Fig. 2 Chronological developenent of the proposed antenna with first four iterative processes.
In section one preliminary design of the UWB notch antenna is presented with four iterative processes. The chronological developenent of the proposed antenna with first four iterative processes is shown in figure2. Figure 3 shows the simulated reflection coefficient (|S11|) of all the first four iterations of the preliminary design of the UWB notch antenna. In first iteration shown in top and bottom view of figure2 (a), a simple circular radiating patch is created which is fed by the microstrip feed line with a partial ground plane as shown. With these configurations a poor impedance matching is obtained as the impedance bandwidth obtained as 6.6 GHz (bandwidth from 3.1 GHz to 9.7 GHz). In the second iteration shown in figure 2 (b), an open-ended rectangular slot is created at the middle of the ground plane just below the microstrip feed line. The open-ended rectangular slot in the middle of the ground plane enhances the bandwidth from 3.1 GHz to 11 GHz with impedance bandwidth 7.9 GHz. The enhancement of bandwidth due to the creation of an open-ended slot in the ground plane helps in proper impedance matching between the microstrip feed line and the circular radiating patch. In the third iteration a square slot is created for more enhanced impedance matching purpose which is shown in the figure 2 (c). After the simulation of the third iteration the bandwidth extended from the 4 GHz to 12 GHz with the impedance bandwidth 8 GHz. Though the impedance bandwidth increases from 7.9 GHz (second iteration) to 8 GHz (third iteration) but the lower frequency is increased to 4 GHz which is not acceptable for UWB operation point of view. To address this problem and for the creation of notch at fn =5.5 GHz to mitigate out the WLAN operation in the UWB band, a thick rectangular stub is erected which is protruding down from the top middle edge of the square slot towards the center of the square slot created in the circular patch which is the fourth iteration as shown in the figure 2 (d). Due to this rectangular thick stub the bandwidth is extended from the 2.8 GHz to 12.4 GHz which has the impedance bandwidth 9.6 GHz which is 1.6 GHz more than the impedance bandwidth obtained in third iteration. But at the same time the protruding rectangular stub creates a notch centered atfns1 =4.7 GHz which is having the notch band extended from 4.1 GHz to 5.3 GHz. The theoretical notch frequency (fnt1 ) can also be expressed in terms of the effective physical electrical current path length (Leff1 ) around the protruding rectangular stub and is given by
\(f_{nt1}=\frac{c}{2L_{eff1}\sqrt{\varepsilon_{\text{eff}}}}\)———————————–(1)
where is the effective permittivity of the substrate whose expression is given by
\(\varepsilon_{\text{eff}}=\frac{\varepsilon_{r}+1}{2}+\frac{\varepsilon_{r}-1}{2}\frac{1}{\sqrt{1+12\frac{h}{W_{f}}}}\)———————————–(2)
εr =4.4 is the permittivity of the substrate andc is the speed of light in vacuum. Putting all the numerical above in the above expression, the effective dielectric constantεeff was found to be εeff=3.324. The effective physical electrical current path length (Leff1 ) around the protruding rectangular stub is found to be 18 mm. Hence applying the equation (1) the theoretical notch frequency is found to be 4.6 GHz, which is very close to the simulated value of the center of notch frequency fns1 =4.7 GHz. The difference between the theoretical fnt1and simulational fns1 is due to the different simulation environment adopted for simulation. Though with the introduction of protruding rectangular stub enhances the band but creates a notch with the center notch frequency offns1 =4.7 GHz which is not the required notch center frequency for WLAN which is fn =5.5 GHz. From figure 3, the sufficient enhancement of the impedance bandwidth is depicted with a notch characteristic at aroundfns1 =4.7 GHz. Hence to obtain the required notch frequency at fn =5.5 GHz and also extended impedance bandwidth, an open-ended slit ring and a SRR is embedded in the proposed antenna structure which is described in section two.