Dual-Band RHCP Stacked Microstrip Antenna for IRNSS Receiver

Transcription

1 Volume 118 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu Dual-Band RHCP Stacked Microstrip Antenna for IRNSS Receiver Praveen Chandran C.R 1, Mudumba Ramesh 2 and S. Raghavan 3 1 Master Control Facility, Indian Space Research Organisation, Hassan, Karnataka , India praveencr1986@gmail.com 2 3 Department of Electronics and Communication Engineering, National Institute of Technology, Tiruchirappalli, Tamilnadu , India mudumba@mcf.gov.in and raghavan@nitt.edu January 2, 2018 Abstract Objectives: This paper presents the design aspects and simulation of a right-hand circularly polarized, dual-band, two layer stacked micro-strip antenna for IRNSS receiver front-end operating at L5 band ( MHz) and S band ( MHz) frequencies. Methods: IRNSS satellites transmit navigational signals in L5 ( MHz) and S ( MHz) frequency bands. Dual-band antenna is preferred to mitigate the effect of Ionosphere on the IRNSS signal. Two layer stacked patch structure is adopted for the proposed antenna with the bottom patch designed for L5 band and top patch for S band. Right-Hand Circularly Polarized operation with Axial Ratio less than 2dB is achieved by introducing a pair of square shaped notches at the diagonally opposite sides of 1 47

2 the patches and a single probe feed along the center of the antenna. Findings: The proposed antenna is compact and exhibits dual frequency RHCP operation with axial ratio less than 2dB. The antenna radiation pattern is hemispherical in nature with broad beam width for good sky visibility. Antenna has reasonably good gain and cross polarisation isolation. Improvements: This antenna design is a good candidate for IRNSS receiver applications. However future research works are planned to introduce the concept of metamaterials using complementary split ring resonators (CSRR) for miniaturization and planar layout. Such a design would be ideal for mobile receiver applications. Key Words : IRNSS, GNSS, Stacked Patch, Corner Notch, Axial Ratio 1 Introduction Global Navigation Satellite Systems (GNSS) is becoming key technological achievement in modern world along with Internet and Mobile communications. GNSS offers a wide variety of services such as navigation, positioning, public safety, surveillance, geographic surveys, time standards, mapping and weather and atmospheric information. The presence of GNSS applications became ubiquitous ever since its integration with portable personal navigations devices and mobile phones. GPS is the pioneer among the GNSS systems, which is 24 MEO satellite constellation and provides 7.1m position accuracy with 95% confidence level. GLONASS is a space based navigation system operated by Russia and provides 7.5 m position accuracy with 95% confidence level. Galileo is the European equivalent GNSS system and provides 4m position accuracy with dual-frequency receiver. BeiDou is a Chinese GNSS system which, upon completion by 2020, is expected to give positional accuracy of 10m 1. Indian Regional Navigation Satellite System (IRNSS) is the new entrant in the domain of Space Based Positioning and Navigational System. IRNSS is an autonomous regional satellite navigation sys- 2 48

3 tem being developed by the Indian Space Research Organization which would be under complete control of the Indian government. The requirement for a completely indigenous navigation system is relevant since access to foreign government controlled navigation systems is not guaranteed during hostile situations in the present global scenario. IRNSS envisages establishment of Regional Navigation Satellite System using a combination of GEO and GSO Space crafts over Indian region. The IRNSS constellation will consist of seven satellites - three Satellites in GEO orbit (at 32.5 o E, 83 o E and o E) and four Satellites in GSO orbit inclined at 29 o to the equatorial plane with their longitude crossings as 55 o E and o E (two in each plane). All the satellites are visible in the Indian region for 24hours. The intended service area for IRNSS is primarily the Indian Land Mass. The service area for IRNSS is in general specified as between longitude 40 o east to 140 o east and between latitude ± 40 o. The IRNSS System is expected to provide two sigma position accuracy better that 20 meters over India and a region extending to the about 1500 kilometers around India. The proposed IRNSS system operation will be in two frequency bands, namely L5 and S. On each of these frequencies, two services are available, Standard Positioning Services (SPS) for civilian users and Restricted Services (RS) exclusively for specific users [2,3,4]. Antenna is the first element of the IRNSS receiver to receive the signals originated from the satellites located at different orbital slots. The integrity of the processed data improves if the receiver antenna is capable of receiving signals from as many satellites as possible. In this paper a dual-band circularly polarized antenna is proposed for IRNSS receiver. The antenna is capable of receiving the signals from IRNSS satellites at both L5 ( MHz) band and S ( MHz) bands. This proposed antenna is designed using Ansoft High Frequency Simulation Software (HFSS) which uses Finite Element Method to generate accurate solution. The simulated antenna parameters are compared against the design requirements and are discussed. This paper is organized as follows. Section 2 describes review of some of the available literature GNSS terminal antennas. Design specifications of the antenna are given in Section 3. Section 4 describes the design methodology used for the proposed stacked patch 3 49

4 antenna. Section 5 gives the simulation results and its comparison again the design values. Finally, the concluding remarks are given in Section 6. 2 Related Works Past decade has seen the proliferation of a variety of electronic terminal devices integrated with global positioning, navigation services and hence much research works had happened in the design of terminal antennas for such devices. Many literature works have been published on GPS receiver antennas due to the wide spread use of GPS based applications. IRNSS is still in development phase and its associated services will be made available for usage only in the upcoming years. Hence only very few technical literatures are available which deals with IRNSS receiver antennas. Still many of the design considerations valid for GPS receiver antennas are more or less equally applicable for IRNSS receiver antennas also. Some of the technical literatures available on other GNSS receiver antennas are reviewed and presented below. A small slot-loaded, proximity-fed patch antenna is designed for GPS operation at L1 (1575 MHz) and L2 (1227 MHz) bands. Highdielectric substrate and meandered slots are employed to reduce the antenna size down to 25.4 mm in diameter and mm in thickness. The thickness is important for achieving the wide bandwidth in support of modern GPS coding schemes. The dual-band coverage is achieved by utilizing the patch mode in L2 band and slot mode in L1 band. This design features additional slot stubs for independently tuning the L1 frequency. The right-hand circularly polarized field property is achieved by connecting two proximity probes to a small surface-mount 0 to 90 degree hybrid chip 5. A circularly polarized microstrip antenna with low wide-angle axial-ratio design is available for tri-band GPS applications. The antenna is designed for global positioning satellite operations at 1227MHz (L2), 1575MHz (L1) and 1176MHz (L5, available after 2007). This antenna has another advantage of a much wider band in both VSWR and 3 db axial-ratio compared with single-fed GPS antennas 6. A spiral antenna array on RT/DUROID Substrate for the oper- 4 50

5 ating frequency range of 1.2 to 1.6GHz uses four spiral elements to provide broadband satellite coverage and can also be used in conjunction with a space-time adaptive processor (STAP) for interference suppression. The antenna and integrated feed parameters are optimized for the L1 and L2 band radiation coverage. This antenna was showing remarkable performance over the frequency L5 (1175 MHz) with high gain of 9 db and high directivity. But the proposed antenna design is for single frequency (L5) operation 7. A triangular fractal patch antenna for IRNSS and GAGAN applications uses substrate with dielectric constant of 4.8 and thickness of 3.05 mm. The antenna exhibits multi-band resonant frequencies at the frequency L5 (1175 MHz), L1 ( MHz) and S ( MHz). The antenna exhibits good return loss at the specified frequencies. At MHz frequency range the antenna has a return loss of about -14dB and at the MHz range it is having a return loss of nearly -17dB while it is having a return loss of -15dB at the frequency range of MHz. The gain of the antenna is 5.052dB and the directivity of the antenna is given as dB 8. 3 Antenna Design Specifications Some of the key requirements and technical challenges involved in designing a suitable antenna for IRNSS receiver are mentioned here. The intended design should be small, lightweight and compact as possible to be accommodated in a terminal device. The fabrication cost also should be minimal. Dual-band antenna is preferred to mitigate the effect of Ionosphere on the IRNSS signal. The major effect of ionosphere on IRNSS signals is frequency-dependent phase shift (group delay) caused by the dispersive characteristics of the ionosphere. This adverse effect is effectively reduced by employing two widely spaced frequencies. Redundancy and increased resistance to jamming are the other benefits offered by dual band operation. Hence the proposed antenna should be capable of dual-band operation 1,2. Impedance bandwidth is the frequency range over which 90% of the incident power will be delivered to the antenna i.e. the reflection coefficient S11 less than -10dB and the VSWR is less than 5 51

6 2:1. IRNSS signals will be occupying a bandwidth of 24 MHz in L5 band ( MHz) and 16.5 MHz in S band ( MHz). The proposed antenna should have signal reception bandwidths accordingly 1,2. According to the ICD of IRNSS Standard Positioning Services, the minimum received power of IRNSS signals shall be dBW and maximum value of the received signal power shall be dbw 2. For any of the modern day GNSS receivers, the antenna gain should be greater than 0 dbi 6. The IRNSS receiver antenna should have good sky visibility so as to receive signals from maximum number of satellites. Hence the radiation pattern should be hemispherical in nature with broad beam width. Elevation angles near and slightly below horizon are of specific interest because intentional jamming or unintentional interference signal sources are most likely to originate. So the antenna radiation pattern should show sharp drop-off for elevation angle near and below zero degree 1. Use of CP for GNSS signals transmission and reception eliminates the need of polarization alignment which would have been needed for Linear or Elliptical Polarization. IRNSS satellites transmit signals in right hand circular polarization hence the use of linearly polarized antenna at receiver end would cause 3dB loss due to polarization mismatch. For optimal performance the proposed receiver antenna should be of RHCP. If antennas are not having perfect polarization, an RHCP antenna will pick up some of the LHCP energy. The quality of circular polarization is described in terms of Axial Ratio. For IRNSS receiver antennas the axial ratio should be less than 2dB 9. Many of the design requirements for IRNSS terminal antennas are often conflicting with one another due to the restrictions imposed by size, weight, compactness and performance characteristics. 4 Design Methodology Helix and Patch antennas are the most widely used types in GNSS receiver applications. When it comes to designing antennas for terminal devices, integrating the antenna into the actual device is of primary concern. Patch antennas are ideal for an application where 6 52

7 the antenna sits on a flat surface. Patch antennas can provide high gain, especially if they are mounted on top of a large ground plane. Fabrication cost can also be reduced by using substrate materials like FR-4 or even air as a dielectric. Hence patch antenna based design is chosen for the intended IRNSS terminal antenna. Microstrip antenna with multi-frequency operation has been achieved by several techniques like single-patch with shorting pins or by introducing slots of different shapes in the patch surface, multi-layer structure and multi-feed planar antenna etc. Using multi-layer structure is the easiest way to achieve multi-frequency operation than using other approaches. The dimensions of each patch determines the individual resonant frequencies and by varying the patch dimensions, the specific resonant frequencies can be changed without affecting the other frequencies. The proposed design has multi-layer square patches where the side length is calculated using the equation given in10. The width of the patch is chosen to be same as that of length in order to have a square patch structure. The upper patch designed to resonate at MHz (S-Band) and the lower patch length is designed for MHz (L5 Band) of operation. RT/Duriod 5880 with dielectric constant of 2.2 is chosen as substrate for upper patch whereas FR4 Epoxy with dielectric constant of 4.4 is selected for lower patch. Thickness of the substrate is chosen as inches (4.75 mm). The ground plane is selected to be 100 mm 100 mm in dimension. Both the upper and lower patches are fed by the same 50 Ohm Co-Axial feed line. Fig 1 shows the top and side views of the proposed antenna. Some of the methods for obtaining CP using single feed are diagonally fed nearly square, square with stubs and notches along the two opposite edges, corner-chopped squares, squares with a diagonal slot etc. The proposed design uses square notch cuts at the diagonally opposite corners of the square patches. The dimensions of the microstrip antennae are modified such that the resonance frequencies of the two orthogonal modes are close to each other. The antenna is excited at a frequency in between the resonance frequencies of these two modes, such that the magnitude of the two excited modes are equal. Also, the feed-point location is selected in such a way that it excites the two orthogonal modes with phase difference of +45 o and -45 o with respect to the feed point, which results in phase quadrature between the two modes. These two con- 7 53

8 ditions are sufficient to yield CP. The size of the square notches at the diagonally opposite corners of the square patches and the single feed location is optimized so as to produce right hand circularly polarized radiation pattern with axial ratio less than 2dB 11. Figure 1: Schematic Layout of Stacked Patch Antenna Table 1: Antenna Dimensions Dimension Upper Patch Lower Patch (S-band) (L5-band) Square Side Length 36.9 mm mm Substrate Thickness 4.75 mm 4.75 mm Corner Notch Size 4.9 mm 6.4 mm 8 54

9 5 Simulation Results Simulation results are found to be satisfying in accordance to the design requirements. Fig 2 shows the measured return loss results of the designed antenna. The measured return loss is below -10dB at both L5 and S bands. Figure 2: Return Loss (S11 in db) Vs Frequency (GHz) The -10dB impedance bandwidths of the antenna, 50 MHz (1.16 to 1.21 GHz) centered around GHz (L5 band) and 90 MHz (2.44 to 2.53 GHz) centered on GHz (S band), meets the design requirement. Use of thicker substrate ensures sufficient impedance bandwidth. The gain of the antenna is found to be 1.02dB and 6.73 db at GHz and GHz respectively. Fig. 3 given below shows the 3D ration pattern measured at L5 and S bands through simulation. The radiation pattern is found to be hemispherical is shape with maximum gain in the direction normal to the plane of the patch. Figure 3: Radiation Pattern (3D) of the Antenna 9 55

10 The antenna produces right-hand circularly polarized waves. Both co-pol (RHCP) and cross-pol (LHCP) radiation patterns at L5 and S bands are measured and are shown in Fig. 4 and Fig. 5 respectively. Figure 4: Radiation Pattern (2D) of the Antenna at L5 band Figure 5: Radiation Pattern (2D) of the Antenna at S band From Fig.4 and Fig. 5, it is clear that the antenna exhibits good cross polarisation separation at both L5 and S band frequencies. The performance of a circularly polarized antenna is characterized by its Axial Ratio (AR). AR is defined as the ratio of orthogonal components of an E-field. For a perfectly circularly polarized antenna AR will be 1 or 0 db and for a linearly polarized antenna AR 10 56

11 will be infinity. Generally AR in the range of 3 to 6 db will be sufficient for most of the practical applications. By carefully choosing proper corner notch dimensions, the proposed antenna has achieved AR values below 2 db in L5 and S bands. 6 Conclusions In this paper, a dual-band right hand circularly polarized stacked patch antenna design for IRNSS receiver is proposed. Accuracy and integrity of received data is highly imperative for IRNSS based positioning and navigational services and hence the antenna design has to be carried out against the stringent design requirements. The proposed antenna characteristics like return loss, gain, impedance bandwidth, cross polarization separation and axial ratio are found to suitable for IRNSS receiver applications. 7 Acknowledgements The first author sincerely thank Prof. S Raghavan and Mr. Mudumba Ramesh for their invaluable support and guidance towards the successful completion of the project. First author thankfully acknowledge the technical and moral support given by the authorities of Master Control Facility, Indian Space Research Organization during the course of project work. References [1] Xiaodong Chen, Clive G Parini, Brian Collins, Yuan Yao. Masood Ur Rehman. Antennas for Global Navigation Satellite Systems. 1st edn. Wiley, [2] IRNSS Signal In Space ICD for Standard Positioning Service. June ISROIRNSS-ICD-SPS-1.0, Indian Space Research Organization, Bangalore. [3] Kibe SV, Gowrishankar D. Indias Satellite Navigation Programme. APRSAF -15: Space for Sustainable Development, December 10th 2008, Vietnam

12 [4] Indian Regional Navigation Satellite System (IRNSS). K.N.Suryanarayana Rao. Project Director-IRNSS. ISAC Bangalore. [5] Ming Chen, Chi-Chih Chen. A Compact Dual-Band GPS Antenna Design. IEEE Antennas Wireless Propagation Letters.2013 vol.12, pp [6] Zhang Y Q. Li X. Yang L. Gong S X. Dual-Band CP Antenna with Low Wide-Angle Axial ratio for Tri-Band GPS Applications. Progress In Electromagnetics Research C Vol. 32, pp [7] B Sada Siva Rao. T Raghavendra Vishnu. Habibulllah Khan. D Venkata Ratnam. Spiral Antenna Array Using RT-Duroid Substrate for Indian Regional Navigational Satellite System. International Journal of Soft Computing and Engineering May, Volume-2, Issue-2. ISSN: [8] Arivazhagan S, Kavitha K, Prashanth HU. Design of a triangular fractal patch antenna with slit IRNSS and GAGAN applications. Proceedings of ICICES, India, [9] Request for Proposal for the Development of IRNSS SPS GPS User Receivers, v.1.0, March 2014, Space Application Center, ISRO, Ahmedabad. [10] Ramesh Garg, Prakash Bhartia, Inder J Bahl, Apisak Ittipiboon, Microstrip Antenna Design Handbook. Artech House, [11] Girish Kumar, KP 0Ray. Broadband Microstrip Antennas. Artech House,

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