I. INTRODUCTION
Electromagnetic bandgap (EBG) structures are periodic structures (periodic arrangement of metallic or nonmetallic elements in one, two, or three dimensions), which on interaction with electromagnetic waves, prohibit/assist the propagation of electromagnetic waves in specific bands of frequencies. Before Sievenpiper et al. introduced a mushroom-like EBG structure [Reference Sievenpiper, Zhang, Broas, Alex́opolous and Yablonovitch1], reported periodic structures had a half wavelength gap between the elements, which made them inconvenient for microwave applications. Since then, various mushroom EBG structures have been reported for different applications in microwave frequency bands, however, with a fabrication challenge. Yang et al. introduced the uniplanar compact EBG structure [Reference Yang, Ma, Qian and Itoh2], which consists of metal patches separated by a gap connected through narrow lines with inset. The current flow through these narrow lines contributes to inductance and gaps contribute to capacitance, thereby forming a distributed Inductance and capacitance L-C (tank) network. Also, the absence of via holes in these structures makes them convenient for fabrication. Uniplanar EBG structures were further modified by increasing the inductance and the capacitance of unit cell [Reference Lin, Liang, Zeng and Zhang3, Reference Lin, Zheng and Yuan4] to make them compact.
It is desirable to have a compact EBG structures for various applications. For example, it is desirable to reject interfering signals in UWB (ultra wideband) communications. UWB BPFs (band pass filters) with a notched band were realized by different techniques such as an embedded open-circuited stub [Reference Sarkar, Ghatak, Pal and Poddar5–Reference Li, Kurita and Matsui11], asymmetric parallel coupled lines [Reference Shaman and Hong12], an asymmetric interdigital coupled feed line [Reference Kim and Chang13], an asymmetric coupling strip [Reference Song and Xue14], embedding L-shaped slots in the stepped impedance resonator (SIR), and loading the SIR with resonators [Reference Lee, Hsu and Chen15], meander line slot for a notch and defected ground structures are used to insert transmission zeros to stop spurious passband [Reference Yang, Jin, Vittoria, Harris and Sun16], a meander slot-line structure embedded in a hybrid microstrip and a CPW (co-planar waveguide) detached mode resonator structure to realize the notch band [Reference Luo, Ma, Ma and Yeo17], based on the waves cancellation theory [Reference Nosrati and Daneshmand18, Reference Wong and Zhu19], loading multimode resonator (MMR) with a parasitic coupled line [Reference Pirani, Nourinia and Ghobadi20], embedded folded SIR [Reference Hao and Hong21] using multilayer liquid crystal polymer (LCP) technology, short-circuited stub resonators that are integrated in the bandpass filter with slotted ground structure are used to realize single or multiple notch bands [Reference Hao, Hong, Parry and Hand22] using multilayer LCP technology, embedding SIR coupled to BPF [Reference Ghatak, Sarkar, Mishra and Poddar23–Reference Wei, Xu, Shi and Liu25], using a defected split-ring resonator ground structure[Reference Li, Li, Liang and Wu26], notch using complementary split ring resonators [Reference Ali and Hu27], dual notched bands by using a simplified composite right/left-handed resonator [Reference Wei, Wu, Shi and Chen28], a dual narrow notched band based on two mushroom-type EBG structures is presented in [Reference Liu, Yin, Yang, Jing and Sun29], multiple frequency notches are generated by loading some SRRs to the input or output microstrips [Reference Wang, Hong, Tang, Zhang, Dong and Wu30]. Tunable and switchable notch bands in UWB BPF are realized in [Reference Wei, Chen, Shi and Liu31–Reference Chun, Shaman and Hong33]. In the literature, mushroom EBG structures have been reported to introduce a transmission notch in the UWB filters but uniplanar EBG structures producing a notch in UWB filter have not been reported to the best of our knowledge.
In this report we present a compact uniplanar EBG structure and its application in a notched-band UWB filter and also proposed switchable and tunable notched UWB filters.
II. EBG STRUCTURE AND CHARACTERIZATION
Figure 1 shows the unit cell of the proposed EBG structure. This is arrived at by appropriately modifying the conventional uniplanar EBG structure [Reference Yang, Ma, Qian and Itoh2] shown in Fig. 2. (All dimensions are in mm.)
Fig. 1. The proposed EBG structure.
Fig. 2. The conventional uniplanar EBG strucure.
The capacitance is obtained by the interdigital lines on the periphery of the unit cell, whereas, the inductance is obtained through the meander lines from the center of the structure to its four corners as shown in Fig. 1. This results in a compact structure as compared to the structure reported in [Reference Yang, Ma, Qian and Itoh2]. Dispersion diagrams for 2-D structures are obtained using CST eigen mode solver. Dispersion diagrams of the proposed structure and the conventional planar EBG are shown in Figs 3(a) and 3(b). It is seen from the dispersion diagrams that the proposed structure offers a bandgap in the 5.48–7.9 GHz band. As compared with the bandgap (9.41–12.41 GHz) of the conventional EBG structure of same size, nearly 38% reduction in the bandgap center frequency is seen in the proposed structure. TE (transverse electric) and TM (transverse magnetic) surface wave measurements [Reference Sievenpiper, Zhang, Broas, Alex́opolous and Yablonovitch1] were carried out on the fabricated 7 × 7 array of proposed EBG cells and the results are shown in Fig. 4, which validate a surface-wave bandgap. The proposed structure is equally compact as the structure reported in [Reference Lin, Liang, Zeng and Zhang3], however, the proposed structure results in good coupling when placed adjacent to the microstrip line.
Fig. 3. Dispersion diagrams of the (a) proposed structure and (b) conventional planar EBG.
Fig. 4. TE and TM surface wave measurement results.
The proposed structure was further characterized by placing 7 × 1 cells symmetrically along a 50Ω transmission line as shown in Fig. 5(a). The spacing between the transmission line and the structure is 0.1 mm. A photograph of the fabricated structure is shown in Fig. 5(b). This arrangement introduces an additional capacitance between the transmission line and the EBG structure, which results in further reduction in the notch frequency band to 4.97–5.37 GHz as shown in Fig. 5(c). The above results from the dispersion diagrams, TE–TM measurements, and with the transmission line indicate that the proposed structure can be used for obtaining a notch in a UWB filter. It was also seen in the simulation that the rejection band can be tuned by properly adjusting the size of the unit cell (p) or by appropriately changing the inductances and the capacitance by varying w and g in the structure (Fig. 1).
Fig. 5. EBG structure coupled to a microstrip line. (a) Schematic, (b) photograph, (c) simulation, and measurement results.
III. APPLICATION OF EBG FOR BAND-NOTCH UWB FILTER
All structures were fabricated on GML 1000 substrate having a dielectric constant (ɛr) 3.2, thickness 0.762 mm and copper metallization of 0.017 mm which is then gold plated. All measurements were done using ZVB20 Rohde & Schwarz vector network analyzer. A Well-known MMR UWB filter [Reference Zhu, Sun and Menzel34] was fabricated with dimensions as shown in Fig. 6(a). A photograph of the fabricated filter is shown in Fig. 6(b) and its measurement results are shown in Fig. 6(c). From the measurement results, the passband of the UWB filter is from 3.47 to 10.23 GHz. The return loss is better than 11.5 dB in the passband. The maximum group delay variation in the passband is 0.366 ns.
Fig. 6. MMR UWB filter: (a) Schematic, (b) photograph, and (c) measurement results.
A) Single notch-band UWB filter
In the proposed method for producing a notch in the passband of the UWB filter, seven unit cells of EBG are placed adjacent to the output line of MMR filter on either side as shown in Fig. 7(a) (Filter A). The measurement results shown in Fig. 7(b) indicate the passband is from 3.37 GHz to 9.97 GHz with a fractional bandwidth of 99%. The insertion loss at the center frequency (6.67 GHz) is 1.35 dB. The notch band is from 4.88 to 5.48 GHz centered at 5.19 GHz with a notch depth of 16.8 dB. The return loss is better than 11 dB in the passband except in the notch region. The maximum group delay variation in the passband is 0.28 ns before the notch and 0.54 ns after the notch.
Fig. 7. Notch UWB filter with EBG coupled to output line (Filter A). (a) Photograph, (b) measurement results, and (c) simulation results of notch band UWB filter with Et = 0 and OAS boundary conditions.
When the UWB filter is cascaded with the EBG, we have observed that the result is the sum of individual responses. With the effect of EBG a wide upper stop band with good depth is achieved, which can be seen by comparing the results in Figs. 6(c) and 7(b). This is because of higher bandgaps of the EBG falling in the upper stop band of the UWB filter. It is also seen from the results in Fig. 7(b) that even though good |S 12| is achieved after 10 GHz, |S 22| is not close to 0 dB around 11.5 GHz and 13 GHz. This is because the structure is radiating at higher frequencies. The same is not seen in |S 11| as the filter structure is preceding the EBG structure in the case of input in port 1. To study the interference of these radiations, simulations were done in CST microwave studio with open add space (OAS) as boundary conditions and Et = 0 as boundary conditions in all directions. Simulations results are shown in Fig. 7(c). It is seen from the simulations that there is no difference in the performance of the filter, which shows that radiations due to the EBG structures are not affecting the filter performance.
The limitation of this filter is that if the first band gap of the EBG falls below 5 GHz then the second band gap will fall below 10.6 GHz. Thus getting a notch frequency below 5 GHz without affecting the upper side of passband using this procedure is not possible. Fortunately, WLAN frequencies interfering UWB band are above 5 GHz.
The depth of the notch in the Filter ‘A’ can be further improved by placing the EBG on either side of the filter. The photograph of the filter with the EBG on input and output lines is shown in Fig. 8(a) (Filter B) and the measurement results are shown in Fig. 8(b). With the addition of EBG cells along the input line, the size of the filter became larger than Filter ‘A’. The 3 dB passband is from 3.38 to 9.79 GHz with a fractional bandwidth of 97.27%. There is a slight decrease in the bandwidth. The insertion loss at the center frequency (6.59 GHz) is 1.51 dB. The notch band is from 4.78 to 5.6 GHz centered at 5.19 GHz with an improved notch depth to 32.3 dB. The return loss is better than 13 dB in the passband except in the notch region. The maximum group delay variation in the passband is 0.42 ns before the notch and 0.49 ns after the notch.
Fig. 8. Notch UWB filter with EBG coupled to input and output line (Filter B). (a) Photograph and (b) measurement results.
To reduce the size of the overall structure, the EBG cells are distributed along the filter with some cells along the low impedance line of the MMR filter as shown in Fig. 9(a) (Filter C). Here, three unit cells of EBG are placed on either side of the line at three different places. The measurement results are shown in Fig. 9(b). The 3 dB passband is from 3.3 to 9.99 GHz with fractional bandwidth of 100%. The insertion loss is 1.26 dB at the center frequency (6.63 GHz). The 3 dB notch is from 4.69 to 5.55 GHz centered at 5.16 GHz with a notch depth of 18.8 dB. The return loss is better than 9.73 dB throughout the passband except in the notch region. The maximum group delay variation in the passband is 0.34 ns before the notch and 0.38 ns after the notch. This filter ‘C’ is the optimal solution in terms of size and notch depth.
Fig. 9. Notch UWB filter with EBG coupled to input line, output line and on low impedance line of the filter (Filter C). (a) Photograph and (b) measurement results.
B) Dual notch-band UWB filter
Dual notch bands can be obtained with two different sized EBG cells. EBG with unit cell size of 4.32 mm is used in all the above circuits. In the next circuit, seven unit cells of size 4.32 mm are placed on either sides of the output line and seven EBG cells of 3.65 mm are placed on either sides of the input line. The photograph of the dual notch band filter is shown in Fig. 10(a) (Filter D). Measurement results are shown in Fig. 10(b). The 3 dB passband is from 3.43 to 9.84 GHz with a fractional bandwidth of 96.61%. The insertion loss at the center frequency (6.635 GHz) is 1.59 dB. The first 3 dB notch band is from 4.85 to 5.46 GHz centered at 5.16 GHz with a notch depth of 16 dB. The second 3 dB notch band is from 7.45 to 9.03 GHz centered at 8.24 GHz with a notch depth of 23 dB. The return loss is better than 11.5 dB throughout the passband except in the notch regions. The maximum group delay variation in the passband is 0.23 ns before the first notch, 0.4 ns in between the notches and 0.15 ns after the second notch.
Fig. 10. Dual notch UWB filter with EBG coupled to input and output line (Filter D). (a) Photograph and (b) measurement results.
C) Reconfigurability and tunability of the notch UWB filter
The notch frequency can be changed by changing the unit cell dimension, based on this, reconfigurability and tunability can be achieved. Figure 11(a) shows a photograph of the switchable filter in ideal on state. Coupled lines with two small pads are connected to the unit cell by a connecting strip as shown in the inset of Fig. 11(a). When the strip is connected, the unit cell dimension increases thereby notch frequency decreases. This is observed in simulations and measurements were done with an ideal on state (with a connecting strip) and an ideal off state (without a connecting strip). The measurement results of the ideal on and off states are shown in Fig. 11(b). From the results it is seen that the notch frequency changes from 8.35 GHz in the off state to 7.4 GHz in the on state. Thus by using a diode in the place of strip, a switchable notch filter can be achieved. If varactors are used in the place of connecting strips, tuning of the notch can be achieved.
Fig. 11. Switchable notch band UWB filter. (a) Photograph of an ideal on state and (b) measurement results of on state and off state.
IV. CONCLUSION
The paper presents a new compact uniplanar EBG structure as compared with the conventional uniplanar EBG structure. The proposed structure is suitable for obtaining a bandgap by placing adjacent to a microstrip line. This method of obtaining a notch by coupling a uniplanar EBG structure to a microstrip line is the first of its kind and it is successfully applied to obtain a notched UWB filter. All single-notched UWB filters have shown the notch, centered around 5.19 GHz. The dual notch filter presented has the second notch centered at 8.24 GHz with a notch depth of 23 dB in addition to the notch at 5.16 GHz with a depth of 16 dB. Switchable and tunable-notched UWB filters are proposed and a switchable notch filter is verified with ideal diode conditions. The important characteristics of various notched UWB filters in this paper are summarized in Table 1.
Table 1. Summary of the characteristics of band notch UWB filter
Lalithendra Kurra received B.E degree from Arunai Engineering College, Tamil Nadu in 2001 and M.Tech degree from College of Engineering, Andhra University, Andhra Pradesh in 2004. He worked as Assistant Professor for five and half years during 2004–2009 in CVR college of Engineering, Hyderabad, Andhra Pradesh. Currently he is pursuing his Ph.D. with teaching assistantship from the Center for Applied Research in Electronics (CARE), IIT Delhi, New Delhi, India. His main research interests are EBG structures, microwave integrated circuits, and planar antennas.
Mahesh P. Abegaonkar received his Ph.D. in Physics (Microwaves) from the University of Pune in 2002. He worked as a post-doctoral researcher and Assistant Professor at Kyungpook National University Daegu, South Korea during 2002–2005. Currently he is an Assistant Professor at Centre for Applied Research in Electronics (CARE), Indian Institute of Technology (IIT) Delhi. His current research activities include microwave and millimeter wave antennas, EBG and DGS structures and microwave engineering. He is currently the Secretary and Treasurer of IEEE MTT-S Chapter under Delhi Section.
Ananjan Basu (born August 12, 1969) did his B.Tech. in Electrical Engineering and M.Tech in Communication and Radar Engineering from IIT, Delhi in 1991 and 1993, respectively, and Ph.D. in Electrical Engineering from the University of California, Los Angeles in 1998. He has been employed at the Centre for Applied Research in Electronics, IIT Delhi as Assistant Professor from 2000 to 2005 and as Associate Professor from 2005. His specialization is in microwave and millimeter-wave component design and characterization.
S. K. Koul received the B.E. degree in Electrical Engineering from the Regional Engineering College, Srinagar, India, in 1977, and the M.Tech. and Ph.D. degrees in Microwave Engineering from the Indian Institute of Technology Delhi, India. He is a Professor with the Centre for Applied Research in Electronics, Indian Institute of Technology Delhi where he is involved in teaching and research activities. His research interests include: RF MEMS, device modeling, millimeter wave IC design and reconfigurable microwave circuits including antennas. He is the Chairman of M/S Astra Microwave Pvt Ltd, a major private company involved in the development of RF and microwave systems in India. He is author/co-author of 192 research papers and seven state-of-the art books. He has successfully completed 25 major sponsored projects, 50 consultancy projects, and 30 Technology Development Projects. He holds seven patents and four copyrights.
Dr. Koul is a Fellow of IEE, USA, Fellow of the Indian National Academy of Engineering (INAE) India and Fellow of the Institution of Electronics and Telecommunication Engineers (IETE) India, He has received Gold Medal presented by Institution of Electrical and Electronics Engineers Calcutta (1997), S.K.Mitra Research Award from IETE for the best research paper; Indian National Science Academy (INSA) Young Scientist Award (1986); International Union of Radio Science (URSI) Young Scientist Award (1987) the top Invention Award (1991) of the National Research Development Council for his contributions to the indigenous development of ferrite phase shifter technology; VASVIK Award (1994) for the development of Ka-band components and phase shifters; Ram Lal Wadhwa Gold Medal (1995) from the IETE; Academic Excellence award (1998) from Indian Government for his pioneering contribution to phase control module for the Rajendra Radar Shri Om Prakash Bhasin Award (2009) in the field of Electronics and Information Technology, and teaching excellence award from IIT Delhi in 2012.
Dr Koul is a distinguished IEEE Microwave Theory and Techniques Lecturer for the years 2012–2014.
