# Planar UWB Monopole Antenna with Tri-Band Rejection Characteristics at 3.5/5.5/8 GHz
## 4 Results and Discussion  
To verify the proposed design, the antenna was manufactured on a FR-4 substrate. The dimensions of the substrate was 35.5 mm \(\times\) 30 mm \(\times\) 1.6 mm. The photo of the fabricated antenna is shown in Figure 8. The return loss of the antenna was measured by using an Agilent N5230A vector network analyzer. The measurement and simulation results of the return loss are shown in Figure 9, which shows a good agreement between the simulation results and the measurement results. The slight deviation may be due to fabrication imperfection or SMA connector error. The results show that the presented design had triple band-notched function, i.e., band I from 2.59-4.01 GHz, band II from 4.73-6.11 GHz, and band III from 7.68-8.40 GHz, which obviously avoid WiMAX, WLAN interference, and X-band satellite system.  
The proposed antenna can be equivalent to the circuit model shown in Figure 10, which contains three parallel RLC resonant loops. When the circuit was operating, the first RLC parallel resonant loop resonated at 3.5 GHz, corresponding to the H-shaped slot on the radiating patch; the second RLC parallel resonant loop resonated at 5.6 GHz, corresponding to the L-shaped slot on the ground plane; the third RLC parallel resonant loop resonated at 8.1 GHz, corresponding to the rectangular slot below the microstrip line. When the circuit operates at the notched band frequency, the value of the input resistance of the circuit becomes maximum. The RLC parameters' value can be given by the following formulas [26]:  
\[Q_{0}=\frac{f}{BW} \tag{2}\]  
\[Q_{0}=2\pi f_{0}RC \tag{3}\]  
\[f_{0}=\frac{1}{2\pi\sqrt{LC}} \tag{4}\]  
where \(f_{0}\) is the resonant frequency, \(Q_{0}\) is the quality factor, BW is the RLC bandwidth, \(R\) is the resistance value, \(C\) is the capacitance value, and \(L\) is the inductance value in the RLC parallel resonant loop.  
Figure 8: Photo of the fabricated antenna: (**a**) front view; (**b**) back view.  
Figure 9: Simulated and measured results of the return loss of the proposed antenna.  
Firstly, we obtained the curve of the input impedance with the frequency as shown in Figure 11. The resistance values of the three RLC parallel resonant loops resonance were obtained from the peak values of the real component curve. Then, we calculated the quality factors by Formula (2). Finally, we calculated other parameter values by Formula (3) and Formula (4). Table 2 shows the specific parameter values of the RLC equivalent circuit. The equivalent circuit was simulated by ADS 2017. The return loss of the equivalent circuit was compared with measurement and simulation results of the designed antenna in Figure 12, which showed the good consistency between the equivalent circuit and the designed antenna.  
Figure 13 shows the variation of realized gain and antenna efficiency with frequency. It can be observed that the realized gain and antenna efficiency decrease significantly at the three stopbands. Figure 14a-d shows the 2-D far-field simulation radiation patterns in the yz-plane and xz-plane at sampling frequencies of 2.5 GHz, 4.5 GHz, 7.0 GHz, and 8.6 GHz respectively. In these figures, it was found that the omnidirectional radiation characteristics were good. The results show that  
Figure 11: Input Impedance results of the proposed antenna.  
Figure 12: The comparison of equivalent circuit’s return loss with simulated and measured results of return loss of the proposed antenna.  
Figure 10: Schematic of the equivalent RLC resonant circuit model of the proposed antenna.