I. INTRODUCTION
High-quality factor, low insertion loss, low cost, and small size are the features desired in any microwave system. Microstrip ring resonator (RR) has met all these requirements and has found many potential applications in microwave systems. The last decade has witnessed continuous and intense research to reduce overall loss and to enhance the Q. Microstrip lines combined with variants of Yablonovite structures [Reference Yoblonovictch1, Reference John2] have been very useful in the pursuit, which are also called photonic band gap (PBG) or electromagnetic band gap (EBG) structures. Other variants of PBG structures are periodic defect ground structures that have demonstrated remarkable potential to reduce the loss and increase the Q [Reference Radisic, Qian, Coccioli and Itoh3–Reference Ahn, Park, Kim, Kim, Qian and Itoh5]. However, these structures, when combined with planar microstrip line devices, have their own advantages and disadvantages. The common disadvantages are as follows:
– Slow wave effect is dependent on the angle and positions of the microstrip line with respect to the principal axis of their periodic structures [Reference Yang, Ma, Qian and Itoh6, Reference Lim, Park, Lee, Ahn and Nam7].
– It is difficult to realize EBGs as the width of the line is generally very narrow [Reference Xue, Shum and Chan8].
– Etched defect also acts as a scattering point [Reference Lim, Park, Lee, Ahn and Nam7].
– Defects on the back of the substrate alter the performance of the line [Reference Ritika and Daya9].
– These structures are not uniplanar as the periodic structures are patterned at the back, making analysis of the combined system very complex [Reference Lim, Park, Lee, Ahn and Nam7].
In 2002, Yunbo Pang presented a simple and new technique of sandwich structure of PBG where square metallic pads were printed in the midmost of the substrate which is suspended by microstrip line structure. This technique can tune the width and depth of the forbidden gap easily [Reference Yunbo and Baoxin10].
The present paper reports the same technique of suspended periodic defect structure (SPDS) that effectively overcomes the aforementioned difficulties and enhances the device performance without modifying the structure or its ground plane. Circular dielectric basis points were spread periodically over the suspended structure, which effectively enhanced the quality factor of the device. This paper also presents a similar technique wherein effects of SPDS have been investigated on a planar ring resonator.
II. DESIGN AND MEASUREMENT
Circular dielectric basis points with square and triangular lattice pattern of radius r = 1.57 mm and lattice constant 8.00 and 12.00 mm, respectively, were simulated on RF band solve at 1.5 GHz. Two medium-sized printed circuit boards of ɛ r = 4.5 were used to realize the structure where square and triangular SPDS were periodically etched over the top face of the substrate and inserted at 2 mm distance beneath the ground plane of RR as shown in Fig. 1. RR was designed at 1.5 GHz of width 1.77 mm corresponding to 50 Ω characteristic impedance and satisfies the resonance condition that the perimeter of ring is an integral multiple of guided wavelength:
where r is the angular radius and λ g is a function of the microstrip parameter. RR with SPDS was simulated on CST microwave studio and fabrication of the device and EBG patterns were carried out by the photolithography process and wet etching was adopted for the removal of copper. Measurements have been taken on a vector network analyzer (Rohde & Schwarz ZVA 50). The performance of RR with SPDS was compared with the performance of RR without SPDS.
Fig. 1. (a) Top view of microstrip RR with SPDS, (b) square SPDS, and (c) triangular SPDS.
III. RESULTS
Figure 2 shows simulation versus measurement results of RR with and without square and triangular SPDS. Simulation and experimental results are in agreement. It was observed that RR without SPDS was resonant at 1.515 GHz with 17.08 dB insertion loss and quality factor of 43. On inserting SPDS near the ground plane the frequency switched to 1.546 GHz for square symmetry and 1.638 GHz for triangular symmetry with considerable reduction in loss and increase in quality factor. The loaded quality factor is obtained from measured resonant frequency f and half power (−3 dB) bandwidth Δf of the resonance:
Fig. 2. Simulated and measured performance of RR with and without SPDS. ———, Measured result; – – – –, simulated result.
Results are tabulated in Table 1.
Table 1. Performance comparison of RR with and without SPDS.
Shift in resonance is caused due to additional capacitive and inductive components introduced from SPDS [Reference Ritika and Daya9] which also facilitate in system performance enhancement by suppressing the surface wave.
A) Effect of radius of SPDS
Further the simulation study, using CST microwave studio, was carried out to understand the switching behavior of RR by varying the radius of circular patterns on SPDS from r = 1.57 mm to 1.47 mm and 1.67 mm, respectively; shift in resonance has been observed. A 128 MHz of resonance shift was observed toward lower frequency in the case of decreasing the radius by R = 1.47 whereas 100 MHz of shift in resonance toward higher frequency value was observed by changing the radius to R = 1.67 mm. Resonance shift was also observed in simulation results at different radii of triangular SPDS. By changing the radius of triangular SPDS from 1.57 to 1.44 and 1.67 mm, 61 and 86 MHz resonance shift was demonstrated toward lower- and higher-frequency sides, respectively. The reason for shift in resonance is the energy localization in square and triangular SPDS in the form of capacitance due to the suspended structure. It is clear from Fig. 3 that shift in resonance varies with radius because energy localization differs due to the changing radius of the suspended structure.
Fig. 3. Simulated response of square SPDS at different radii.
On comparing responses of square SPDS with triangular SPDS, the localization of energy is found to be greater in square SPDS resulting in larger resonance shift. Triangular SPDS can be attributed to the triangular dielectric basis points on the EBG plane. These EBG planes act as scattering centers and thus lesser shift is observed due to less energy localization (Fig. 4).
Fig. 4. Simulated response of triangular SPDS at different radii.
In order to observe the field behavior in RR with SPDS, these SPDS are compared with ring RR with suspended plane ground plane (PGP). Figure 5(a) clearly depicts that the fields are fully contained within the RR and do not even come outside the RR in the case of RR with suspended PGP. However, in the case of an RR with SPDS fields are coming outside the RR (in suspended region) and through dielectric basis points, these fields are localizing in SPDS as shown in Figs 5(b) and 5(c).
Fig. 5. Cross-sectional view of (a) field distribution in the suspended region of RR with PGP, (b) field distribution in the suspended region of RR with SPDS, and (c) top view of the field in SPDS.
This paper reports an interesting study of the effect of SPDS structure. This structure when inserted near the ground plane of the device can switch the frequency that can be controlled by choosing the radius as shown in Fig. 6. These SPDS facilitate loss suppression and increase in quality factor.
Fig. 6. Response of change in frequency versus ratio of radius r and lattice constant a for (a) square SPDS and (b) triangular SPDS.
IV. EFFECT OF EBG ON A MICROSTRIP LINE
To realize the behavior of square and triangular EBG structures, these EBG structures are simulated with a microstrip line etched on the back plane of these EBG structures instead of PGP. Since the EBGs are designed at pass band frequency of 1.5 GHz, the stop band region is observed at a higher frequency range and at 1.5 GHz both square and triangular EBG show the pass band property. Square EBG demonstrated remarkable band gap of 2.7 GHz from 7.152 to 9.852 GHz (Fig. 7a), whereas triangular EBG demonstrated two band gap regions of 1.5 GHz from 4.94 to 6.53 GHz and 2.2 GHz from 10.31 to 12.53 GHz, respectively, as shown in Fig. 7b. Hence, the above-analyzed SPDS has shown the effect of EBG in improving the performance of RR in terms of loss reduction with increase in quality factor.
Fig. 7. Performance of (a) square EBG structure and (b) triangular EBG structure with a transmission line.
V. CONCLUSION
To conclude, present paper reports the work on SPDS and their role in controlling the performance of planar device without changing the initial design. The adopted technique is a potential tool for three-fold increase in Q and performance enhancement of the device without altering the system design. Further, another attractive feature of this work is the ability to switch the frequency by just inserting SPDS or by changing the radius of SPDS in the near vicinity of the device. Results presented in Section 3 clearly show SPDS to be a more efficient tool in suppressing the surface wave with ability to switch the resonance frequency. This easy-to-implement and easy-to-analyze technique will open a new horizon to seamlessly enhance the performance of existing and new planar microwave systems without the need to modify the device design.
Tulika Giri received her B.Sc. degree from Dayalbagh Educational Institute, Agra, India, in 2007, M.Sc. degree in Pure Physics from the Dayalbagh Educational Institute, Agra, India, in 2009, and M.Phil. degree in Electronics from Dayalbagh Educational Institute, Agra, India, in 2010. Currently, she is working as a Research Associate with Microwave group, Dayalbagh Educational Institute, Agra, India, where she is involved with a project on the low-cost synchronization oscillator, supported by the Ministry of Human Resource and Development, India. Her current research interests include microwave oscillators and EBG structures.
K. S. Daya did her Ph.D. from Dayalbagh Educational Institute in 2002. From 2002–3, she did her post-doctoral work at Research Center Juelich, Germany. Later she joined the Center for Applied Research in Electronics, IIT Delhi, India as Project Scientist where her main research interests were MEMS and Photonic crystals. In 2005, she joined Nokia Siemens Networks as an RF R&D Scientist, where she worked on the development of low-cost and high-gain antenna for standalone mobile network. Since 2007, she is a lecturer at Dayalbagh Educational Institute with her current areas of interest being properties of EBG structures, microwave medical bio-sensors, and low-cost RF devices for wireless communication networks.
