I. INTRODUCTION
In the era of technological advancement, there has been a tremendous increase in the demand of small-sized high-performance multiband personal communication handsets capable of covering various standards developed in wireless communication simultaneously [Reference Balanis1]. It has led to the research and studies of microstrip antennas as a viable solution to this interesting problem due to its advantages such as low-profile, conformability, low-fabrication cost, and ease of integration with feed networks [Reference Islam, NazmusShakib, Misran and Sun2].
The rapid developments of wireless communication systems, especially for WLAN, worldwide interoperability for microwave access (WiMAX), and long term evolution (LTE) applications, which cover all the bands of 2.4/5.2/5.8 GHz, 2.5/3.5/5.5 GHz, and 3.4/4.4 GHz, respectively, have aroused much interest in the research of multi band and broadband antennas [Reference Chair, Mak, Lee, Luk and Kishk3]. In [Reference Parkash and Khanna4], the authors have referred to various papers pertaining to coplanar waveguide(CPW)-fed design configurations such as a monopole antenna fed with a meandered coplanar waveguide, CPW-fed tapered bent folded monopole antenna, printed monopole antenna with a trapezoid conductor-backed plane, CPW-fed monopole antenna with a cross-slot for WLAN operation, CPW-fed Koch-fractal slot antenna, dual-band annular-ring slot antenna, and to obtain broadband characteristics in antenna operation [Reference ThiruvalarSelvan and Raghavan5–Reference Raj, Joseph, Aanandan, Vasudevan and Mohanan10].
In this paper, we have combined two techniques i.e. modified CPW feed and slotting and applied them on the radiating rectangular patch in a different fashion to create multiple bands and widen the bandwidth of each band. The design of wideband microstrip patch antenna is presented in Section II. The results and discussion are given in Section III.
II. ANTENNA DESIGN
In the proposed design, the antenna patch is made of perfect electric conductor mounted on FR4 substrate with relative permittivity, ε r = 4.4, tan delta = 0.027 at 2 GHz and height, h = 1.6 mm.
The antenna structure consists of a rectangular patch with E-slot in its center, rotated 90° anticlockwise. The geometry of the antenna patch is depicted in Fig. 1, where L p (length of patch) and W p (width of patch) are connected at the end of the CPW feed line.
Fig. 1. Geometry of the antenna patch.
It has been noticed in the simulation that the operating bandwidth of the rotated E-shaped microstrip patch antenna is critically dependent on the length (L p) and width (W p) of the rectangular patch of the antenna. The optimization is done in terms of length and width of patch to obtain optimal band of operation for the WiMAX and LTE systems and good impedance matching at each resonating centre frequency. The final optimized geometric parameters of the proposed antenna are: L p = 43.75 mm, W p = 51.95 mm, substrate length L s = 90 mm, and substrate width W s = 90 mm. Four slots are etched in the radiating patch to control the current flow on the antenna surface across the targeted frequency bands. The optimized slot dimensions obtained are: slot length L s1 = 24 mm, L s2 = 4 mm, L s3 = 4 mm, and L s4 = 4 mm; slot width w s1 = 12 mm, w s2 = 10 mm, and w s3 = 12 mm.
A CPW transmission line is used for feeding the antenna. It consists of a signal strip of width w f = 4.94 mm with a gap distance, d = 0.175 between the signal strip and the coplanar ground plane. Two equal finite ground planes are placed symmetrically on each side of the CPW feed-line to surround the rectangular patch. The fabricated design structure of the antenna is shown in Fig. 2. The width and position of the stripline feed determine the impedance matching of the antenna at a particular resonant frequency as shown by the smith chart in Fig. 3.
Fig. 2. Fabricated antenna structure.
Fig. 3. Smith chart for impedance matching of the proposed antenna structure.
III. PARAMETRIC STUDY OF ANTENNA
Parametric study is investigated and it demonstrates that the following parameters influence the performance of the proposed antenna in terms of return loss, impedance bandwidth, gain, etc.
A) CPW ground plane structure
A CPW ground plane structure surrounds the rectangular patch on all sides in such a manner to create a spacing between the patch edge and the CPW ground referred to as “x” on X-axis and “v” on vertical i.e. Y-axis. Along the X-axis the initial value of the gap is fixed at 12 mm and sweep function is applied by varying x from 0 to 15 in steps of 3. It is observed that at x = 0, we get a triple band. When the value of x is increased it leads to merging of 2nd and 3rd band generating a broadband antenna, the best value being x = 6 or the total gap value of 18 mm for which the bandwidth is 1.15 GHz. The antenna responds symmetrically both along the positive and negative X-axis as shown in Fig. 4.
Fig. 4. Sweep results for variation of spacing between patch edge and CPW ground along X-axis.
The vertical spacing “v” is also analyzed along Y-axis for broadbanding and multibanding characteristics. “v” is varied from 0 to 45 mm with step size of 5. Starting with the initial gap of 1 mm, it is observed that v = 0, i.e. the vertical gap value of 1 mm yields best results as shown in Fig. 5.
Fig. 5. Sweep results for variation of spacing between patch edge and CPW ground along +ve Y-axis.
B) Slotting on radiating patch
The rectangular patch is slotted as shape of “E” rotated 90° anticlockwise and the effect of variation of the position and dimension of each slot on the bandwidth is studied and plotted in the return loss characteristics. A collective sweep is applied to all slots by a variable parameter b, which gives best bandwidth at value −16 mm as shown in Fig. 6. This value is in addition to the independent initial value for each slot which gives the optimized slot dimensions as: slot length L s1 = 24 mm, L s2 = 4 mm, L s3 = 4 mm, and L s4 = 4 mm; slot width w s1 = 12 mm, w s2 = 10 mm, and w s3 = 12 mm.
Fig. 6. Sweep results for variation of slot dimensions.
IV. RESULTS AND DISCUSSION
The optimized antenna results obtained after applying sweep to various design parameters are presented below.
A) Return loss
The return loss for the first band so obtained covering frequencies from 1.56 to 1.69 GHz is −14.6 dB and as that of second band in the frequency range from 2.6 to 3.75 GHz is −19.76 which is well below −10 dB level required for the antenna to operate efficiently (Figs 7(a) and 7(b)).
Fig. 7. (a) Return loss/impedance bandwidth for first band. (b) Return loss/impedance bandwidth for second band.
B) Current distribution and radiation pattern
The current density and radiation patterns are analyzed using CST MWS. With a series of simulations it is seen that the surface current on the patch region of the antenna around the edges cause resonance at 1.62 and 3.61 GHz (Figs 8(a) and 8(b)) and the E-slots contribute to the radiation at 2.71 and 3.14 GHz (Figs 8(c) and 8(d)).
Fig. 8. Simulated current density on the surface of the antenna at (a) 1.62 GHz, (b) 3.61 GHz, (c) 2.71 GHz, and (d) 3.14 GHz.
The radiation pattern is plotted at the different resonances. Almost Omni-directional radiation patterns at the resonant bands are observed in Fig. 9(a–d).
Fig. 9. Radiation pattern of the antenna at (a) 1.62 GHz, (b) 2.71 GHz, (c) 3.14 GHz, and (d) 3.61 GHz.
The gain of the antenna is found to be constant throughout the resonant bands which make the antenna stable as seen from the graph plotted between gain versus frequency in Fig. 10.
Fig. 10. Variation of gain with frequency.
The simulated and measured results are compared in Fig. 11 which verifies the designed structure in terms of return loss and impedance bandwidth obtained for different bands. The shift in resonant frequency between measured and simulated results is due small errors in fabricated structure.
Fig. 11. Simulated and measured result.
V. CONCLUSION
In this paper, we present a design of a CPW-fed rotated E-slot microstrip patch antenna which is capable of operating at multiple bands with enhanced bandwidth at each band to be used in LTE/WiMAX applications. Measured results indicate that the antenna exhibits a good impedance bandwidth of 143.86 MHZ, 1.147 GHz at different bands. A return loss S11 parameter is found to be well below −10 dB, and considerable antenna gain with constant value is observed.
Geetanjali Singla received her Masters in Electronics and Communication Engineering from BBSBEC, Fatehgarh Sahib in 2010 and is doing her Ph.D. degree from Thapar University. Presently, she is a lecturer at Thapar Polytechnic College, Patiala. Her main research interests are design and optimization of microstrip antenna, multiband and wideband antenna, and wireless communication networks. She has published many papers in journals and conferences on microstrip antenna and wireless communications networks.
Rajesh Khanna received his B.Sc. (Engg.) degree in ECE in 1988 from REC, Kurukshetra and M.E. degree in 1998 from IISc., Bangalore. He was with Hartron R&D center till 1993. Until 1999, he was in AIR as AS Engg. Presently, he is working as Professor and HOD in the ECED at Thapar University, Patiala. He has published 80 papers in National and International journal/Conferences. He has worth Rs 1.5 crore projects to his credit. His main research interest includes antennas, wireless communication, MIMO, and FFT.
