3. Results and discussion

The intermediate antenna design steps are compared in terms of their S11 curves 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 higher frequency with an additional resonance.

#### 3.1 Frequency domain

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

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 dimensions than other structures.

Figure 3.

44

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.

UWB Technology - Circuits and Systems

Figure 4. Reflection coefficient of intermediate antenna design steps.

Figure 5.

Reflection coefficient versus frequency characteristic of single antenna element.


Figure 8 illustrates that the peak realized gain of Antenna I is varying from 0.52

to 4.98 dB with an average of 3.5 dB over the operating frequency band. It also presents that the gain of Antenna II is varying between 1.43 and 3.5 dB with an

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

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

The variations of radiation and total efficiencies with frequency for both antenna elements are shown in Figure 9. The radiation efficiencies of both antenna elements are more than 0.7 with an average of more than 0.86. Similarly, the total efficiencies have an average of 0.83. The efficiencies are decreasing with an increase

in frequency due to the use of lossy FR-4 substrate.

Scattering parameters versus frequency characteristics of the designed antenna.

average of 2.45 dB.

Figure 6.

47

#### Table 2.

Bandwidth comparison of single antenna element with and without ground notch.

The simulated radiation patterns of two antenna elements at the resonance frequencies of 6, 8, 10.8, 15.8 and 18.8 GHz in all three planes are illustrated in Figure 7. From Figure 7, it is observed that the antenna structures have bidirectional and omnidirectional patterns at lower frequencies. At higher frequencies, the patterns are distorted omnidirectional in nature due to the excitation of higher-order modes at those frequencies. It is also clearly observable that the patterns of both antenna structures have a phase difference of 90° in each plane as desirable.

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

Figure 6. Scattering parameters versus frequency characteristics of the designed antenna.

Figure 8 illustrates that the peak realized gain of Antenna I is varying from 0.52 to 4.98 dB with an average of 3.5 dB over the operating frequency band. It also presents that the gain of Antenna II is varying between 1.43 and 3.5 dB with an average of 2.45 dB.

The variations of radiation and total efficiencies with frequency for both antenna elements are shown in Figure 9. The radiation efficiencies of both antenna elements are more than 0.7 with an average of more than 0.86. Similarly, the total efficiencies have an average of 0.83. The efficiencies are decreasing with an increase in frequency due to the use of lossy FR-4 substrate.

The simulated radiation patterns of two antenna elements at the resonance frequencies of 6, 8, 10.8, 15.8 and 18.8 GHz in all three planes are illustrated in Figure 7. From Figure 7, it is observed that the antenna structures have bidirectional and omnidirectional patterns at lower frequencies. At higher frequencies, the patterns are distorted omnidirectional in nature due to the excitation of higher-order modes at those frequencies. It is also clearly observable that the patterns of both antenna structures have a phase difference of 90° in each plane as

Reflection coefficient versus frequency characteristic of single antenna element.

Reflection coefficient of intermediate antenna design steps.

UWB Technology - Circuits and Systems

Bandwidth comparison of single antenna element with and without ground notch.

desirable.

46

Figure 5.

Figure 4.

Table 2.


F ¼ max

than 1 ns over the entire band of operation.

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

3.3 Diversity performance

Figure 7.

49

Radiation patterns of the two antenna elements in XY, YZ and ZX planes.

Ð <sup>∞</sup>

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

Ð <sup>∞</sup> �<sup>∞</sup> j j stð Þ<sup>t</sup> <sup>2</sup>

�<sup>∞</sup> stð Þ<sup>t</sup> srð Þ <sup>t</sup> <sup>þ</sup> <sup>τ</sup> <sup>d</sup><sup>τ</sup>

dt Ð <sup>∞</sup>

where st(t) and sr(t) are the transmitted and received pulses and τ is the group delay. The variations of group delay with respect to frequency for all four cases are illustrated in Figure 12. It is observed that the group delay has its variations less

To analyse the diversity performance of the designed antenna, various parameters like envelope correlation coefficient, diversity gain (DG) and mean effective gain (MEG) are to be calculated from s-parameters or farfield patterns. The envelope correlation coefficient (ECC) signifies the correlation between the radiation patterns of two antenna elements. For the designed antenna structure, ECC (ρe) is

�<sup>∞</sup> j j srð Þ<sup>t</sup> <sup>2</sup>

dt " # (3)

Table 3. Bandwidth of two ports.


#### Table 4.

Comparison of designed antenna with other UWB diversity antenna.
