dielectric substrate used has εr = 2.2 and the thickness is 0.787 mm. Measured and simulated return loss and gain, respectively, are shown in Figure 7.184 and Figure 7.185. The figure demonstrates a wide bandwidth covering about 1.35 GHz to 3.2 GHz for the

Figure 7.184 Measured and simulated return loss of H-shaped antenna ([91], copyright C 2007 IEEE).

4

2

Frequency (GHz)

Figure 7.185 Comparison of measured and simulated gain ([90], copyright C 2003 IEEE).

return loss below –10 dB with the substrate εr = 2.2, whereas when εr = 1, meaning no dielectric substrate, a considerably wider bandwidth (1.3–3.9 GHz) is obtained. The gain obtained is about 1 dBi over the frequency range of 1.3 GHz to 3.5 GHz.

7.2.4.1.3 A half-circular disk quasi-self-complementary antenna

Antenna geometry with dimensional parameters is illustrated in Figure 7.186 [92], showing that a printed semi-circular disk on a dielectric substrate is combined with its