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7.2 Design and practice of ESA

203

 

 

S11 (dB)

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40

 

 

 

 

 

 

 

Measurement DEA

 

 

 

 

 

 

 

 

 

 

 

4

5

6

7

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9

 

Simulation DEA

 

50

 

Measurement SEA 2

860

880

900

800

 

 

820

 

840

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.

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Design and practice of small antennas I

 

 

l

w

w

h

h

w

l

H

L

Figure 7.181 L-shaped self-complementary antenna [from 87].

The self-complementary structure has an inherent frequency independent property, which is infinite bandwidth, when it is constituted with an infinite structure. Unfortunately, practical antennas can never be realized in an infinite structure, as a truncation can never be evaded, thus bandwidth must always be finite. Although the bandwidth would be limited, antennas with complementary structure may still have wide enough bandwidth for practical applications; that is, an antenna is practically useful when the bandwidth is reasonably wide to satisfy the requirement. Hence, the complementary concept, even with truncation in the antenna structure – always encountered in small antennas – is adapted as a useful means to attain an appreciable bandwidth for an antenna of very small size.

7.2.4.1.1 L-shaped quasi-self-complementary antenna

One of the most simple self-complementary antennas is a monopole combined with a dual slot (Figure 6.34) introduced in [88, 89], in which wideband performance was demonstrated, even with an antenna of small size. Since a practical antenna can never be composed with infinite structure, it should be referred to as quasi self-complementary. Instead of a monopole, an Inverted-L antenna is combined with a dual L-slot to compose a quasi self-complementary structure (Figure 7.181) [89, 90]. The input impedance of the antenna with dimensions of l = 15 mm, h = 15 mm, and w = 4 mm, shows broad frequency characteristics covering about 2 GHz to 10 GHz for VSWR less than two (Figure 7.182(a)). However, the ground plane size (X × Y) was 46 mm × 60 mm, much smaller than one wavelength at the lowest frequency. Efficiency is sacrificed to a certain measure by using a load resistance 188 at the side opposite of the feed

7.2 Design and practice of ESA

205

 

 

VSWR

 

 

 

 

 

 

 

100

 

 

 

 

 

2

 

 

 

 

 

 

80

 

 

 

 

 

 

 

 

 

 

(%)

 

 

 

 

 

 

 

 

 

 

 

 

60

 

 

 

 

 

 

 

 

 

 

 

Efficiency

 

 

 

 

 

1.5

 

 

 

 

 

40

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20

 

 

 

 

 

1

 

 

 

 

 

 

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0

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Frequency (GHz)

 

 

 

 

 

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(b)

 

 

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).

40

 

 

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0

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y (mm)

 

20

–20

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–20

x (mm)

 

–40

 

 

–40

 

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

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Design and practice of small antennas I

 

 

S11 (dB)

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Measurement εr = 2.2

 

 

 

 

 

Simulation εr = 2.2

 

 

 

 

 

 

 

35

 

 

 

 

Simulation εr = 1.0

 

1.5

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4.5

1

Frequency (GHz)

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

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2

(dBi)gain

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Antenna

–2

 

 

 

 

 

 

–4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

–6

 

 

 

 

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–8

 

 

 

 

Simulation

 

 

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1

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

7.2 Design and practice of ESA

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W

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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L2

 

 

 

 

 

L1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Microstrip line

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

wf

 

 

 

 

 

Ground plane

 

 

 

 

 

 

ws

 

 

 

 

 

 

in back

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y

H

substrate

 

 

 

 

εr

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7.186 Geometry of half-disk shaped quasi self-complementary antenna ([92], copyrightC 2009 IEEE).

 

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Simulated

 

 

 

 

 

 

 

 

 

 

 

 

 

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Measured

 

(dB)loss

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25

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

30

 

 

 

 

 

 

 

 

 

 

 

 

 

35

 

 

 

 

 

 

 

 

 

 

 

 

 

40

1

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0

Frequency (GHz)

Figure 7.187 Simulated and measured return loss ([92], copyright C 2009 IEEE).

dual slot to construct a quasi-self-complementary structure. A triangular notch is adopted at the feed point on the ground plane to improve the impedance matching. The substrate has thickness H = 1.6 mm and the relative permittivity εr = 3.0. Measured and simulated return loss are shown in Figure 7.187, which indicates fairly wide bandwidth, covering 3 GHz to 10.7 GHz for the return loss less than –10 dB. Simulated gain is depicted in Figure 7.188, in which a gain of 3 dB over the wide frequency range is observed.

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