I. INTRODUCTION
The ultra-wide bandwidth (UWB) technology has attracted great attention in the wireless world due to their advantages, including high-speed data rate, low-power consumption, high capacity, low-cost, and low complexity. Due to extremely large bandwidth, UWB promises to modernize high data rate transmission at power level below noise floor, efficient usage of the spectrum, easy connection and data exchange among a large number of multimedia devices such as PCs, high definition TVs, digital cameras, vehicular radar systems, imaging systems, surveillance systems, medical systems, etc. UWB systems necessitate UWB antennas with the appealing features, such as high bandwidth, simple structure, and omni-directional radiation pattern. Many UWB antenna designs have been developed recently [Reference Gautam, Yadav and Kanaujia1–Reference Gautam, Chandel and Kanaujia3], among those designs, planar monopole antennas are promising for UWB wireless communication applications. However, there still exist several narrow bands for other communication systems over the designated frequency band, such as: IEEE 802.16 WiMAX system operating at 3.3–3.7 GHz and the wireless local area network (WLAN) for IEEE 802.11a operating at 5.15–5.35 and 5.725–5.825 GHz, which may cause severe electromagnetic interference to the UWB system. Therefore, it is desirable to design UWB antennas with band-notched performance in those frequency bands to avoid potential interference. Many researches have been carried out to design an UWB antenna with band-notched characteristics to minimize the interference between UWB and narrow band system without mounting an additional bandstop filter [Reference Zhang, Hong, Yu, Kuai, Don and Zhou4, Reference Ma and Wu5]. Different methods to get band-notched structure embrace cutting slot of different shapes in radiator and ground plane [Reference Gautam, Indu and Kanaujia6–Reference Mandal and Das8].
A number of antenna configurations have been suggested for WLAN and WiMAX bands rejection in recent years [Reference Lee, Yang and Cho9–Reference Li, Zhai, Li, Liang and Han17]. A UWB operation with a notched frequency band can be obtained by inserting an Archimedean spiral-shaped slot into a microstrip open-circuit circular stub of microstrip–slotline transition [Reference Lee, Yang and Cho9]. Two split-ring resonator slots are etched on the radiators to achieve band-notched characteristics [Reference Gao, He, Wei, Xu, Wang and Zheng10]. In [Reference Gao, Zhu, Ho, Abd-Alhameed, See, Li and Xu12], the notched characteristics are as achieved by etching one quasi-complementary split-ring resonator in the feed line. An arc-shaped parasitic strip is printed inside a circular slot to achieve band-notched characteristics [Reference Li, Hei, Feng and Shi11]. In [Reference Li, Hong and Wang13], a switched band-notched UWB is proposed using S-shaped slots and switch. The compact folded stepped impedance resonator is located beside the feed line to achieve the band rejection characteristics [Reference Sung14]. A λ/3 rectangular metal strip with the antenna element and λ/4 open slots are used to obtain notched characteristics [Reference Li, Chu, Li and Xia15]. In [Reference Li, Zhai, Li, Liang and Han16], the open-loop resonators and etching C-shaped slot on radiating patch and L-shaped stub on the ground plane [Reference Li, Zhai, Li, Liang and Han17] are used to achieve band-notched characteristics. From the review of the above papers, it is evident that there are many techniques to achieve band-notched characteristics. Table 1 illustrates the comparison of the size and obtained fractional bandwidth of the recently published ultra-wideband antennas with band-notched characteristics. It is clearly seen from the comparison that the proposed antenna shows a compact size with comparable fractional bandwidth.
Table 1. Dimensions and fractional bandwidth comparison of recently reported antennas with band-notched characteristics.
In this paper, a coplanar waveguide (CPW)-fed lamp-shaped UWB microstrip antenna with dual band-notched characteristics is designed which successfully rejects the electromagnetic interference at WiMAX (3.3–3.7) and WLAN (5.15–5.825) bands. The proposed antenna possesses a lamp-shaped radiator patch with two rectangular ground planes on both the sides of the radiating patch. Two triangular strips one at each of the ground plane are added and a rectangular slot in the radiating patch is etched to improve impedance mismatch at middle and higher frequencies of the UWB band, respectively. Furthermore, an L-shaped slot in the radiator and two L-shaped slots in the ground plane are used to restrict electromagnetic interference (EMI) at WiMAX and WLAN bands, respectively, without affecting the electrical performance of the UWB antenna. Detailed design of the antenna is discussed in the following sections:
II. ANTENNA DESIGN AND PARAMETRIC STUDY
A) Ultra wide band antenna
Figure 1 shows the geometry with detailed design parameters of the proposed UWB microstrip antenna. The UWB antenna is fabricated on a 1.6-mm-thick FR4 substrate with dielectric constant ε r = 4.4 and loss tangent tan δ = 0.02. The overall size of the antenna is 15 × 28 × 1.6 mm3, and each of the embedded ground planes is separated from the feed line with a gap of 0.57 mm, respectively. The antenna consists of a simple lamp-shaped radiating patch with rectangular slot of L p2 × W p2 at the top and two rectangular ground planes embedded with two triangular-shaped structures to enhance the impedance bandwidth. The width of 8.9 mm long feed line is kept constant at 3 mm for providing the 50 Ω characteristics impedance. The length L p1 of the radiator, length L g3 of the triangular-shaped structure and gap between the feed line and ground planes are used for the parametric study and the optimized values of all parameters are listed in Table 2.
Fig. 1. Geometry of the proposed dual band-notched antenna with UWB characteristics.
Table 2. Design parameters (mm) of the proposed dual band-notched antenna with UWB characteristics.
Figure 2 shows the variation of length (L p1) of the lamp-shaped radiating patch from 5.5 to 9.5 mm. As the value of L p1 increases from 5.5 to 7.5 mm, the bandwidth of antenna increases greatly with an improvement in impedance mismatch furthermore, the bandwidth starts decreasing with a high-impedance mismatch. Therefore, the value 7.5 mm is chosen as an optimum value for L p1. Figure 3 shows the variation of length L g3 of the triangular-shaped structure attached to the ground plane on the voltage standing wave ratio (VSWR) of the antenna. As the values of L g3 change from 0.9 to 2.9 mm, the impedance bandwidth of the antenna increases greatly with an improvement in impedance mismatch at all the frequency bands and decreases furthermore. Therefore, L g3 = 2.9 mm is chosen as an optimum value.
Fig. 2. Simulated VSWR against frequency for the proposed UWB microstrip antenna with various L p1; other parameters are the same as listed in Table 2.
Fig. 3. Simulated VSWR against frequency for the proposed UWB microstrip antenna with various L g3; other parameters are the same as listed in Table 2.
The influence of the gap between two ground planes and single radiator patch is depicted in Fig. 4. Since in the present design, the radiator is surrounded by a metal ground plane for reducing the antenna area, the small gap between the radiator and the ground plane is a major factor to cause over-strong capacitive coupling. Thus a small change in the gap between ground planes will adversely affect the impedance matching of the structure. As the gap between ground plane and feed line increases from 0.37 to 0.57 mm, the impedance bandwidth remains almost constant but as it affects the tuning of feed line and ground planes, the impedance mismatch at middle and higher frequency bands starts improving. This mismatch increases with furthermore change. Therefore, the gap between feed line and ground planes is chosen 0.57 mm as an optimum value.
Fig. 4. Influence of the gap between two ground planes and feeding line patch on antenna performance.
B) Band-notch WiMAX band
The band-notch at 3.35 GHz occurs due to the narrow (0.27 mm wide) L-shaped slits in the radiating patch. This slot will disturb the impedance matching between the feed line and radiating patch as the field across the slits will be out of phase. As a result, the VSWR in this band is increased and notched band is created. This can also be established by studying the current distribution as shown in Fig. 7(a). In this figure, it is clearly illustrated that the current maxima occurs at the end edge of this slit. At resonant frequency, the length of the current path would be half of the guided wavelength. Therefore,
$${L_{n1}} = {W_{ps}} + {L_{ps}}.$$
In the proposed antenna design, this L-shaped slit acts as a quarter-guided wavelength resonator, and thus a center-rejected frequency f n may be empirically approximated by [Reference Li, Hei, Feng and Shi11]
$${f_n} = \displaystyle{c \over {4{L_{n1}}\sqrt {\displaystyle{{{\epsilon} _{\rm r} + 1} \over 2}}}},$$
where L n1 is the total length of the L-shaped slit, ε r is the relative dielectric constant, and c is the speed of the light. Given a desired resonance frequency, one can then use numerical simulations to adjust the dimensions of the L-slot to obtain the final design. The total length of the L-slot is 12.87 mm corresponding to the center frequency of the band-notch at 3.35 GHz. The effectiveness of the design method is further validated by predicting the notch frequency for the data presented in Fig. 5. In Table 3, the notched resonant frequency as a function of L ps is compared with the full-wave simulated data.
Fig. 5. Simulated VSWR against frequency for the proposed band-notched UWB microstrip antenna with length L ps of L-slot in the radiating patch.
Table 3. Comparison of notched resonant frequency as a function of Lps with the full-wave simulated data.
From Fig 5, it can be seen that the resonant frequencies calculated from the design method are agreeing well with the simulated data. From the above result, it is concluded that the notch frequency is controllable by changing the overall length of the L-slot.
C) Band-notch at WLAN band
Further, to reject the interference of the WLAN band (from 5.1 to 5.8 GHz), L-shaped slots (0.45 mm wide) are etched on the right-hand side of the ground plane as a quarter-guided wavelength resonator to generate band-notched. As discussed above, this slot in the right-hand side of the ground plane will disturb the impedance matching between the feed line and radiating patch as the field across the slits will be out of phase. As a result, the VSWR in this band is increased and notched band is created. This can also be established by studying the current distribution as shown in Fig. 7(b). In this figure, it is clearly illustrated that the current maxima occurs at the end edge of this slit; whereas the L-slit (0.2 mm wide) in the left-hand side ground is used to restrict the bandwidth of the band notch from 4.9 to 5.8 GHz only. At resonant frequency, the length of the current path would be half of the guided wavelength. Therefore,
$${L_{n2}} = {W_{gs}} + {L_{gs}}.$$
In the proposed antenna design, this L-shaped slit acts as a quarter-guided wavelength resonator and thus a center-rejected frequency f n may be calculated by (2). Fig. 6 shows, the simulated VSWR characteristics for the length L gs of the L-shaped slot etched in the right-hand side of the ground plane from 5 to 8 mm. It is observed that the change in slot length causes the change in current path, in turn, additional stop band occurs at WLAN band. Furthermore, it is evident from Fig. 6 that the center frequency of the band-notch shifts toward lower frequency. Therefore, the length of L-shaped slit filter is chosen 7 mm as an optimum value for the band-notch rejection at 5.51 GHz. The effectiveness of the design method is further validated by predicting the notch frequency for the data presented in Fig. 6. In Table 4, the notched resonant frequency as a function of L gs is compared with the full-wave simulated data.
Fig. 6. Simulated VSWR against frequency for the proposed band-notched UWB microstrip antenna with length L gs of L-slot in the ground plane.
Table 4. Comparison of notched resonant frequency as a function of Lgs with the full-wave simulated data.
From Fig. 6, it can be seen that the resonant frequencies calculated from the design method are agreeing well with the simulated data. From the above result, it is concluded that the notch frequency is controllable by changing the overall length of the L-slot.
D) Current density
The simulated current densities at various frequencies are plotted in Fig. 7 to understand the ultra-wideband and band-notched behavior of the antenna. It is seen from Fig. 7(a) that the current is mainly distributed on the lower part of the radiator and the ground planes for the UWB antenna, and it is also concentrated around the L-shaped slit in the lamp-shaped radiator for the band-notched antenna, as the total current is dissipated within the slit. The electric field across the L-shaped slit is out of phase, thus no radiation will take place. This clearly indicates that the L-shaped slit in the radiator effectively provides the electrical current path for producing the 3.43 GHz rejection frequency band. Figure 7(b) shows that the surface currents are mainly distributed over the ground plane and the lower part of the radiator for the UWB antenna, and it is also concentrated around the L-shaped slits in the ground plane for the band-notched antenna. It clearly indicates that the L-shaped cuts in the ground plane dissipate the total current within the slits. Again the electric field across the slits are out of phase, thus no radiation will take place at the frequencies around 5.5 GHz. At the other passband frequency of 13 GHz, the surface current is uniformly distributed over the radiator and ground planes (see Fig. 7(c)). From the above Figs 7(a)–(c), it is evident that the L-shaped slits in radiator and ground planes affects the performance of UWB antenna at notch frequencies only otherwise they offer no effect on the antenna as expected.
Fig. 7. Surface current densities for the proposed antennas.
III. RESULTS AND DISCUSSION
After optimization, the proposed antenna was fabricated with the MITS-Eleven Laboratory PCB machine. Figure 8 shows the simulated and measured VSWR of the proposed antennas. The measurement was performed with an Agilent N5230A vector network analyzer. Measured data show good agreement with the simulated one. It is found that the proposed antenna shows two-designed band notches at 3.0–3.9 and 4.9–5.8 GHz, while maintaining broadband performance from 2.7 to 14.4 GHz with VSWR less than 2, covering the entire UWB frequency band.
Fig. 8. Measured and simulated VSWR for the proposed CPW-fed UWB microstrip antenna with band-notched characteristics and UWB antenna.
The measured and simulated radiation patterns of UWB antenna with dual band-notched characteristics, in the E-(xz-) and the H-(xy-) planes at (a) 2.78, (b) 3.43, (c) 5.51, and (d) 12.4 GHz, are shown in Figs 9(a)–(d), respectively. It is found that the antenna has nearly good omni-directional radiation patterns at all frequencies in the E- (xz-) plane and the H- (xy-) plane. However, at the band-notched frequencies (3.43 and 5.51), the antenna displays unstable radiation patterns with degradation in gain as shown in Fig. 10 which illustrates the measured gain of UWB antenna and UWB antenna with dual band-notched characteristics. It is observed from Fig. 10 that the gain of the antenna drops down (~6.5 dB) at both the band-notches and remains constant for the rest of the band. Thus, the design shows the ultra-wideband performance with band-notched operation at WiMAX and WLAN bands.
Fig. 9. Radiation pattern for various resonance frequencies for the proposed lamp-shaped UWB antenna with dual band-notched characteristics with – measured – simulated at (a) 2.78 GHz, (b) 3.43 GHz, (c) 5.51 GHz, and (d) 12.4 GHz.
Fig. 10. Gain for various resonance frequencies for the proposed UWB antenna and UWB antenna with dual-notched characteristics.
IV. TIME-DOMAIN CHARACTERISTICS
To investigate the performance of UWB antenna in transmitter and receiver UWB systems, two identical antennas are placed at some distance from each other in three different orientations (face-to-face, side-by-side-x, and side-by-side-y) in the far-field region as shown in Fig. 11 using CST MWS [18]. A well-defined parameter named fidelity is proposed to access the quality of received signal waveform regarding the input signals in (4):
$$F = \max \left \vert {\displaystyle{{\int_{ - \infty} ^\infty {s_t}(t){s_r}(t + \tau )dt} \over {\sqrt {\left( {\int_{ - \infty} ^\infty {s_t}{{(t)}^2}dt} \right)\left( {\int_{ - \infty} ^\infty {s_r}{{(t)}^2}dt} \right)}}}} \right \vert,$$
where s t (t) and s r (t) are the input and received signals. The fidelity (F) is the maximum correlation coefficient of the two signals by varying the time delay [Reference Quintero, Zurcher and Skrivervik19]. The fidelity factor in three different orientations (face-to-face, side-by-side-x, and side-by-side-y) comes out to be 66.82, 72.55, 71.26 %, respectively. In Fig. 11, the time-domain transmitted and received pulses for three different orientations confirm the fidelity factor results.
Fig. 11. Input and received pulses in different orientations of the proposed antenna.
Group delay is another parameter in time-domain analysis used to measure the pulse distortion. Generally, it is desirable to have a linear phase response, i.e. constant group delay. Figure 12 describes the simulated group delay of the UWB antenna and the UWB antenna with band-notched characteristics. The variation of the group delay of the UWB antenna is less than 1 ns, while group delay of the UWB antenna with notch characteristics exceed 1 ns at the notch band, which distorts the minimal phase linearity. However, the group delay variations are small in rest of the frequency band of the UWB antenna with notch characteristics. The group delay characteristics discussed above demonstrate that the proposed antennas exhibit phase linearity at desired UWB frequencies. It is found from aforesaid studies that the antenna has good pulse handling capability in the UWB frequency band.
Fig. 12. Group delays for the proposed compact UWB microstrip antenna – and UWB microstrip antenna with dual band-notched characteristics.
V. CONCLUSION
A compact CPW-fed UWB-printed monopole antenna with dual band-notched characteristics is proposed, fabricated, and discussed. The two designed band notches were realized by etching quarter-wavelength L-shaped slot in radiating patch and two quarter-wavelength L-shaped slots in both the ground planes. To get the desired band rejection, the effects of the overall length of the slots were analyzed. Surface current distributions were used to show the effect of these L-shaped slots in getting the notched bands. The fabricated antenna showed good agreement between the measured and simulated results with a wide bandwidth from 2.7 to 14.40 GHz and two intended notched bands in a small size.
Swati Yadav was born in Roorkee, India, in 1988. She completed her B. Tech. in Electrical and Electronics Engineering and M. Tech. in Digital Signal Processing from Uttarakhand Technical University, Dehradun, India in 2010 and 2013, respectively. Presently, she is working toward her doctorate degree from G B Pant Engineering College, Pauri Garhwal, India. She has published many research papers in peer-reviewed journals/conferences. Her main research interest is designing and modeling of ultra wideband microstrip antenna.
A K Gautam was born in Noida, Uttar Pradesh, India. He received his B.E. degree in Electronics and Communication Engineering from Kumaon Engineering College, Almora, India and his Ph.D. degree in Electronic Engineering from Indian Institute of Technology, Banaras Hindu University, Varanasi, India, in 1999 and 2007, respectively. He joined the Department of Electronics and Communication Engineering, G B Pant Engineering College, Pauri Garhwal, India, in 2000, as an Assistant Professor and he has been an Associate Professor there since 2009. Dr. Gautam is an active member of Board of Study (BOS), Academic council, and many other academic committees of G B Pant Engineering College (GBPEC), Pauri. He is also a member of BOS of HNB Garhwal Central University, India and Uttarakhand Technical University, Dehradun, India. He is nominated as Nodal Officer, TSP and SCSP Grants by Government of Uttarakhand and executed several projects under these grants. He has supervised 20 M. Tech. and one Ph.D. thesis and currently supervising nine Ph.D. theses in the area of microstrip antenna. Dr. Gautam is the author/co-author of more than 70 research papers published in the refereed international journals and conferences such as IEEE, Microwave, and Optical Technology Letters, Springer, etc. He is the author of 13 books in the field of Electronics Engineering, Digital Electronics, Antenna, and Microwave Engineering. He is a member of IEEE (USA) and many other technical societies. He is also in reviewers panel of IEEE, Transaction on Antenna and Propagation, IEEE, Antenna and Wave Propagation Letters, IET Microwaves, Antennas and Propagation, Personal and wireless communication, Springer, International Journal of Electronics, International Journal of Microwave and Wireless Technologies, International Journal of Antenna and Propagation, etc. His main research interests are in design and modeling of active microstrip antenna, microstrip antennas with defected ground structure, ultra wide bandwidth antennas and reconfigurable antennas, reconfiguration antenna array, circular polarized antenna, etc.
Binod Kumar Kanaujia is presently working as an Associate Professor in the Department of Electronics and Communication Engineering in Ambedkar Institute of Advanced Communication Technologies and Research (formerly Ambedkar Institute of Technology), Geeta Colony, Delhi. Dr. Kanaujia joined this Institute as an Assistant Professor in 2008 through selection by Union Public Service Commission, New Delhi, India and served on various key portfolios, i.e. Head of Department, In-charge Central Library, Head of Office, etc. Before joining this institute he served in the M.J.P. Rohilkhand University, Bareilly, India as a Reader in the Department of Electronics and Communication Engineering and also as the Head of the Department. He has been an active member of Academic Council and Executive Council of the M.J.P. Rohilkhand University and played a vital role in academic reforms. Prior, to his career in academics, Dr. Kanaujia worked as an Executive Engineer in the R&D division of M/s UPTRON India Ltd. He completed his B.Tech. in Electronics Engineering from KNIT Sultanpur, India in 1994. He did his M.Tech. and Ph.D. in 1998 and 2004, respectively from the Department of Electronics Engineering, Indian Institute of Technology, Banaras Hindu University, Varanasi, India. He has been awarded Junior Research Fellowship by UGC Delhi in the year 2001–2002 for his outstanding work in electronics field. He has keen research interest in design and modeling of microstrip antenna, dielectric resonator antenna, left-handed metamaterial microstrip antenna, shorted microstrip antenna, ultra wideband antennas, and reconfigurable and circular-polarized antenna for wireless communication. He has been credited to publish more than 105 research papers with more than 200 citations with h-index of 10 in peer-reviewed journals and conferences. He has supervised 45 M. Tech. and three Ph.D. research scholars in the field of microwave engineering. He is a reviewer of several journals of international repute, i.e. IET Microwaves, Antennas and Propagation, IEEE Antennas and Wireless Propagation Letters, Wireless Personal Communications, Journal of Electromagnetic Wave and Application, Indian Journal of Radio and Space Physics, IETE Technical Review, International Journal of Electronics, International Journal of Engineering Science, IEEE Transactions on Antennas and Propagation, AEU-International Journal of Electronics and Communication, International Journal of Microwave and Wireless Technologies, etc. He has successfully executed four research projects sponsored by several agencies of Government of India, i.e. DRDO, DST, AICTE, and ISRO. He is also a member of several academic and professional bodies, i.e. IEEE, Institution of Engineers (India), Indian Society for Technical Education, and the Institute of Electronics and Telecommunication Engineers of India.
