Analysis and design of a compact ultra-wideband antenna with WLAN and X-band satellite notch

Received Jul 11, 2019 Revised Feb 27, 2020 Accepted Mar 8, 2020 A compact design of ultra-wideband (UWB) antenna with dual band-notched characteristics are investigated in this paper. The wider impedance bandwidth (from 2.73 to 11.34 GHz) is obtained by using two symmetrical slits in the radiating patch and another slit in the partial ground plane. The dual band-notch rejection at WLAN and X-band downlink satellite communication system are obtained by inserting a modified U-strip on the radiating patch at 5.5 GHz and embedding a pair of single rectangular split ring resonators (SR-SRR) on both sides of the microstrip feed line at 7.5 GHz, respectively. The proposed antenna is simulated and tested using CST MWS high frequency simulator and exhibits the advantages of compact size, simple design and each notched frequency band can be controlled independently by using the geometrical parameters of the corresponding resonator. Therefore, the parametric study is carried out to understand the mutual coupling between the dual band-notched elements. To validate simulation results of our design, a prototype is fabricated and good agreement is achieved between measurement and simulation. Furthermore, a radiation patterns, satisfactory gain, current distribution and VSWR result at the notched frequencies make the proposed antenna a suitable candidate for practical UWB applications.


INTRODUCTION
The Federal Communications Commission (FCC) has authorized the ultra-wide band (UWB) spectrum from 3.1 GHz to 10.6 GHz with signal bandwidth more than 500 MHz for applications in commercial UWB communication systems [1]. UWB technology offers attractive features which include a high data transmission rate of more than 100 Mbps, low power and multi-path communication. Many techniques in the literature have been applied to design planar wideband antennas using microstrip [2,3] or coplanar waveguide (CPW) slot antennas [4][5][6].
In recent years, a large number of interfering narrow-band communication systems operate in this broad bandwidth, like WiMAX (3.3-3.6 GHz), C-band systems (3.7-4.2 GHz), WLAN (5.15-5.825 GHz), and downlink of X-band satellite . So designing the UWB antennas without interfering with these narrow bands are required. Among the techniques that have been investigated to provide band-notched characteristic in order to meet this requirement, we find the following: embedding slots with different shapes in the radiating patch [7,12], using quarter wavelength stubs attached in the ground plane [13], employing  [14], using a parasitic rectangular strip [15], inserting split ring resonators (SRR) on the ground plane [16,17] or using coplanar waveguide technology [18]. Other techniques have also been investigated by engraving two slots on radiating patch and a pair of rotated V-strip slot on the back side of the substrate [19], using slotted planar structure [20], using capacitor [21] however it needs to be reconfigurable. In [22], a slotted strip line structure is proposed.
In this work, a compact design of planar UWB antenna with dual-notch bands is investigated. The bandwidth and the adaptation of proposed UWB antenna are enhanced using two techniques: The first is loading the patch antenna by two symmetrical slits and the second one by introducing the slit in the partial ground plane. In order to create the dual band-notch rejection at WLAN and X-band downlink satellite, we introduce a modified U-strip on the radiating patch to have the 5.5 GHz notch and by embedding a pair of rectangular SRRs on both sides of the microstrip feed line, we obtain the second notch at 7.5 GHz. Therefore, we simulate and discuss the parameters of the band notch antenna such as the VSWR, current distribution, radiation patterns and gain response. The size of the antenna is 24×15×1.6 mm and it's adapted for WLAN and X-band downlink satellite communication services. Figure 1 shows the procedure to designing UWB antenna, which is printed on Fr-4 substrate with a dimension of 24*15 mm 2 , relative dielectric constant εr = 4.4, thickness of 1.6 mm and loss tangent of 0.02. Figure 2 shows the VSWR simulation for each step of presented antenna Figure 1 (from (Ant. 1) to (Ant. 3)) that allows us to obtain an UWB antenna as a result Figure 1   In the first step, and based on theory and recent studies, the initial design is the square antenna with symmetrical bevel, as shown in Figure1(Ant. 1). In the second step, The design was developed and optimised by considering different aspects such as the improvement of impedance matching and bandwidth. The radiating element is loading by two symmetrical slits to obtain better impedance matching and by introducing a slit on the partial ground plane, broadband impedance bandwidth can be achieved, as shown in

UWB antenna with dual band notch geometry
After the success in creating a compact printed monopole UWB antenna (without a band notch). Figure 3 illustrates two steps followed in producing dual-notched band characteristic on UWB radiator. The first one consists on creating a slit on the radiating patch and after that, adding a modified U-strip resonator inside the previous slit that leads to provide the first notch at 6 GHz. However, the second one consists on embedding a pair of SR-SRR on each side of the feed line which leads to provide another notch at 8.2 GHz. The detailed values of the two resonators are displayed in Table 1.

VSWR
The simulated VSWR of the proposed antenna is shown in Figure 4, where VSWR of UWB antenna (without any resonator) is compared to VSWRs of the antenna having the single and dual band-reject performance, respectively. The designed UWB antenna has broadband performance from 2.78 to 11.34 GHz with VSWR < 2, which covers all of the UWB frequency band. Furthermore, it can be clearly seen that each resonator creates a corresponding notched band with VSWR >2 at these notches. These results mean that there are no power transmissions at notches frequencies. However, to better explain the correlation between the band-notched elements and tuned each rejected band to the desired value, a parametric study is carried out by varying one parameter at a time and fixing the others.

The WLAN rejected band
The first notch can be generated by adding a modified U-strip resonator inside a slit in the radiator. The length of this resonator is calculated as Lnotch1=2LR1+ 2LR2 +WR1. The following formula [23] can explain the dependence between the overall length (Lnotch1) of the modified U-strip resonator and the first notch-band frequency (f notch1).
where C is velocity of light through free space and ɛr is dielectric constant. Figure 5 shows the simulated VSWR parameters for different values of "LR2". It can be seen that the VSWR at the notched bands is dependent on "LR2" values. The notched bands shift toward the lower frequency band when "LR2" increases. The optimum value of "LR2" to cover the desired notched band centred at 5.5 GHz (WLAN) band is 5.7 mm.

The X-band rejected band
Also the second frequency notch is generated by embedding the SR-SRR on side the microstrip feed line. The length of this resonator is calculated as Lnotch2=2a+2b-g. By using the formula (2), the relationship between the total radiator length (Lnotch2) of the SR-SRR and the second notch-band frequency (f notch2) is calculated using the following formula  Figure 6 depicts the influence of "g" the rectangular SRR-Split width to the second-band reject. It indicates that when the dimension of "g" is varied from 0.8 to 0.4, only the second notch band shifts, so the optimum value of "g" to obtain a notch band at 7.5 GHz is 0.4 mm. According to the parametric study above, it is observed that the first frequency notch (WLAN) is achieved at 5.5 GHz and the second (X-band) is achieved at 7.5 GHz. It also confirmed that the geometrical parameters of each resonator are used to adjust the position of the corresponding unwanted band. Moreover, the optimal result of VSWR of the suggested antenna is illustrated in Figure 7.

Current distribution and gain
To further understand the phenomenon behind dual notch performance, the surface current is also investigated at frequencies of 5.5 and 7.5 GHz. As shown in Figure 8(a), at 5.5 GHz notched frequency, the stronger current is mainly focused over the modified U-strip resonator inside the radiation patch. Figure 8(b) shows the current distribution at 7.5 GHz, which corresponds to the upper notched frequency. The current distribution is more concentrated over a pair of SR-SRR and very week on radiating patch and on modified U-strip. Therefore, at the notched-bands, the energy cannot be radiated and it is stored around the resonators. Figure 9 shows the simulated antenna gain versus frequency over the whole impedance bandwidth. The gain at the notched frequency bands is significantly reduced. It clearly indicates that these resonators can perfectly prevent the propagation of the signal close to its resonant frequencies.

FABRICATION AND MEASUREMENT
In order to validate the design of dual band-notch UWB antenna, a prototype has been successfully fabricated using LPKF ProtoMat E33 and tested using A Rohde and Schwarz ZVB 20 vector network analyzer. Figures 10 (a) and (b) show a photograph of fabricated antenna, The simulated and measured results of our antenna are plotted in Figure 11 and listed in Table 2.  The fabricated antenna has a large bandwidth from 3.38 GHz to10.5 GHz for VSWR<2 with dual notched bands in the range of WLAN and X-band. A slight discrepancy exists among the rejected bands. Some of these slight discrepancies can be attributed to connector mismatches and manufacturing tolerances. The radiation patterns of the designed antenna at the frequencies 4 GHz, 6.5 GHz and 9 GHz in the E and H planes are simulated, measured and compared, as shown in Figure 12. A nearly omnidirectional radiation characteristics are observed in the H-plane. While in E-plane, the radiation pattern is similar to that of the dipoles. Table 3 summarized the comparison of proposed design and published works in terms of frequency notches, peak VSWR at center notched frequency, paek gain and full size. As observed, the proposed design is compact in size as compared to all the references taken.  Table 3 summarized the comparison of proposed design and published works in terms of frequency notches, peak VSWR at center notched frequency, paek gain and full size. As observed, the proposed design is compact in size as compared to all the references taken.

CONCLUSION
In this paper, A compact planar UWB antenna with dual band-notched characteristics has been presented. The desired stop bands with centre frequencies at 5.5 GHz and 7.5 GHz are obtained by inserting a modified U-strip on the radiating patch and incorporating a SR-SRR on each side of the microstrip feed line, respectively. Moreover, the central frequency of the notches could be easily adjusted by altering the geometrical parameters of corresponding resonator. VSWR, current distribution, gain and radiation pattern are taken into account to analyse the performance of the proposed antenna. In addition, The H-plane shows nearly omnidirectional radiation patterns over the operating frequency range. From the above results, it can be concluded that the performance of proposed antenna is adequate for UWB wireless communications applications except WLAN and downlink signals of the X-band satellite communication services.  Hanae Elftouh was born in 1987, Tetouan, Morocco. She received a Master degree in Telecommunication systems engineering from Abdelmalek Essaâdi University, Tetuan, Morocco, in 2010. She received PHD degree in microwave circuits in 2016 at the same university. Her research interests on miniaturization of printed microwave circuits (antennas and filters). She has authored and co-authored several papers in different international indexed journals and conferences in the field of microwave communications Technology.