Antenna length (L/λ)

Figure 7.34 Radiation efficiency of a folded MLA ([14], (copyright C 2005 IEEE).

Lf = 10 mm

L = 43 mm

Figure 7.35 A trial model of 0.1λ folded MLA ([13], copyright C 2004 IEICE).

W = 35 mm (0.08λ), Lf = 10 mm (0.02λ), d = 1.7 mm (4.0 × 10−3 λ), s = 1.4 mm (3.3 × 10−3 λ), and the thickness of the element made with thin copper plate t = 0.1 mm (2.3 × 10−4 λ) (Figure 7.35). Measured impedance characteristics of both dipole and folded types are shown in Figure 7.36. The folded type is shown to have higher impedance than that of the dipole type as was mentioned before. Radiation patterns of both dipole and folded types are given in Figure 7.37. Gain is evaluated as –5.99 dBd for the dipole type and –2.28 dBd for the folded type, respectively. The gain obtained by using the IE3D simulator was –5.2 dBd for the dipole type and –1.5 dBd for the folded type, showing higher gain of about 3.7 dB in the folded type [13].

7.2.1.1.1.1.6 MLAs of length L = 0.05 λ [13, 14]

Smaller antennas with length 0.05λ were studied and shown to be practically useful, with appreciably high gain of –12 dBd for the dipole type and –10 dBd for the folded type, respectively, even in this small size [14]. A type of MLA, which is sandwiched by two planar dielectric substrates of high dielectric constant εr (Figure 7.38), was developed, and the antenna performances were studied. Figure 7.39 provides dimensional

Figure 7.39 Dimensional parameters and radiation performances of 0.05λ MLAs for cases εr = 1 and 10 ([14], copyright C 2005 IEEE).

Figure 7.40 A trial 0.05λ folded MLA model ([13], copyright C 2004 IEICE).

parameters and antenna performances of 0.05λ MLAs of both dipole type and folded type. In the figure, (a) gives results when the substrate εr = 1, while (b) represents the substrate εr = 10. A trial model is depicted in Figure 7.40, which has an asymmetric structure, with the element width d of the driven element (0.2 mm) different from that of the folded element (0.4 mm) in order to keep the loss resistance as small as possible. Antennas are designed for the resonance frequency of 700 MHz for both the

Figure 7.45 Radiation patterns of 0.05λ MLA for different N ([14], copyright C 2005 IEEE).

200 400 600 800 1000

Frequency (MHz)

Figure 7.46 Return-loss characteristics of a MLA shown in Figure 7.6 ([5], copyright C 2006 IEICE).

pattern parallel to the ground plane on the front side, while vertical on the back side. The return-loss characteristics, gain, and calculated radiation patterns, respectively, at 600 MHz are given in Figures 7.46–48. In Figure 7.48, (a) and (b) illustrate radiation patterns on the azimuth plane (y–z plane) and the elevation plane (x–z plane), respectively, for the antenna on the front side, and (c) and (d) show those on the azimuth plane and the elevation plane, respectively, for the antenna on the back side. The gain is evaluated to be greater than –3 dBd in the frequency range between 550 MHz and 800 MHz, and the return-loss characteristic is less than –5 dBd for the frequency range of 500 MHz to 700 MHz. This antenna is designed to receive UHF band terrestrial TV broadcasting.