Compact filtering monopole patch antenna with dual-band rejection

In this paper, a compact ultra-wideband patch antenna with dual-band rejection is proposed. The proposed antenna filters 3.3–3.8 GHz WiMAX and 5.15–5.85 GHz WLAN by respectively rejecting these bands through a C-shaped slit and a λg/4 resonator. The λg/4 resonator is positioned as a pair, centered around the microstrip line, and a C-type slit is inserted into an elliptical patch. The impedance bandwidth of the proposed antenna is 2.9–9.3 GHz, which satisfies the bandwidth for ultra-wideband communication systems. Further, the proposed antenna provides dual-band rejection at two bands: 3.2–3.85 and 4.7–6.03 GHz. The radiation pattern of the antenna is omnidirectional, and antenna gain is maintained constantly while showing −8.4 and −1.5 dBi at the two rejected bands, respectively.

λ g /4 resonators is centered on the microstrip line, and a C-shaped slit is inserted into an elliptical patch. The proposed antenna satisfied the required bandwidth for UWB communication systems specified by the Federal Communications Commission, while maintaining dual-band rejection to prevent interference between bands.
The remainder of the paper is organized as follows: the "Background" section briefly introduces the proposal and design of the tapered slot antenna. The section, "Methodology and analyses of experimental data, " deals with the characteristics of the antenna, which were analyzed through a simulation and measurement process. The "Result and discussion" section presents the comprehensive results for the proposed antenna. Finally, in the "Conclusion", we draw conclusions regarding the proposed antenna.

Antenna design
For the structure of the proposed antenna, a λ g /4 resonator and a C-shaped slit were inserted into an antenna. The antenna was designed with an elliptical patch structure, in order to reject both WiMAX and WLAN. It was fabricated using the Taconic TRF-45 substrate, which is 1.62 mm in thickness and offers a relative permittivity of 4.5 and a loss tangent of 0.0035. The structure of the antenna is compact, with a total size is 40 × 35 mm 2 .
The design and analysis of the antenna were facilitated with HFSS, a commercial simulator tool available from Ansys. Its structure and design parameters are shown in Fig. 1 and Table 1 (Weng et al. 2010a, b).
Two bands coexist for unlicensed use in the UWB:WiMAX (3.3-3.8 GHz) and . The proposed antenna rejects both of these bands using a λ g /4 resonator and a C-shaped slit (Sarkar et al. 2014;Wu et al. 2013).
The equivalent circuit for the proposed λ g /4 resonator is shown in Fig. 2. The capacitance C 0 is the coupling generated between the microstrip line and the λ g /4 resonator. The capacitance C 1 is derived from the voltage gradient between the λ g /4 resonator and the ground plane, whereas inductance L 1 is generated due to the current flowing through the pin. The rejection frequency is derived as follows (Trinh-Van and Dao-Ngoc 2011).
In order to reject WiMAX, the following equation is used: where L c1_1 is the length of the resonator, and λ g is a guided wavelength.
For the guided wavelength λ g , an effective dielectric constant ε eff must first be determined, along with a suitable length for the microstrip line. Here, f denotes the frequency, and c denotes the speed of light in a free space. An effective dielectric constant ε eff can be derived with the following equation: (1)  . 2 The mechanism of the proposed λ g /4 resonator where ε r is the relative permittivity of the substrate, and h and w denote the substrate thickness and the width of the microstrip, respectively. Next, L c1 and L c2 are calculated such that the C-shaped slits reject WLAN, and these are derived as follows (Xu et al. 2012;Hong and Lancaster 2004): Figure 3 depicts the size and look of the proposed UWB monopole antenna with dualband rejection.

Methodology and analyses of proposed antenna
The antenna is expressed by the reflection coefficient Γ, which is the amount of reflected signal due to the impedance mismatch between the source and the antenna.
The VSWR is calculated as follows (Chang 2000): When |Γ| = 0, optimal VSWR is obtained. This means that all power is transmitted to the antenna, and that there is no reflection. The impedance bandwidth of the antenna is defined at VSWR ≤ 2, and it is the reflected value of approximately 11 % input power (Chang 2000). Therefore, the proposed antenna has wide impedance bandwidth and powerful rejection-band characteristics.
We analyzed the characteristics of the proposed antenna in terms of its ability to perform dual-band rejection by simulating its current distribution, as shown in Fig. 4. Figure 4 shows the field distribution pattern on the patch, along with its modification with a C-shaped slit and λ g /4 resonators. The proposed antenna had a concentrated current at the λ g /4 resonator over the 3.5 GHz band (WiMAX), and a further concentration at the C-shape slits over the 5.5 GHz band (WLAN). The dual-band rejection and impedance bandwidth for the proposed antenna were analyzed for each structure using the voltage standing-wave ratio (VSWR). All three structures and their corresponding VSWR are shown in Figs. 5 and 6, respectively. As shown in Fig. 6, Structure-1 converged with a VSWR ≤ 2 over the 3.27-10.48 GHz band, thereby satisfying the impedance bandwidth for standard UWB communication systems. Structure-2 successfully rejected the 3.27-3.87 GHz band, and its impedance bandwidth converged with a VSWR ≤ 2 at the 2.99-9.50 GHz band. Structure-3 successfully rejected both the 3.28-3.85 GHz band and the 4.7-6.03 GHz band, and its impedance bandwidth converged with a VSWR ≤ 2 at the 2.94-9.63 GHz band. Therefore, we demonstrated that dual-band rejection is feasible, and we verified that the proposed structure offers suitably high bandwidth for UWB communications.
Either the WLAN band or the WiMAX band can be rejected with physical changes to the C-shape slits or the λ g /4 resonator of the proposed antenna, respectively. Thus, the WiMAX band can be rejected through physical changes to the λ g /4 resonator, as shown in Fig. 7. Figure 7 shows that, as the length of the λ g /4 resonator increased from 9.6 to 13.6 mm, the rejected band shifted to a higher frequency. At 11.6 mm, the WiMAX band was exclusively rejected.
The WLAN band can likewise be rejected by making physical changes to the C-shaped slits, as shown in Fig. 8.  Figure 8 shows that, as the length of the C-shaped slit increased from 6.4 to 7.4 mm, the rejected band shifted to a lower frequency. At 6.8 mm, the WLAN band was exclusively rejected.
In order to confirm the reliability of the band rejections with the proposed antenna, we analyzed the gaps between the antenna and the λ g /4 resonator and between the antenna and the C-shaped slit. Our analysis showed that a higher VSWR resulted in superior band rejections, owing to an impedance mismatch. We performed this analysis by varying the gap between the λ g /4 resonator and the antenna to 1.2 mm, the results for which (i.e., exclusively rejecting WiMAX) are shown in Fig. 9.
As shown in Fig. 9, as the gap narrowed between the antenna and the λ g /4 resonator, the VSWR increased and antenna gain reduced to below −10 dBi.
We then modified the gap between the C-shaped slit and the antenna to 1 mm, and the results of this modification (i.e., exclusively rejecting WLAN) are shown in Fig. 10.
As shown in Fig. 10, as the gap narrowed between the antenna and the C-shaped slit, the VSWR increased and antenna gain reduced to below −8 dBi.
The overall characteristics of the proposed antenna in terms of its ability to reject WLAN and WiMAX are provided in Table 2.  Table 2 shows that, for WiMAX, the antenna gain was −10.7 dB when Gap 1 was 0.6 mm. For WLAN, the antenna gain was −8.7 dB when Gap 2 was 0.75 mm. These results confirm the feasibility of the proposed antenna with dual-band rejection.  The UWB monopole patch antenna was also evaluated using a simulation, and these results are shown in Fig. 11.
As shown in Fig. 11, the impedance bandwidth of the proposed antenna satisfied dualband rejection at two bands: 3.3-3.85 and 4.8-6.1 GHz. Its impedance bandwidth converged with a VSWR ≤ 2 over the 2.9-9.3 GHz band. Therefore, the simulation results are consistent with the measurement results.
To further evaluate the proposed antenna, we used a simulation to analyze the radiation pattern along the E-plane (XZ-plane) and H-plane (YZ-plane) over two bands, 4 and 7 GHz, the results for which are shown in Fig. 12.
As shown in Fig. 12, the analysis of the radiation pattern of the proposed antenna demonstrated its omnidirectional characteristics at the 4 and 7 GHz bands.
The antenna gain of the proposed antenna over all bands was also analyzed, and the results from this analysis are shown in Fig. 13.
As shown in Fig. 13, the analysis of the proposed antenna's gain revealed that the simulation and measurement results were similar. However, the simulation analysis results and measurement results at 5.5 GHz were different by approximately 6 dBi. This was because of the loss in the physical size of the C-shaped slot during the production process. However, the band-rejection proceeded downward below 0 dBi, which is a suitable value.

Results and discussion
The overall analysis results in Table 3 show that the proposed antenna offers the appropriate bandwidth for UWB communication systems, coupled with dual-band rejection to avoid interference between communication systems. These results also show that the radiation pattern of the antenna is omnidirectional.
A mismatch was observed between the simulation results and the measured results for the proposed antenna. This occurred in two forms: the first pertained to errors during the manufacturing process, and the second to loss between the antenna and the connector. However, this mismatch is not a major problem with the proposed performance. On this basis, the impedance bandwidth was achieved with a VSWR ≤ 2, and the rejected band proceeded downward below 0 dBi.  The proposed antenna is compared to other antennas with rejected-band characteristics in Table 4. The advantage of the proposed antenna lies in its compact design and the fact that it has a dual-band rejection characteristic.

Conclusion
In this paper, we proposed a UWB monopole patch antenna with dual-band rejection. The impedance bandwidth of the proposed antenna satisfied VSWR ≤ 2 at the 2.9-9.3 GHz band and dual-band rejection from an impedance mismatch at the 3.3-3.85 and 4.8-6.1 GHz bands. Furthermore, we demonstrated that the antenna's radiation pattern is omnidirectional, and that the antenna gain proceeded downward to below 0 dBi for dual-band rejection. Furthermore, the proposed antenna offers the advantage of dualband rejection and a compact design, compared with different antennas.
The design of the proposed antenna was optimized through HFSS, a commercial electromagnetic simulator provided by Ansys. The antenna was designed using the Taconic TRF-45 substrate, which is 1.62 mm thick with a relative permittivity of 4.5 and a loss tangent of 0.0035.