- Email: [email protected]
A novel single feed frequency and polarization reconfigurable microstrip patch antenna
Accepted Manuscript Regular paper A Novel Single Feed Frequency and Polarization Reconfigurable Microstrip Patch Antenna Bharathi Anantha, Lakshminarayana Merugu, Somasekhar Rao PII: DOI: Reference:
S1434-8411(16)30624-0 http://dx.doi.org/10.1016/j.aeue.2016.11.012 AEUE 51728
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
1 September 2016 17 October 2016 16 November 2016
Please cite this article as: B. Anantha, L. Merugu, S. Rao, A Novel Single Feed Frequency and Polarization Reconfigurable Microstrip Patch Antenna, International Journal of Electronics and Communications (2016), doi: http://dx.doi.org/10.1016/j.aeue.2016.11.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Novel Single Feed Frequency and Polarization Reconfigurable Microstrip Patch Antenna Bharathi, Anantha a , M.E(Ph.D), Lakshminarayana, Merugub , Ph.D and Somasekhar Rao, PVD c , Ph.D. a. Assistant Professor, ECE Dept., UCE, Osmania University, Hyderabad, Telangana State, India. e-mail: [email protected] (Corresponding Author) b. Scientist ‘H’(Retd.), DLRL (DRDO) & Prof. and Principal, Kshatriya College of Engg., Armoor, Nizamabad Dist., Telangana State, India. e-mail : [email protected]. c. Professor(Retd.), ECE Dept., JNTUH and Dean Academics, G. Narayanamma Institute of Technology and Science, Shaikpet, Hyderabad, India. e-mail : [email protected].
Abstract— In this paper, a novel single feed frequency and polarization reconfigurable microstrip patch antenna is presented. This antenna mainly comprises of a corner truncated square patch with a rectangular ring slot, eight PIN diodes and six conductive pads. Four PIN diodes are placed symmetrically in the rectangular ring slot to bridge the gap and to switch the frequency between WLAN bands resonating at 5.2 GHz and 5.8 GHz. Four PIN diodes connect the corner truncated square patch to parasitic triangular conductors. PIN diodes are used to Axial Ratio (AR), square microstrip antenna (A1), square ring antenna (A2)
switch the polarization between linear, right hand circular and left hand circular at each frequency. When compared to conventional patch, the proposed design provides a size reduction of 12% at 5.2 GHz, and 30% at 5.8 GHz. The simulated reflection coefficient and radiation patterns are presented and compared with the experimental data. This antenna finds applications for modern wireless communication system. Keywords — frequency reconfigurable, microstrip antenna, PIN diodes, polarization reconfigurable, WLAN. 1. Introduction Reconfigurable microstrip antennas are gaining much attention due to their numerous advantages. They minimize fading effects caused by multipath, increase the system capacity by frequency reuse, provide immunity to interfering signals, increase the communication link quality and reduce co-channel interference. They offer more functionalities than conventional antennas [1][2]. The parameters of the antenna that can be reconfigured are frequency, radiation pattern, polarization or combinations of them. Many researchers have presented work on single parameter reconfiguration [3][4][5]. Recently, several antennas are designed to achieve compound reconfiguration; which enables polarization and pattern reconfiguration [6], frequency and pattern reconfiguration [7], polarization and frequency reconfiguration [8]. The compound reconfiguration can provide significant advantages over single reconfiguration design but the designer has to face harder challenges in (a) modeling reconfiguration mechanism, (b) implementing reconfigurable mechanism without severely affecting the performance of the antenna, (c) matching single antenna geometry to operate with multiple modes of antenna
configurations. Moreover, while optimizing the antenna for one parameter, the other characteristics may deviate, posing additional problems in analysis and design. Frequency and Polarization reconfigurable antennas are of interest for many advanced communication systems like cognitive radio, frequency hopping spread spectrum and software defined radios, which are reconfigured to communicate using diverse protocol operating at different frequencies and polarizations. White et al. [9] demonstrated a slot ring antenna where in, each polarization of dual polarized antenna is tuned independently. Recently, a polarization diversity slot antenna with switchable frequency has been reported [10]. However, it has complex feed structure. A K band patch antenna with reconfigurable frequency and polarization properties is proposed by Benjamin Rohrdantz et al. [11]. It can switch between two distinct frequency bands, but it performs polarization switching between right hand circular polarization (RHCP) and left hand circular polarization (LHCP). The present paper demonstrates a frequency and polarization reconfigurable microstrip patch antenna with simple feed structure. It can switch between linear polarization (LP), RHCP and LHCP at dual frequency. The work on single-feed frequency and polarization reconfigurable microstrip patch antenna is reported earlier [12]. The concept is validated by simulation results using ideal switches. Although, the conceptual view of bias circuit including lumped elements is demonstrated, the real behavior of PIN diode switch and its associated bias circuit was not considered in the simulation. This paper demonstrates the frequency and polarization reconfigurable ring slot microstrip patch antenna incorporating the practical diode and its associated bias circuit for accurate prediction of antenna characteristics. The concept is also validated by simulation and experimental results. The design is simple and compact generating LP, RHCP and LHCP states at dual frequencies. 2. Antenna Geometry
Fig.1 shows the geometry of proposed frequency and polarization reconfigurable antenna with detailed dimensions. A square patch of side a is printed on RT Duroid substrate (εr = 2.2) with thickness h = 0.8 mm. A λ/4 impedance transformer connects a 50 Ω feed line to patch for impedance matching. A rectangular ring slot is etched in the patch with outer dimensions l × w. Four PIN diodes D1, D2, D3 and D4 are symmetrically placed in ring slot to bridge the gap between center patch and outer ring patch. Four parasitic triangular conductors are formed, by etching slots at four corners of the radiating patch. They are connected to the radiating patch through PIN diodes D5, D6, D7 and D8. PIN diode used in the proposed antenna is SMP1320 011F, and is modeled with the manufacturer’s specified diode equivalent values in the simulation. According to the datasheet [13], ON state of the PIN diode has 0.75Ω resistance and OFF state has 0.23pF capacitance. PIN diode in general requires a bias network to realize ON/OFF switching states. A square slot etched on the center patch aids bias circuit implementation for diodes D1, D2, D3 and D4. As shown in the inset, capacitor C1 is connected between two narrow conducting strips to maintain RF continuity. An inductor L5 linking the narrow conductive strip and bias line isolates RF and DC source. This bias line is connected by a plated through hole (PTH) to the center bias line located on the substrate floor as shown in Fig. 1(c). Four small conductive pads are connected to parasitic triangular conductors through inductors (L1=L2=L3=L4=47nH) to isolate RF and DC. They are also connected through PTH to the four DC bias lines located on substrate floor. They supply DC voltage to diodes D5, D6, D7 and D8. The DC bias lines on the substrate floor are isolated from RF ground by circular ring slots. DC ground is achieved by shorting a λ/4 bias line with the ground. The length of the λ/4 bias line corresponds to wavelength of lowest operating frequency of the proposed antenna. Same line is used to achieve DC ground for highest operating
frequency. This however, does not affect antenna characteristics due to its low frequency ratio. A DC voltage of ±0.8V is used to control the operation of PIN diode switch.
(a)
(b)
(c) Fig.1. Geometry of the antenna (a) Front view (b) Side view (c) Rear view , a = 16, l = 6.36, w = 6.31, l1 = 9, l2 = 8, t = 2.43(All dimensions in mm) 3. Working Principle and Parametric Analysis
3.1 Principle of operation A rectangular ring slot etched in a quarter wave fed square patch antenna realizes frequency reconfigurability with the aid of four PIN diodes D1, D2, D3 and D4. With all these diodes ON, the structure operates as square microstrip antenna (A1) resonating at frequency, f1. With all these diodes OFF, it operates as a square ring antenna (A2) resonating at frequency, f2. Polarization reconfigurability in A1 or A2 is realized with diodes D5, D6, D7 and D8. With all the diodes ON, the geometry is symmetric generating LP. With D5, D7 ON and D6, D8 OFF, RHCP is achieved. With D5, D7 OFF and D6, D8 ON, LHCP is achieved. Fig. 2 depicts the equivalent structure of A1 for different diode configurations. When D5 to D8 are ON it behaves as conventional square microstrip antenna as shown in Fig. 2(a) and radiates linearly polarized waves. The operating frequency is determined by patch side length a, and effective permittivity of the substrate εeff and is given by
f1 =
c 2a ε eff
(1)
A single feed microstrip patch can radiate circular polarization (CP) if two orthogonal patch modes TM10 and TM01 are simultaneously excited with equal amplitude and ± 90° out of phase with the sign determining the sense of rotation. This can be accomplished by perturbing the patch at appropriate locations with respect to the feed. In the proposed design corner truncated perturbation is used to generate CP because there is only one parameter to deal with; the depth of truncation. Fig. 2(b) shows the perturbation (broken lines) required for generating RHCP waves. The position of the feed with respect to the perturbation plays an important role in determining the sense of rotation (RHCP/LHCP). For LHCP, the truncations are made on orthogonal corners. With perturbation, the fundamental mode is split into 2 orthogonal modes TM10 and TM01with slightly different frequencies fa and fb [14].
f a = f1(1 − 2∆S / S ) f b = f1
(2) (3)
where ∆S is the total area of perturbations of A1. S is the area of square patch (S = a2) In order to achieve two orthogonal modes with equal amplitude and + 90º phase shift, corner truncated perturbation has to satisfy the following condition ∆S / S = 1 / 2Qo
(4)
Q o is unloaded quality factor of two orthogonal modes. Fig. 3 depicts the equivalent structure of A2 for different diode configurations. When D5 to D8 are ON, it behaves as a square ring microstrip antenna as shown in Fig. 3(a) and radiates linearly polarized signals. However, when compared to A1, the electrical length of the current increases by w as it flows around the slot. The operating frequency is determined by patch length a and slot width w and is expressed as
f2 =
c 2(a + w) ε eff
(5)
It radiates LP waves at a lower frequency when compared with A1. The square ring microstrip antenna can be made to radiate CP with the corner perturbation.
(a)
(b)
Fig. 2 Equivalent antenna structure for A1 (a) LP (b) RHCP
(a)
(b)
Fig. 3 Equivalent antenna structure for A2 (a) LP (b) RHCP
When A2 is corner truncated as shown in Fig. 3(b), two orthogonal modes are achieved with resonant frequencies f 'a and f 'b given by f ' a = f 2 (1 − 2∆S1 / S1 ) < f a
(6)
f 'b = f 2 < fb
(7)
where, S1 = S - (l x w) is the area of square ring patch effectively used for radiation and ∆S1= ∆S. Frequencies f′a and f′b are observed to be lower than fa and fb because S1 < S. The CP frequency corresponding to the mean of f′a and f′b also reduces. Hence it generates CP at lower frequency compared to A1. For A2, the condition in (4) satisfies with ∆S1, S1, Qo1. where, Qo1 is unloaded quality factor of two orthogonal modes in A2. 3.2 Parametric Analysis
The significant parameters, which affect reflection coefficient and axial ratio (AR) of the antenna are patch length, ring slot dimensions and corner truncation depth. The parametric studies of ring slot gap g for RHCP state in A1 and A2 are shown in Fig. 4. As g increases, the outer dimensions of ring slot increases with inner dimensions fixed. The increase in outer dimensions of ring slot reduces the resonant frequency of both A1 and A2 as shown in Fig. 4(a). The solid lines show A1 and broken lines show A2. The rate of reduction is observed to be more in A2 than in A1. It can be seen from Fig. 4(b) that outer dimensions of ring slot has little effect on AR characteristics of A1 but has significant effect on A2. This is because larger ring dimensions need enhanced perturbations to achieve good AR characteristics. However, no shift in resonant frequency with g is observed in AR characteristics. The variation in inner slot ring dimension is found to have negligible effect on antenna characteristics. The crucial parameter for achieving good AR is the corner truncation depth t. Fig. 5 shows the affect of t on reflection coefficient and AR characteristics of A1 and A2 for RHCP state of the antenna. As shown in Fig. 5(a) with increase in t the axial ratio frequency decreases for A1 and increases for A2. As shown in Fig. 5(b), the minimum value of AR is significantly affected with
increase in t. An optimum value of t is chosen to achieve good impedance and AR characteristics for CP along with good impedance characteristics for LP. Rigorous parametric analysis is performed on the structure using Computer Simulation Technology (CST) Microwave Studio [15] to match the single antenna structure to all the polarizations at dual frequencies.
(a)
(b) Fig. 4 Parametric analysis of g on A1 and A2 (a) Reflection coefficient (b) Axial ratio The following two limitations are observed. First, enhanced perturbations are needed for A2 to achieve good CP bandwidth. However, in order to achieve CP at dual frequencies, the ring slot dimensions are optimized to have same perturbation as A1; ∆S1 = ∆S. This condition limits the CP bandwidth of A2. Second, obtaining larger frequency ratio depends on dimensions and location of the ring slot. Frequency reconfigurable antenna with large frequency ratio using
offset ring slot has been reported[16]. In the proposed design, addition of polarization reconfiguration limits the usage of offset ring slot, resulting in small frequency ratio limited to 1.11. However, when compared to standard conventional patch, the antenna provides an area
(a)
(b) Fig. 5 Parametric analysis of t on A1 and A2 (a) Reflection coefficient (b) Axial ratio reduction of 12% at 5.2 GHz and 30% at 5.8 GHz. Moreover, good impedance and axial ratio characteristics can be achieved by increasing the substrate thickness. 4. Simulated and Experimental Results
In order to verify the operation of proposed design, a prototype is fabricated as shown in Fig. 6. The antenna is characterized using Vector Network Analyzer (8720ET, Agilent Technologies).
Fig. 7 shows the simulated (solid line) and measured (broken line) reflection coefficient and AR results for A1 and A2. As seen from fig. 7(a), structure A1 resonates at 5.8GHz and -10dB impedance bandwidth for LP1, RHCP1 and LHCP1 are 93MHz (5.65–5.75 GHz), 227MHz (5.68–5.9 GHz) and 204MHz (5.68–5.88 GHz) respectively. Configuration A2 resonates at 5.2GHz and the impedance bandwidth for LP2 and RHCP2 is 103MHz (5.16–5.26 GHz) and it is 75MHz (5.18–5.26GHz) for LHCP2. The measured results show an operating frequency shift of 80 MHz (1.3%) in LP and 110MHz (1.8%) in CP states of A1. A shift of 140 MHz (2.6%) is observed in all the polarization states of A2. The shift in resonant frequency is attributed to fabrication errors (unstable substrate parameters, etching and soldering errors) and packaging affects of the diodes.
(a)
(b)
Fig. 6 Photograph of fabricated antenna (a) Top view (b) Rear view Fig. 7(b) shows that measured results have similar shift in AR characteristics. AR < 3dB is achieved for A1 and A2 and the characteristics remain the same in both the CP states. AR bandwidth is observed to be less for A2 as expected. Fig. 8 shows the current distribution of the antenna A1 at 5.8GHz in RHCP state. It shows current variations at different instants of time. The current rotates in anticlockwise direction and is clearly seen on the outer periphery of the
ring slot.
(a)
(b) Fig. 7 Simulated and measured results of A1, A2 (a) Reflection coefficient (b) Axial ratio
t=0
t=T/2 Fig. 8 Simulated surface current distribution in RHCP state of A1.
t=T/4
t=3T/4
The radiation pattern of the proposed antenna is measured in anechoic chamber. Fig. 9 depicts the simulated and measured normalized radiation patterns for LP state at 5.6 GHz and 5.1GHz for A1 and A2 respectively. The measured results are in good agreement with simulation results. Symmetrical radiation performance is observed. Simulated and measured cross polarization levels are observed to be less than -17dB for A1 and A2. Fig. 10(a) and Fig. 10(b) depicts the simulated radiation patterns of A1 and A2 respectively for CP states. Good co polarization (Co pol) and cross polarization (X pol) characteristics are observed. The cross polarization level at bore sight is observed to be < -15dB for A1 and A2. Fig. 11 shows the measured normalized radiation pattern characteristics of RHCP state for A1 and A2. Due to symmetry in the structure, similar characteristics are also observed for LHCP. The E plane and H plane patterns are observed to be similar resulting in good AR over a broader beam from +30 º to -30º. The simulated gain of A1 and A2 is observed to be above 7dBi in all the polarizations in the operating frequency band. The gain of the antenna is measured using gain comparison method. A 4-6GHz Scientific Atlanta standard gain pyramidal horn antenna is used as reference antenna. For A1, the measured boresight gain is observed to vary from 6.31 – 7.3dBic for CP and 6.42 –7dBi for LP in the operating frequency band.
E plane
(a)
H plane
E plane
(b)
H plane
Fig. 9 Simulated and measured radiation patterns of LP state (a) A1 (b) A2 For A2, it varies from 6.47–7.4dBic for CP and 6.58 –7.15dBi for LP. The gain is observed to be same for both RHCP and LHCP. The gain for A1 is different from gain for A2. This is because of additional losses due to the resistance in the ON state of four additional PIN diodes considered in A1. The gain in LP is observed to be less compared to gain in CP state for both A1 and A2.
E plane
(a)
H plane
E plane
(b)
H plane
Fig. 10 Simulated 2D radiation patterns of CP state (a) A1 (b) A2
The difference is due to the fact that LP state has 4 PIN diodes in the ON state while CP state has only two PIN diodes in the ON state. The measured broadside gain is lower than the simulated result. The difference may be attributed to PIN diode ohmic losses, fabrication errors and measurement errors like mounting and alignment errors in the anechoic chamber.
(a)
(b)
Fig. 11 Radiation patterns of RHCP state measured at frequency of minimum AR (a) A1 (b) A2
5. Conclusion
This paper introduced a novel frequency and polarization reconfigurable ring slot patch antenna. Eight PIN diodes are placed at appropriate locations to achieve polarization reconfigurability at two frequencies. The antenna is fabricated and results validated by measurement. Simulation and measured results are in good agreement. The proposed antenna has added advantage of size reduction hence can be used for space borne applications. When compared to standard conventional patch, the design provides an area reduction of 12% at 5.2 GHz and 30% at 5.8 GHz. The proposed antenna can be used in WLAN software defined radio or cognitive radio applications where in, both bands can be selected to function depending on the interference level. This is because the design has similar radiation characteristics at dual frequencies. The future work is targeted to improve the bandwidth of the proposed antenna. Acknowledgment
The authors are thankful to Dr. D.R. Jahagirdar, Sc-G, DRFS, RCI, DRDO, Hyderabad for extending the Antenna Test Facility for evaluating the proposed antenna. We further would like to thank his staff for their support. References
[1] K. Alameddine, S. Abou Chahine, M. Rammal, Z. Osman. Wideband patch antennas for mobile communications. Int. J. Electron. Commun. 2006; 60(8): 596-598. [2] J.A. Ansari, Anurag Mishra, N.P. Yadav, P. Singh, B.R. Vishvakarma. Analysis of W-slot loaded patch antenna for dual band operation. Int. J. Electron. Commun. 2012; 66(1): 32-38. [3] Huda A. Majid, Mohamad Kamal Abdul Rahim, Mohamad Rijal Hamid Noor Asniza Murad and Mohd Faizal Ismail. Frequency-Reconfigurable Microstrip Patch-Slot Antenna. IEEE Antennas Wirel Propag Lett 2013; 12: 218-20.
[4] Zi-Xian Yang, Hong-Chun Yang, Jing-Song Hong and Yang Li. Bandwidth enhancement of a polarization-reconfigurable patch antenna with stair-slots on the ground. IEEE Antennas Wirel Propag Lett 2014; 13:579 - 82. [5] D. Rama Krishna, M. Muthukumar, V.M. Pandharipande. Design and development of reconfigurable rectangular patch antenna array for tri-band applications. Int. J. Electron. Commun. 2015; 69: 56–61. [6] S. Raman, P. Mohanan, Nick Timmons and Jim Morrison. Microstrip fed pattern and polarization reconfigurable compact truncated monopole antenna. IEEE Antennas Wirel Propag Lett 2013; 10: 710-13. [7] Huff G.H., Feng,J., Zhang. S., Bernhard J.T. A novel radiation pattern and frequency reconfigurable single turn square spiral microstrip antenna. IEEE Microw and Wirel Comp Lett 2003; 13 (2): 57- 9. [8] Pei-Yuan Qin, Y. Jay Guo, Yong Cai, M Eryk Dutkiewicz, and Chang-Hong Liang. A reconfigurable antenna with frequency and polarization reconfigurability. IEEE Antennas Wirel Propag Lett 2011; 10:1373-6. [9] C. R. White and G. M. Rebeiz. A differential dual-polarized cavity backed microstrip patch antenna with independent frequency tuning. IEEE Antennas Wireless Propag. Lett. 2011; 58(11): 3490-8. [10]
Jeen-Sheen Row and Chuang-Jiashih Shih. Polarization-diversity ring slot antenna with frequency agility. IEEE Trans on Antennas and Propaga. 2012; 60(8): 3953-7.
[11]
Benjamin Rohrdantz, Chinh Luong, and Arne F. Jacob. A frequency and polarization reconfigurable patch antenna at K-band. In Proc 44th European Microwave Conference, Rome, Italy. 2014: 49-52.
[12]
A. Bharathi, M. Lakshminarayana, P.V.D. Somasekhar Rao. Design of polarization reconfigurable antenna with frequency tuning. In: International Workshop on Antenna Technology, Florida, USA. 2016: 150-3.
[13]
M/A-COM Technology Solutions, SMP1320 series surface mountable pin diode datasheet.
[14]
Ramesh Garg, Prakash Bhartia, Inder Bahl, and Apisak Ittipihoon. Circularly Polarized Microstrip Antennas and Techniques in Microstrip Antenna Design Handbook, Artech House, Norwood, MA, 2001: 505-9.
[15]
CST Microwave Studio, version 2011.
[16]
Ibrahim Tekin and Michael Knox. Reconfigurable microstrip patch antenna for WLAN Software Defined Radio applications. In Proc ICWITS, Honolulu, HI. 2010: 1-4. A. Bharathi was born in Telangana, India in 1978. She completed B. Tech degree with distinction in Electronics & Communication Engineering (ECE) from Jawaharlal Nehru Technological University Hyderabad (JNTUH) in 2001 and M.E in Microwave and Radar
Engineering from University College of Engineering, Osmania University, Hyderabad, Telangana, India in 2004. She is persuing Ph.D. in Dept. of ECE, JNTUH, Hyderabad. She is presently working as an Assistant Professor in ECE Dept., University College of Engineering, Osmania University, Hyderabad, India. She has 12 years of teaching experience. Her research interests include Antenna Theory, Reconfigurable Antennas, Radar Systems, Microwave and RF circuits, Ultra wideband (UWB) antennas. She is a Member of IEEE, IETE.
Merugu Lakshminarayana was born in Telangana, India in 1955. He graduated in Electronics & Communication Engineering from Jawaharlal Nehru Technological University, Hyderabad in 1978, completed his M.Tech in Microwave and Radar Engineering in 1980 from Indian Institute Of Technology (I.I.T.), Kharagpur and obtained Ph.D. from Queens University of Belfast, U.K. in 1992. He joined Defence Electronics Research Laboratory under Defence Research Development Organisation, India, as Scientist "B" in 1981 and retired in 2015 as Scientist "H". He is working presently as Prof. and Principal, Kshatriya College of Engg., Armoor, Nizamabad Dist., Telangana State, India. Dr. Merugu is a Sr. Member of IEEE, Fellow of IETE (India), Fellow of Institute of Engineers (IE), Member of AOC. Dr. P. V. D. Somasekhar Rao obtained his B.E. Degree in Electronics & Communication Engg., from Sri Venkateswara University College of Engg., Tirupati (1977), M.Tech. Degree in Microwave and Radar Engg., from I.I.T., Kharagpur, India (1979). He earned his Ph.D. Degree in Electronics and Communication Engg., from I.I.T. Kharagpur. He worked at Radar Centre, I.I.T. Kharagpur, and Tata Institute of Fundamental Research (TIFR/ RAC Group), Ooty. Later, he joined as the Faculty of ECE Dept. of Jawaharlal Nehru Technological University Colleges of Engg., at Anantapur and Hyderabad. He has recently retired as Professor from Dept. of ECE, JNTUH College of Engg., Hyderabad. Presently he is with G Narayanamma Inst. Of Technology& Science, Hyderabad, as Professor of ECE and Dean of Academics. He is Sr. Member of IEEE, Life Member of ISTE (India), and Fellow of IETE (India).
S1434-8411(16)30624-0 http://dx.doi.org/10.1016/j.aeue.2016.11.012 AEUE 51728
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
1 September 2016 17 October 2016 16 November 2016
Please cite this article as: B. Anantha, L. Merugu, S. Rao, A Novel Single Feed Frequency and Polarization Reconfigurable Microstrip Patch Antenna, International Journal of Electronics and Communications (2016), doi: http://dx.doi.org/10.1016/j.aeue.2016.11.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Novel Single Feed Frequency and Polarization Reconfigurable Microstrip Patch Antenna Bharathi, Anantha a , M.E(Ph.D), Lakshminarayana, Merugub , Ph.D and Somasekhar Rao, PVD c , Ph.D. a. Assistant Professor, ECE Dept., UCE, Osmania University, Hyderabad, Telangana State, India. e-mail: [email protected] (Corresponding Author) b. Scientist ‘H’(Retd.), DLRL (DRDO) & Prof. and Principal, Kshatriya College of Engg., Armoor, Nizamabad Dist., Telangana State, India. e-mail : [email protected]. c. Professor(Retd.), ECE Dept., JNTUH and Dean Academics, G. Narayanamma Institute of Technology and Science, Shaikpet, Hyderabad, India. e-mail : [email protected].
Abstract— In this paper, a novel single feed frequency and polarization reconfigurable microstrip patch antenna is presented. This antenna mainly comprises of a corner truncated square patch with a rectangular ring slot, eight PIN diodes and six conductive pads. Four PIN diodes are placed symmetrically in the rectangular ring slot to bridge the gap and to switch the frequency between WLAN bands resonating at 5.2 GHz and 5.8 GHz. Four PIN diodes connect the corner truncated square patch to parasitic triangular conductors. PIN diodes are used to Axial Ratio (AR), square microstrip antenna (A1), square ring antenna (A2)
switch the polarization between linear, right hand circular and left hand circular at each frequency. When compared to conventional patch, the proposed design provides a size reduction of 12% at 5.2 GHz, and 30% at 5.8 GHz. The simulated reflection coefficient and radiation patterns are presented and compared with the experimental data. This antenna finds applications for modern wireless communication system. Keywords — frequency reconfigurable, microstrip antenna, PIN diodes, polarization reconfigurable, WLAN. 1. Introduction Reconfigurable microstrip antennas are gaining much attention due to their numerous advantages. They minimize fading effects caused by multipath, increase the system capacity by frequency reuse, provide immunity to interfering signals, increase the communication link quality and reduce co-channel interference. They offer more functionalities than conventional antennas [1][2]. The parameters of the antenna that can be reconfigured are frequency, radiation pattern, polarization or combinations of them. Many researchers have presented work on single parameter reconfiguration [3][4][5]. Recently, several antennas are designed to achieve compound reconfiguration; which enables polarization and pattern reconfiguration [6], frequency and pattern reconfiguration [7], polarization and frequency reconfiguration [8]. The compound reconfiguration can provide significant advantages over single reconfiguration design but the designer has to face harder challenges in (a) modeling reconfiguration mechanism, (b) implementing reconfigurable mechanism without severely affecting the performance of the antenna, (c) matching single antenna geometry to operate with multiple modes of antenna
configurations. Moreover, while optimizing the antenna for one parameter, the other characteristics may deviate, posing additional problems in analysis and design. Frequency and Polarization reconfigurable antennas are of interest for many advanced communication systems like cognitive radio, frequency hopping spread spectrum and software defined radios, which are reconfigured to communicate using diverse protocol operating at different frequencies and polarizations. White et al. [9] demonstrated a slot ring antenna where in, each polarization of dual polarized antenna is tuned independently. Recently, a polarization diversity slot antenna with switchable frequency has been reported [10]. However, it has complex feed structure. A K band patch antenna with reconfigurable frequency and polarization properties is proposed by Benjamin Rohrdantz et al. [11]. It can switch between two distinct frequency bands, but it performs polarization switching between right hand circular polarization (RHCP) and left hand circular polarization (LHCP). The present paper demonstrates a frequency and polarization reconfigurable microstrip patch antenna with simple feed structure. It can switch between linear polarization (LP), RHCP and LHCP at dual frequency. The work on single-feed frequency and polarization reconfigurable microstrip patch antenna is reported earlier [12]. The concept is validated by simulation results using ideal switches. Although, the conceptual view of bias circuit including lumped elements is demonstrated, the real behavior of PIN diode switch and its associated bias circuit was not considered in the simulation. This paper demonstrates the frequency and polarization reconfigurable ring slot microstrip patch antenna incorporating the practical diode and its associated bias circuit for accurate prediction of antenna characteristics. The concept is also validated by simulation and experimental results. The design is simple and compact generating LP, RHCP and LHCP states at dual frequencies. 2. Antenna Geometry
Fig.1 shows the geometry of proposed frequency and polarization reconfigurable antenna with detailed dimensions. A square patch of side a is printed on RT Duroid substrate (εr = 2.2) with thickness h = 0.8 mm. A λ/4 impedance transformer connects a 50 Ω feed line to patch for impedance matching. A rectangular ring slot is etched in the patch with outer dimensions l × w. Four PIN diodes D1, D2, D3 and D4 are symmetrically placed in ring slot to bridge the gap between center patch and outer ring patch. Four parasitic triangular conductors are formed, by etching slots at four corners of the radiating patch. They are connected to the radiating patch through PIN diodes D5, D6, D7 and D8. PIN diode used in the proposed antenna is SMP1320 011F, and is modeled with the manufacturer’s specified diode equivalent values in the simulation. According to the datasheet [13], ON state of the PIN diode has 0.75Ω resistance and OFF state has 0.23pF capacitance. PIN diode in general requires a bias network to realize ON/OFF switching states. A square slot etched on the center patch aids bias circuit implementation for diodes D1, D2, D3 and D4. As shown in the inset, capacitor C1 is connected between two narrow conducting strips to maintain RF continuity. An inductor L5 linking the narrow conductive strip and bias line isolates RF and DC source. This bias line is connected by a plated through hole (PTH) to the center bias line located on the substrate floor as shown in Fig. 1(c). Four small conductive pads are connected to parasitic triangular conductors through inductors (L1=L2=L3=L4=47nH) to isolate RF and DC. They are also connected through PTH to the four DC bias lines located on substrate floor. They supply DC voltage to diodes D5, D6, D7 and D8. The DC bias lines on the substrate floor are isolated from RF ground by circular ring slots. DC ground is achieved by shorting a λ/4 bias line with the ground. The length of the λ/4 bias line corresponds to wavelength of lowest operating frequency of the proposed antenna. Same line is used to achieve DC ground for highest operating
frequency. This however, does not affect antenna characteristics due to its low frequency ratio. A DC voltage of ±0.8V is used to control the operation of PIN diode switch.
(a)
(b)
(c) Fig.1. Geometry of the antenna (a) Front view (b) Side view (c) Rear view , a = 16, l = 6.36, w = 6.31, l1 = 9, l2 = 8, t = 2.43(All dimensions in mm) 3. Working Principle and Parametric Analysis
3.1 Principle of operation A rectangular ring slot etched in a quarter wave fed square patch antenna realizes frequency reconfigurability with the aid of four PIN diodes D1, D2, D3 and D4. With all these diodes ON, the structure operates as square microstrip antenna (A1) resonating at frequency, f1. With all these diodes OFF, it operates as a square ring antenna (A2) resonating at frequency, f2. Polarization reconfigurability in A1 or A2 is realized with diodes D5, D6, D7 and D8. With all the diodes ON, the geometry is symmetric generating LP. With D5, D7 ON and D6, D8 OFF, RHCP is achieved. With D5, D7 OFF and D6, D8 ON, LHCP is achieved. Fig. 2 depicts the equivalent structure of A1 for different diode configurations. When D5 to D8 are ON it behaves as conventional square microstrip antenna as shown in Fig. 2(a) and radiates linearly polarized waves. The operating frequency is determined by patch side length a, and effective permittivity of the substrate εeff and is given by
f1 =
c 2a ε eff
(1)
A single feed microstrip patch can radiate circular polarization (CP) if two orthogonal patch modes TM10 and TM01 are simultaneously excited with equal amplitude and ± 90° out of phase with the sign determining the sense of rotation. This can be accomplished by perturbing the patch at appropriate locations with respect to the feed. In the proposed design corner truncated perturbation is used to generate CP because there is only one parameter to deal with; the depth of truncation. Fig. 2(b) shows the perturbation (broken lines) required for generating RHCP waves. The position of the feed with respect to the perturbation plays an important role in determining the sense of rotation (RHCP/LHCP). For LHCP, the truncations are made on orthogonal corners. With perturbation, the fundamental mode is split into 2 orthogonal modes TM10 and TM01with slightly different frequencies fa and fb [14].
f a = f1(1 − 2∆S / S ) f b = f1
(2) (3)
where ∆S is the total area of perturbations of A1. S is the area of square patch (S = a2) In order to achieve two orthogonal modes with equal amplitude and + 90º phase shift, corner truncated perturbation has to satisfy the following condition ∆S / S = 1 / 2Qo
(4)
Q o is unloaded quality factor of two orthogonal modes. Fig. 3 depicts the equivalent structure of A2 for different diode configurations. When D5 to D8 are ON, it behaves as a square ring microstrip antenna as shown in Fig. 3(a) and radiates linearly polarized signals. However, when compared to A1, the electrical length of the current increases by w as it flows around the slot. The operating frequency is determined by patch length a and slot width w and is expressed as
f2 =
c 2(a + w) ε eff
(5)
It radiates LP waves at a lower frequency when compared with A1. The square ring microstrip antenna can be made to radiate CP with the corner perturbation.
(a)
(b)
Fig. 2 Equivalent antenna structure for A1 (a) LP (b) RHCP
(a)
(b)
Fig. 3 Equivalent antenna structure for A2 (a) LP (b) RHCP
When A2 is corner truncated as shown in Fig. 3(b), two orthogonal modes are achieved with resonant frequencies f 'a and f 'b given by f ' a = f 2 (1 − 2∆S1 / S1 ) < f a
(6)
f 'b = f 2 < fb
(7)
where, S1 = S - (l x w) is the area of square ring patch effectively used for radiation and ∆S1= ∆S. Frequencies f′a and f′b are observed to be lower than fa and fb because S1 < S. The CP frequency corresponding to the mean of f′a and f′b also reduces. Hence it generates CP at lower frequency compared to A1. For A2, the condition in (4) satisfies with ∆S1, S1, Qo1. where, Qo1 is unloaded quality factor of two orthogonal modes in A2. 3.2 Parametric Analysis
The significant parameters, which affect reflection coefficient and axial ratio (AR) of the antenna are patch length, ring slot dimensions and corner truncation depth. The parametric studies of ring slot gap g for RHCP state in A1 and A2 are shown in Fig. 4. As g increases, the outer dimensions of ring slot increases with inner dimensions fixed. The increase in outer dimensions of ring slot reduces the resonant frequency of both A1 and A2 as shown in Fig. 4(a). The solid lines show A1 and broken lines show A2. The rate of reduction is observed to be more in A2 than in A1. It can be seen from Fig. 4(b) that outer dimensions of ring slot has little effect on AR characteristics of A1 but has significant effect on A2. This is because larger ring dimensions need enhanced perturbations to achieve good AR characteristics. However, no shift in resonant frequency with g is observed in AR characteristics. The variation in inner slot ring dimension is found to have negligible effect on antenna characteristics. The crucial parameter for achieving good AR is the corner truncation depth t. Fig. 5 shows the affect of t on reflection coefficient and AR characteristics of A1 and A2 for RHCP state of the antenna. As shown in Fig. 5(a) with increase in t the axial ratio frequency decreases for A1 and increases for A2. As shown in Fig. 5(b), the minimum value of AR is significantly affected with
increase in t. An optimum value of t is chosen to achieve good impedance and AR characteristics for CP along with good impedance characteristics for LP. Rigorous parametric analysis is performed on the structure using Computer Simulation Technology (CST) Microwave Studio [15] to match the single antenna structure to all the polarizations at dual frequencies.
(a)
(b) Fig. 4 Parametric analysis of g on A1 and A2 (a) Reflection coefficient (b) Axial ratio The following two limitations are observed. First, enhanced perturbations are needed for A2 to achieve good CP bandwidth. However, in order to achieve CP at dual frequencies, the ring slot dimensions are optimized to have same perturbation as A1; ∆S1 = ∆S. This condition limits the CP bandwidth of A2. Second, obtaining larger frequency ratio depends on dimensions and location of the ring slot. Frequency reconfigurable antenna with large frequency ratio using
offset ring slot has been reported[16]. In the proposed design, addition of polarization reconfiguration limits the usage of offset ring slot, resulting in small frequency ratio limited to 1.11. However, when compared to standard conventional patch, the antenna provides an area
(a)
(b) Fig. 5 Parametric analysis of t on A1 and A2 (a) Reflection coefficient (b) Axial ratio reduction of 12% at 5.2 GHz and 30% at 5.8 GHz. Moreover, good impedance and axial ratio characteristics can be achieved by increasing the substrate thickness. 4. Simulated and Experimental Results
In order to verify the operation of proposed design, a prototype is fabricated as shown in Fig. 6. The antenna is characterized using Vector Network Analyzer (8720ET, Agilent Technologies).
Fig. 7 shows the simulated (solid line) and measured (broken line) reflection coefficient and AR results for A1 and A2. As seen from fig. 7(a), structure A1 resonates at 5.8GHz and -10dB impedance bandwidth for LP1, RHCP1 and LHCP1 are 93MHz (5.65–5.75 GHz), 227MHz (5.68–5.9 GHz) and 204MHz (5.68–5.88 GHz) respectively. Configuration A2 resonates at 5.2GHz and the impedance bandwidth for LP2 and RHCP2 is 103MHz (5.16–5.26 GHz) and it is 75MHz (5.18–5.26GHz) for LHCP2. The measured results show an operating frequency shift of 80 MHz (1.3%) in LP and 110MHz (1.8%) in CP states of A1. A shift of 140 MHz (2.6%) is observed in all the polarization states of A2. The shift in resonant frequency is attributed to fabrication errors (unstable substrate parameters, etching and soldering errors) and packaging affects of the diodes.
(a)
(b)
Fig. 6 Photograph of fabricated antenna (a) Top view (b) Rear view Fig. 7(b) shows that measured results have similar shift in AR characteristics. AR < 3dB is achieved for A1 and A2 and the characteristics remain the same in both the CP states. AR bandwidth is observed to be less for A2 as expected. Fig. 8 shows the current distribution of the antenna A1 at 5.8GHz in RHCP state. It shows current variations at different instants of time. The current rotates in anticlockwise direction and is clearly seen on the outer periphery of the
ring slot.
(a)
(b) Fig. 7 Simulated and measured results of A1, A2 (a) Reflection coefficient (b) Axial ratio
t=0
t=T/2 Fig. 8 Simulated surface current distribution in RHCP state of A1.
t=T/4
t=3T/4
The radiation pattern of the proposed antenna is measured in anechoic chamber. Fig. 9 depicts the simulated and measured normalized radiation patterns for LP state at 5.6 GHz and 5.1GHz for A1 and A2 respectively. The measured results are in good agreement with simulation results. Symmetrical radiation performance is observed. Simulated and measured cross polarization levels are observed to be less than -17dB for A1 and A2. Fig. 10(a) and Fig. 10(b) depicts the simulated radiation patterns of A1 and A2 respectively for CP states. Good co polarization (Co pol) and cross polarization (X pol) characteristics are observed. The cross polarization level at bore sight is observed to be < -15dB for A1 and A2. Fig. 11 shows the measured normalized radiation pattern characteristics of RHCP state for A1 and A2. Due to symmetry in the structure, similar characteristics are also observed for LHCP. The E plane and H plane patterns are observed to be similar resulting in good AR over a broader beam from +30 º to -30º. The simulated gain of A1 and A2 is observed to be above 7dBi in all the polarizations in the operating frequency band. The gain of the antenna is measured using gain comparison method. A 4-6GHz Scientific Atlanta standard gain pyramidal horn antenna is used as reference antenna. For A1, the measured boresight gain is observed to vary from 6.31 – 7.3dBic for CP and 6.42 –7dBi for LP in the operating frequency band.
E plane
(a)
H plane
E plane
(b)
H plane
Fig. 9 Simulated and measured radiation patterns of LP state (a) A1 (b) A2 For A2, it varies from 6.47–7.4dBic for CP and 6.58 –7.15dBi for LP. The gain is observed to be same for both RHCP and LHCP. The gain for A1 is different from gain for A2. This is because of additional losses due to the resistance in the ON state of four additional PIN diodes considered in A1. The gain in LP is observed to be less compared to gain in CP state for both A1 and A2.
E plane
(a)
H plane
E plane
(b)
H plane
Fig. 10 Simulated 2D radiation patterns of CP state (a) A1 (b) A2
The difference is due to the fact that LP state has 4 PIN diodes in the ON state while CP state has only two PIN diodes in the ON state. The measured broadside gain is lower than the simulated result. The difference may be attributed to PIN diode ohmic losses, fabrication errors and measurement errors like mounting and alignment errors in the anechoic chamber.
(a)
(b)
Fig. 11 Radiation patterns of RHCP state measured at frequency of minimum AR (a) A1 (b) A2
5. Conclusion
This paper introduced a novel frequency and polarization reconfigurable ring slot patch antenna. Eight PIN diodes are placed at appropriate locations to achieve polarization reconfigurability at two frequencies. The antenna is fabricated and results validated by measurement. Simulation and measured results are in good agreement. The proposed antenna has added advantage of size reduction hence can be used for space borne applications. When compared to standard conventional patch, the design provides an area reduction of 12% at 5.2 GHz and 30% at 5.8 GHz. The proposed antenna can be used in WLAN software defined radio or cognitive radio applications where in, both bands can be selected to function depending on the interference level. This is because the design has similar radiation characteristics at dual frequencies. The future work is targeted to improve the bandwidth of the proposed antenna. Acknowledgment
The authors are thankful to Dr. D.R. Jahagirdar, Sc-G, DRFS, RCI, DRDO, Hyderabad for extending the Antenna Test Facility for evaluating the proposed antenna. We further would like to thank his staff for their support. References
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[4] Zi-Xian Yang, Hong-Chun Yang, Jing-Song Hong and Yang Li. Bandwidth enhancement of a polarization-reconfigurable patch antenna with stair-slots on the ground. IEEE Antennas Wirel Propag Lett 2014; 13:579 - 82. [5] D. Rama Krishna, M. Muthukumar, V.M. Pandharipande. Design and development of reconfigurable rectangular patch antenna array for tri-band applications. Int. J. Electron. Commun. 2015; 69: 56–61. [6] S. Raman, P. Mohanan, Nick Timmons and Jim Morrison. Microstrip fed pattern and polarization reconfigurable compact truncated monopole antenna. IEEE Antennas Wirel Propag Lett 2013; 10: 710-13. [7] Huff G.H., Feng,J., Zhang. S., Bernhard J.T. A novel radiation pattern and frequency reconfigurable single turn square spiral microstrip antenna. IEEE Microw and Wirel Comp Lett 2003; 13 (2): 57- 9. [8] Pei-Yuan Qin, Y. Jay Guo, Yong Cai, M Eryk Dutkiewicz, and Chang-Hong Liang. A reconfigurable antenna with frequency and polarization reconfigurability. IEEE Antennas Wirel Propag Lett 2011; 10:1373-6. [9] C. R. White and G. M. Rebeiz. A differential dual-polarized cavity backed microstrip patch antenna with independent frequency tuning. IEEE Antennas Wireless Propag. Lett. 2011; 58(11): 3490-8. [10]
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[11]
Benjamin Rohrdantz, Chinh Luong, and Arne F. Jacob. A frequency and polarization reconfigurable patch antenna at K-band. In Proc 44th European Microwave Conference, Rome, Italy. 2014: 49-52.
[12]
A. Bharathi, M. Lakshminarayana, P.V.D. Somasekhar Rao. Design of polarization reconfigurable antenna with frequency tuning. In: International Workshop on Antenna Technology, Florida, USA. 2016: 150-3.
[13]
M/A-COM Technology Solutions, SMP1320 series surface mountable pin diode datasheet.
[14]
Ramesh Garg, Prakash Bhartia, Inder Bahl, and Apisak Ittipihoon. Circularly Polarized Microstrip Antennas and Techniques in Microstrip Antenna Design Handbook, Artech House, Norwood, MA, 2001: 505-9.
[15]
CST Microwave Studio, version 2011.
[16]
Ibrahim Tekin and Michael Knox. Reconfigurable microstrip patch antenna for WLAN Software Defined Radio applications. In Proc ICWITS, Honolulu, HI. 2010: 1-4. A. Bharathi was born in Telangana, India in 1978. She completed B. Tech degree with distinction in Electronics & Communication Engineering (ECE) from Jawaharlal Nehru Technological University Hyderabad (JNTUH) in 2001 and M.E in Microwave and Radar
Engineering from University College of Engineering, Osmania University, Hyderabad, Telangana, India in 2004. She is persuing Ph.D. in Dept. of ECE, JNTUH, Hyderabad. She is presently working as an Assistant Professor in ECE Dept., University College of Engineering, Osmania University, Hyderabad, India. She has 12 years of teaching experience. Her research interests include Antenna Theory, Reconfigurable Antennas, Radar Systems, Microwave and RF circuits, Ultra wideband (UWB) antennas. She is a Member of IEEE, IETE.
Merugu Lakshminarayana was born in Telangana, India in 1955. He graduated in Electronics & Communication Engineering from Jawaharlal Nehru Technological University, Hyderabad in 1978, completed his M.Tech in Microwave and Radar Engineering in 1980 from Indian Institute Of Technology (I.I.T.), Kharagpur and obtained Ph.D. from Queens University of Belfast, U.K. in 1992. He joined Defence Electronics Research Laboratory under Defence Research Development Organisation, India, as Scientist "B" in 1981 and retired in 2015 as Scientist "H". He is working presently as Prof. and Principal, Kshatriya College of Engg., Armoor, Nizamabad Dist., Telangana State, India. Dr. Merugu is a Sr. Member of IEEE, Fellow of IETE (India), Fellow of Institute of Engineers (IE), Member of AOC. Dr. P. V. D. Somasekhar Rao obtained his B.E. Degree in Electronics & Communication Engg., from Sri Venkateswara University College of Engg., Tirupati (1977), M.Tech. Degree in Microwave and Radar Engg., from I.I.T., Kharagpur, India (1979). He earned his Ph.D. Degree in Electronics and Communication Engg., from I.I.T. Kharagpur. He worked at Radar Centre, I.I.T. Kharagpur, and Tata Institute of Fundamental Research (TIFR/ RAC Group), Ooty. Later, he joined as the Faculty of ECE Dept. of Jawaharlal Nehru Technological University Colleges of Engg., at Anantapur and Hyderabad. He has recently retired as Professor from Dept. of ECE, JNTUH College of Engg., Hyderabad. Presently he is with G Narayanamma Inst. Of Technology& Science, Hyderabad, as Professor of ECE and Dean of Academics. He is Sr. Member of IEEE, Life Member of ISTE (India), and Fellow of IETE (India).