2. Antenna design

The geometry of the antenna structure is demonstrated in Figure 1, and its optimized dimensions are listed in Table 1. It is etched on a 1.6 mm thick FR-4

epoxy substrate having a relative permittivity of 4.4 and loss tangent of 0.02. All the dimensions are optimized by using finite element method (FEM)-based Ansoft's high-frequency structure simulator (HFSS) [30]. During simulation, the radiating patch and ground planes are assumed to be perfect electrical conductors. The

The fourth iterative fractal geometry of radiator is derived from a rectangular monopole by loading it with a pair of triangular notches on its edges. The flow chart of designing the intermediate design steps is presented in Figure 2. The intermediate design steps for radiator geometry are shown in Figure 3. The iteration structure

Rn ¼ Rn�<sup>1</sup>r

<sup>n</sup> (1)

antenna structure is designed in two steps.

Flow chart of designing initial radiator geometry.

dimensions are governed by Eq. (1):

Table 1.

Figure 2.

43

Dimensions of the designed polarization diversity antenna.

DOI: http://dx.doi.org/10.5772/intechopen.86071

Inner Tapered Tree-Shaped Ultra-Wideband Fractal Antenna with Polarization Diversity

Figure 1. Geometry of the antenna.

Inner Tapered Tree-Shaped Ultra-Wideband Fractal Antenna with Polarization Diversity DOI: http://dx.doi.org/10.5772/intechopen.86071


#### Table 1.

For good diversity performance, the received signals should have very low correlation between them [7]. An increase in correlation reduces the combining efficiency. In a spatial diversity scheme, a large separation (compared to wavelength) between the antennas is used to achieve decoupling between signals. This large space requirement limits the use of this diversity method. To overcome this drawback, other techniques such as pattern or polarization diversity [8, 9] are investigated. These alternate techniques involve the use of two or more antenna elements with different radiation patterns [10]. An UWB system with polarization diversity technique has potential applications in advanced instruments used for microwave imaging, radar and high-speed data transfer. Some UWB polarization diversity antennas are already reported in the literature [11–28]. However, the application of those available structures is limited due to their large dimensions,

Among the various bandwidth enhancement techniques, the use of fractal geometries is proven to be a good method. Fractal antenna structures have a compact size and wideband performance due to properties of self-similarity, space

In this chapter, a compact CPW-fed UWB fractal antenna with polarization diversity performance is presented. The bandwidth of the antenna structure [29] is enhanced by loading the coplanar ground planes with a quarter wavelength long rectangular notches. Two identical copies of this antenna structure are arranged orthogonally to achieve good interport isolation and orthogonal polarization diversity performance without affecting the UWB performance. In the following sections, antenna design description is followed by discussion of frequency domain analysis results, time domain analysis results and diversity performance parameter calcula-

The geometry of the antenna structure is demonstrated in Figure 1, and its optimized dimensions are listed in Table 1. It is etched on a 1.6 mm thick FR-4

multilayer structure, complex feedline, complex geometries, etc.

tion. Finally, it is concluded with major findings of this chapter.

filling and effective energy coupling properties [29].

UWB Technology - Circuits and Systems

2. Antenna design

Figure 1.

42

Geometry of the antenna.

Dimensions of the designed polarization diversity antenna.

#### Figure 2.

Flow chart of designing initial radiator geometry.

epoxy substrate having a relative permittivity of 4.4 and loss tangent of 0.02. All the dimensions are optimized by using finite element method (FEM)-based Ansoft's high-frequency structure simulator (HFSS) [30]. During simulation, the radiating patch and ground planes are assumed to be perfect electrical conductors. The antenna structure is designed in two steps.

The fourth iterative fractal geometry of radiator is derived from a rectangular monopole by loading it with a pair of triangular notches on its edges. The flow chart of designing the intermediate design steps is presented in Figure 2. The intermediate design steps for radiator geometry are shown in Figure 3. The iteration structure dimensions are governed by Eq. (1):

$$R\_n = R\_{n-1}r^n \tag{1}$$

where:

CST MWS [31].

3. Results and discussion

3.1 Frequency domain

sions than other structures.

45

n = iteration number = 2 or 3.

r = iterative ratio = 0.4.

Rn = dimension of the nth iteration.

DOI: http://dx.doi.org/10.5772/intechopen.86071

Lr = electrical length of slot for resonance. fr = resonance frequency = 18.6 GHz.

higher frequency with an additional resonance.

R1 = dimension of the first iteration, i.e. L1, W1 and Wm1.

Lr <sup>¼</sup> <sup>c</sup> 4f <sup>r</sup>

In the first step, the dimensions of radiating patch are unaltered. The bandwidth

In the second step, two identical structures designed in the first step are placed orthogonal to each other. The air gap between the two structures and their locations are optimized. All the frequency domain results are calculated by using HFSS. The time domain results and diversity performance parameters are analysed by using

The intermediate antenna design steps are compared in terms of their S11 curves

The designed diversity antenna structure is simulated by using HFSS and CST MWS simulators. The variations of simulated scattering parameters with frequency are demonstrated in Figure 6. The quantitative analyses of bandwidth for two antenna elements used in designed antenna structure are presented in Table 3. From Figure 6 and Table 3, it is observed that there are some discrepancies among the two simulation results. These discrepancies can be attributed to the different mesh size suitable for numerical techniques on which the simulators are designed. In addition to mesh size, it is also important to mention that in CST MWS the structure can be solved in single pass instead of solution for different frequency spectrum, i.e. 1–2, 2–4, 4–8, 8–16 and 16–32 GHz in HFSS. The differences between the S11 and S22 characteristics are due to asymmetrical structure with respect to substrate. A good isolation of more than 15 dB is achieved. The designed antenna

The comparison among the designed antenna and previously reported polarization/pattern diversity antenna structures is listed in Table 4. It is observed that the designed antenna has wider bandwidth, good isolation and miniaturized dimen-

has resonances at the frequencies of 6, 8, 10.8, 15.8 and 18.8 GHz.

in Figure 4. It is observed that the bandwidth is increasing with an increase in iteration. For further increase in iteration, no significant improvement is observed. The reflection coefficient curves for the initial antenna structure with and without ground notches are illustrated in Figure 5. Its quantitative analysis is listed in Table 2. It is observed that the lower band edge frequency is negligibly changed, whereas the higher band edge frequency is shifted from 16.4 GHz to 19.4 GHz in the case of notch-loaded ground plane. The initial resonances are slightly shifted to

≈Lslot þ 2 � Wslot (2)

of inner tapered tree-shaped fractal antenna is enhanced by loading the ground plane with quarter wavelength rectangular notches to excite an additional resonance at 18.6 GHz. The dimensions of notch are governed by Eq. (2):

Inner Tapered Tree-Shaped Ultra-Wideband Fractal Antenna with Polarization Diversity

Figure 3. Designing stages of proposed MIMO antenna element (a) Zeroth iteration, (b) First Iteration (c) Second Iteration (d) Third Iteration (e) Third iteration with ground notch.

Inner Tapered Tree-Shaped Ultra-Wideband Fractal Antenna with Polarization Diversity DOI: http://dx.doi.org/10.5772/intechopen.86071

where:

n = iteration number = 2 or 3.

R1 = dimension of the first iteration, i.e. L1, W1 and Wm1.

Rn = dimension of the nth iteration.

r = iterative ratio = 0.4.

In the first step, the dimensions of radiating patch are unaltered. The bandwidth of inner tapered tree-shaped fractal antenna is enhanced by loading the ground plane with quarter wavelength rectangular notches to excite an additional resonance at 18.6 GHz. The dimensions of notch are governed by Eq. (2):

$$L\_r = \frac{c}{4f\_r} \approx L\_{slot} + 2 \times W\_{slot} \tag{2}$$

Lr = electrical length of slot for resonance.

fr = resonance frequency = 18.6 GHz.

In the second step, two identical structures designed in the first step are placed orthogonal to each other. The air gap between the two structures and their locations are optimized. All the frequency domain results are calculated by using HFSS. The time domain results and diversity performance parameters are analysed by using CST MWS [31].
