The width of the antenna determines the extent of current flow within the antenna as seen in the current distribution plots below (Figure 4 (a, b, c)). The current is maximum along the feedline and the ground plane adjacent to the feedline. Hence, more the width of the ground plane, the current distribution will reduce gradually till it reaches the edges. So, the width of the antenna is always optimized for maximum current distribution. The width of the feedline also is an important parameter which needs to be optimized for an optimum antenna design. As the feedline width increases, the current accumulation becomes more along the feedline and hence, current distribution in the ground becomes insignificant. Basically, the power handling capacity of the feedline increase as the width decreases. So, it is always preferable to have a minimal feedline width. The gap between the ground plane and the feedline and the gap between the radiating patch and the ground also play a major role in antenna design. Increase in the respective lengths result in reduction in the radiated power as can be observed in the radiation patterns. From the current distribution plots in Figure 4(a, b, c), it can be observed that as the frequency increases, the current distribution along the feedline is decreased and spread to the radiating patch. i.e. the current gets distributed as the frequency increases.
The proposed antenna has been optimized with respect to the ground length, ground width, gap between ground and feed and finally, the radius of the patch. To improve the gain of the antenna, the ground is truncated at the edges. The antenna is been simulated in 3D electromagnetic simulator software HFSS .
A. Optimization of the ground length (lg = 25.2mm)
From Figure 5, it can be observed that the variation in the ground length from 24.8mm to 25.4mm results in a slight shift in the resonant frequency in the lower end for a value of 24.8mm. As the length is increased, there is negligible shift in the first resonant frequency. The optimization is limited to 25.4mm because the radiating patch and ground plane overlap each other for larger values. For the optimized values of lg =25.2mm, the harmonics converge in a neat manner.
B. Optimization of the distance between ground plane and feed-line (dgf = 0.5mm)
The distance between the feed and the ground is varied from 0.3mm to 0.6mm considering the feedline width to vary from 2.8mm to 3.4mm as shown in Fig. 6. The first resonant frequency shifts towards lower frequency side is observed when the feedline/ground-plane gap increases. The gap is optimized to dgf =0.5mm between feedline and ground plane for which the resonant frequencies converge to provide UWB characteristics.
C. Ground plane optimization (gp = 18mm)
The ground plane is generally considered to be in rectangular shape which results in harmonic generation. In order to reduce the harmonics, the ground is modified by bending the sharp edges to provide smooth current path to flow in the ground. Hence, the ground is curved at the edges nearer to the radiating patch. The radius of the curve is optimized for values ranging from 16mm to 19mm. The phenomenon observed in Fig.7 was no shift in the first resonant frequency. Except for the optimized value of gp = 18mm, the return loss rises slightly above -10dB in the frequency range of 4 GHz to 5GHz which suggests impedance mismatch. Increasing the radius to 19mm results in a total impedance mismatch in the higher frequency region.
D. Optimization of the radiating patch (r = 11mm)
The radius (r) of the radiating patch is varied from 10.5mm to 11.5mm keeping the ground plane length at its optimized value 25.2mm. The results from Fig. 8 show that, for radius of 10.5mm, there is an impedance mismatch in the frequency range 7GHz to 8GHz. For r = 11.5mm, there is an impedance mismatch for frequencies between 4GHz to 5GHz. The UWB characteristic is observed for the radius of 11mm.
E. Radiating patterns
The radiation patterns of this antenna have been measured at selective frequencies. The radiation patterns were measured in anechoic chamber. The simulated and measured radiation patterns in H-plane are shown in Figure 9 and Figure 10 respectively at frequencies 3.9 GHz, 5.025 GHz, 6.15 GHz, 8.025 GHz, 10.32 GHz, 13.5 GHz and 17.02 GHz respectively. The nature of measured radiation pattern in H-plane is nearly omni-directional. Similarly, simulated and measured radiation patterns in E-plane are also measured at frequencies i.e. 5.7 GHz, 7.5 GHz, 8.35 GHz, 11.85 GHz and 16.08 GHz respectively as shown in Figure 11 and Figure 12 respectively. The nature of radiation patterns in E-plane are nearly bidirectional. It is also observed that the radiation patterns are marginally varied. It may be due to FR4 lossy substrate and edge reflection.
F. Peak gain and radiation efficiency
The peak gain of this antenna has also been simulated as shown in Figure 13. It is noticed, as the frequency increases, the gain increases. This is because, at higher frequencies, the Receiving area becomes more in comparison to short wavelength. The gain at 3GHz is 1.5dB and 10GHz it is 10.5dB. Beyond this, as the frequency increases, the gain more or less remains the same. A plausible reason could be attributed to the loss tangent of the substrate and increase in cross polarization at higher frequencies. The radiation efficiency (RE) is also simulated over the UWB frequency range (Fig. 11). The RE is 96% at 3 GHz, 90% at 7GHz and 85% at 12 GHz. A decrease in RE is observed as w.r.t frequency which can be attributed to the substrate loss tangent.