SAE TECHNICAL PAPER SERIES

2008-36-0047

E

Design of a Patch Antenna Using Photonic Band Gap Technology for GALILEO Next Generation GPS System Juliano Fujioka Mologni Delphi Corporation, Unicamp Frank Kenji Goto Didimo Garcia Neto Antonio Cesar Rosati Delphi Corporation, Delphi Packard Electrical / Electronic Architecture Marco Antonio Robert Alves Edmundo da Silva Braga Douglas de Freitas Takeuti Unicamp, Dept. of Electronics & Microelectronics

FILIADA À

XVII Congresso e Exposição Internacionais da Tecnologia da Mobilidade São Paulo, Brasil 07 a 09 de outubro de 2008

AV. PAULISTA, 2073 - HORSA II - CJ. 1003 - CEP 01311-940 - SÃO PAULO – SP

2008-36-0047

Design of a Patch Antenna Using Photonic Band Gap Technology for GALILEO Next Generation GPS System Juliano Fujioka Mologni Delphi Corporation, Unicamp

Frank Kenji Goto Didimo Garcia Neto Antonio Cesar Rosati Delphi Corporation, Delphi Packard Electrical / Electronic Architecture

Marco Antonio Robert Alves Edmundo da Silva Braga Douglas de Freitas Takeuti Unicamp, Dept. of Electronics & Microelectronics

Copyright © 2008 Society of Automotive Engineers, Inc

ABSTRACT The next generation of satellite system developed by the European Union and the European Space Agency, GALILEO, is schedule to be fully implemented by 2010. The system is being projected to operate at 1.17645 GHz (L5 frequency) for civilian purposes as an alternative of the currently North American system, and will be used as a safety signal on Ground Positioning Systems (GPS) devices. A low-cost microstrip antenna planar patch antenna resonating at L5 frequency is developed with optimized parameters for automotive applications. For the last 15 years, periodic structures are one of the most noticeable topics of research due to their promising applications in microwave circuit and antenna design. A Photonic Band Gap (PBG) dielectric substrate comprised of periodical structures was used during the development of this project intending to minimize the surface wave losses for a given bandwidth, and therefore, a higher performance antenna could be achieved. Analytical calculations were used to simulate the antenna far-field radiation patterns, S parameters, matching impedance network and Voltage Standing Wave Radio (VSWR). Measurements were then performed in order to validate the design of the antenna. A comparison between the measured results and calculated

parameters reports a great accuracy of the design method proposed in this paper. INTRODUCTION The Galileo positioning system is a satellite based system developed by the European Union and the European Space Agency as an alternative to the already known US military controlled GPS system and the Russian GLONASS. Although the acronym GPS refers only to the existing US system, it is generally used worldwide to mention any positioning systems (such as Galileo and GLONASS). Microstrip patch antennas are often used in GPS applications due to their low profile, lightweight, easy and low cost fabrication [1-3]. On the other hand, patch antennas presents some limitations regarding the radiation efficiency due to surface wave losses. The radiation pattern is affected by the surface power diffracted by the edges of the substrate. The excitation of such surface waves also leads to a higher degree of an undesired mutual coupling between antennas positioned far from each other. Another characteristic of patch antennas, noticed mainly on high dielectric constants (εr) substrates (which are extensively used due to the straightforward integration with monolithic microwave integrated circuit), is the very narrow frequency bandwidths. To overcome these issues, PBG structures can

be employed to suppress substrate modes and surface waves [4-6]. PBG structures are an artificial network that acts analogous to the energy bands of a crystalline atom structure. By controlling the physical dimensions of this photonic crystal, one can control the propagation of certain electromagnetic modes, allowing the propagation of the desired modes and rejecting the propagation of the undesired modes. The frequency band gap relies on the dielectric contrast between the atoms of the periodic structures and substrate material and their dimensions. We have introduced periodic metallic cylindrical geometries into a FR4 epoxy substrate (εr = 4.4) in order to create a PBG crystal substrate. A quarter wave transformer was developed in order to create the possibility to feed the antenna using two outputs 90 degrees out of phase, so the antenna can be circularly polarized. A hybrid network was also developed to reduce the input impedance down to 50Ω. The analysis reported in this paper shows an increase in the antenna gain and a suppression of the side lobes of the antenna radiation pattern by making use of the PBG periodic structures as substrates. Simulation and experimental measurements were made comparing the patch antenna with and without the metallic cylindrical structures. MICROSTRIP PATCH ANTENNA AND HYBRID NETWORK DESIGN The requirements of a GPS antenna differ in various applications. For exact surveying applications, the patch antenna should only receive signals above the ground plane and reject all signals below. It should have a stable phase center that is located with the geometrical center of the antenna. It must also present a good circular polarization characteristic in order to maximize the reception of the incoming right-hand polarized (RHP) signal. It was defined that the design of the patch antenna would follow the numerical procedures described below, with no PBG structures being considered. Afterwards, the periodic structures would be incorporated to the antenna and the results would be compared. The width of a rectangular patch antenna can be evaluated using the equation [7]:

W =

c 2f

2 εr +1

(1)

Where W is the antenna width [mm], L is the length [mm], ∆L are the fringing effects, εreff is the effective dielectric constant, Le is the effective length [mm] and h is the height of the substrate [mm].

L=

c 2 f ε reff

where ∆L and εreff are defined by:

− 2∆L

(2)

ε reff =

εr +1 2

+

ε r −1 2

(ε reff ∆L = 0.412 ⋅ h

(ε reff

1

⋅

h 1 + 12  W 

(3)

)

W  + 0.3  + 0.26  h   W + 0.258  + 0.8    h

)

Le = L + 2∆L

(4)

(5)

The approach used to achieve a circular polarization was to excite the patch antenna using two feeds positioned at the center of the two sides (L/2) both delayed by 90 degrees. With this arrangement, when the vertical current flow reaches its maximum value, the horizontal current flow will be null, so the electric field will be fully vertical. One quarter cycle later (λ/4), the amplitude of the current flow will be reversed and the field will be horizontal. Repeating this cycle will radiate circularly polarized waves. The input impedance equation can be reduced to the following equation:

Z 0 = 60

λ W

(6)

The quarter wave transformer design was developed using microstrip lines with an electrical length of λ /4 and given intrinsic impedance. As mentioned earlier, the two antenna feeds should be 90 degrees out of phase. In the quarter wave transformer, the phase shift along the microstrip trace is π/2, corresponding to λ /4.

Length / Width Height Transformer length Transformer width Z0, L (90o phase shift) Z0, W (90o phase shift)

[mm] 60.39 1.40 35.93 1.54 35.93 3.023

Device Antenna Antenna Transformer Transformer Hybrid Network Hybrid Network

Table 1. Design parameters. Using equations 1-6 one can determine the design parameters of the system. The dimensional parameters used for designing our microstrip antenna, quarter wave transformer and hybrid network are shown on table 1. PHOTONIC BAND GAP LATTICE The use of PBG materials as a replacement of the usual dielectrics or metallic materials on patch antennas is currently one of the most intensive frontiers on electromagnetism engineering. It was already shown that two-dimensional PBG structures enhances the directivity of microstrip antennas [8] and suppress surface waves [9].

metallodielectric crystal. In order to prohibit the propagation of TM mode in the substrate, the plane wave expansion method [10] was used yielding the lattice periodicity a=29mm; r/a=0.38 which generates a photonic band gap between 1.16 and 1.18 Ghz. The square lattice was chosen as it provides a forbidden band for any direction in the plane of the substrate due to the highly symmetry with its respective Brillouiz zone (BZ). A forbidden gap will appear when the BZ boundary is β.a=π. Table 2 shows the relationship of the propagation vectors on both real space (R1 and R2) and k-space. Path ΓX XM Figure 1. Analogy between electronic and photonic band gap. As exposed on figure 1, the PBG shows a similarity with electronic band gap. The presence of a photonic crystal modifies the electromagnetic radiation, and for a given spectral bandwidth, the propagation of electromagnetic waves is forbidden inside the crystal. Figure 2 shows the PBG substrate and the patch antenna schematics. As demonstrated before, the square patch antenna has a width of 60.39mm and a resonance frequency of 1.17645 GHz. The material used as a substrate is a FR4 with height h=5mm. The cutoff frequency for the first transverse electric mode (TE1) is given by:

fc =

c 4h εr − 1

(7)

Figure 2. a) PBG substrate and patch antenna representation; b) real space square lattice of periodic cylindrical structures and c) the equivalent distribution on kspace. The resultant fc is 8.13GHz, and considering that fc is placed distant from the L5 working frequency, the only mode that needs to be suppressed is the transverse magnetic (TM) mode. The PBG structure is created by inserting metallic holes (filled with air) resulting on a

MΓ

Range of k [(0,0),(π/a,0) [(π/a,0),(π/a,π/a) ] [(π/a,0),(0,0)]

k||.R1 [0, π] [π, π]

k||.R2 [0,0] [0, π]

[π, 0]

[π, 0]

Table 2. Eigenvalue vectors for a square lattice. SIMULATION AND ANALYSIS OF RESULTS The Numerical Electromagnetics Code (NEC-2) is a computer code for analysis of electromagnetic structures and it was our tool of choice. NEC-2 code is publicly available for general use and it is built based on the numerical solution of integral equations evaluating the currents induced on the model by sources or incident fields. This methodology avoids many of the assumptions required by other solution methods and thus provides an accurate tool for electromagnetic investigation. The code combines an integral equation for smooth surfaces with one specific for wires, enabling a convenient and accurate modeling of a wide range of structures.

Figure 3. Simulated input return loss for the conventional antenna and the PBG substrate antenna. A conventional patch antenna and a patch antenna with PBG substrate were compared using numerical simulations. The first investigation comprises of an analysis of the input return loss (S11) for both antennas. As observed on the graph of figure 3 the resonant frequency of the microstrip

patch antenna is slightly different when the PBG substrate is used. At the L5 frequency, both antennas presents about the same input return loss. The difference on the resonant frequency is explicated by the variation between the effective dielectric constant of the substrate with and without the PBG structures.

Figure 4. Three dimensional total gain pattern for a) conventional antenna and b) antenna with PBG substrate; and axial ratio pattern for c) conventional antenna and d) antenna with PBG substrate. It is possible to notice the improvement on the total gain of the antenna with PBG substrate comparing the graphs on figure 4a and 4b. The antenna gain is defined as the relationship of the radiation strength of an antenna in a given direction compared to the intensity of the same antenna radiating in all directions (isotropic antenna), for that reason, the values for gain are given in dBi. The total gain of the conventional antenna is 9.29 dB, and the antenna with PBG substrate is 10.9. The axial ratio of a microstrip antenna is the ratio of the magnitudes of the major axis and the minor axis of the ellipse described by the electric field vector. As shown on figures 4c and 4d, the use of the PBG substrate also increases the axial ratio of the antenna. In order to simultaneously compare the radiation efficiency of antennas, the charts presented on figure 5 is often used. The use of PBG reduced the side and back lobes by approximately 2dB. If the patch antenna is positioned on the roof of the vehicle, a metallic ground plane will exist and it will further improve the directivity and performance of the antennas.

Figure 5. Radiation pattern (at 1176MHz) comparing the directivity of both antennas. The government standard for GPS applications states that antennas should provide a total gain higher than -3.5 dBi over 95% of the solid angle coverage between the angles of 90 and 100 degrees. As observed the graphs on figure 5, our design fulfills this requirement. The plot of figure 6 shows the electric field distribution on the surface of the conventional antenna and the substrate at 1.176GH. Due to the antenna geometry, the position of the feeds and the quarter wave transformer, the electric field is stronger on the borders of the antenna for a given phase. Besides the desired circularly polarization, the arrangement using two inputs via the quarter wave transformer creates a balanced microstrip antenna, which results in fewer disturbances on the total system.

Signal information was acquired on both ports feeding of the antenna using a network analyzer (2nd and 3rd ports connected to the hybrid network). Figure 8 shows the phase shift when the system operates on frequencies above 1.18GHz. Although the quarter wave transformer was projected to operate at exactly 1.176GHz, small variances occurs during the manufacturing process of the antenna, the quarter wave transformer and the hybrid network, leading to small deviations on the parameters of the system. The measured phase shift on the second port was 90.38 degrees. Figure 6. Conventional antenna electric field distribution. Experimental measurements were performed in order to compare the simulated results to a real life condition. Parameters like VSWR (voltage stand wave ratio) and match impedance were experimentally obtained. The VSWR indicates the amount of signal that is reflected back to the source in a RF (radio frequency) circuit. The hybrid network was used to step down the high antenna impedance down to 50Ω so the antenna can be connected to the source connector without any reflectance. Ideally, the antenna should resonate at 50Ω for our L5 frequency and this would lead the impedance directly to middle of the Smith chart; however, the conditions of test are not ideal and at 1.176GHz the resonant impedance is around 70Ω. As the frequency varies, the VSWR as well as the S11 changes, and they should be considered optimal at the resonance working frequency. One of our objectives was to have the VSWR less than 2. Figure 7 shows the Smith chart for our system with the Hybrid network where is possible to observe that the VSWR for our RF system is 1.4.

Figure 7. Smith chart showing the S11 impedance and VSWR value for the antenna with the hybrid network.

Figure 8. Experimental data of S21 parameter on second and third ports of the hybrid network. All simulations and numerical studies considered in this work assumed that the antenna was placed over a flat 30x30 meters ground plane instead of the actual vehicle roof. This concept is based on the assumption that the roof of the vehicle is a perfect ground, being large enough so the radiation pattern is not affected. Nevertheless, the real vehicle roof is found to have some irregularity on geometries. Also, if the antenna is placed near the edge of the roof, the respective radiation pattern will be affected. The distance from the edge of the vehicles roof can be measured as a multiple value of the operational wavelength. When the antenna is placed near the edges, the resonant frequency slightly changes and a reduction on the antenna efficiency is observed. Figure 9 shows the model of a car made of wires. The number of wires is limited in this plot for a better clarity of the model. The wire grid surface concept is frequently used on antenna electromagnetic radiation and scattering problems, and is capable to provide a very good accuracy if the wire diameter and the grid size are correctly chosen [11]. At 30mm (0.25λ) from the edge of the vehicles roof, the directivity radiation pattern presents a reduction on the directivity. Also, the lateral and back side lobes increase due to the absence of the ground plane in this region. Hence, the patch antenna itself, with or without PBG substrate, will present a reduced performance as it is placed close to the edge of the vehicles roof.

[3] K.L. Wong, Compact and Broadband Microstrip Antennas, New York: Wiley-InterScience, 2002. [4] J. Y. Park et al., An Improved low profile cavity-backed slot antenna loaded with 2D UC-PBG Refector, Proc. IEEE Antennas and Propagation Society Int. Symp., 2001, pp. 194–197. [5] M. Rahman and M. A. Stuchly, Circularly polarized patch antenna with periodic structure, Proc. Inst. Elect. Eng. Microw. Antenna Propag., vol. 149, no. 3, pp. 141–146, Jun. 2002. [6] Y. Qian et al., Microstrip patch antennas using novel PBG structures, Microwave Journal, vol. 42, pp 66-76, Jan. 1999. [7] C.A. Balanis, Antenna Theory: Analysis and Design, 2nd edition, John Wiley &Sons, New York. 1997. [8] M. Thevenos et al., Design of a new photonic cover to increasy antenna directivity, Microwave Technol. Opt. Lett., vol 22, pp 136-139. 1999. Figure 9. Total gain pattern of the patch antenna with PBG substrate placed at 30mm from the edge of the car roof.

[9] E.R. Brown, C.D. Parker and E. Yablonovitch, Radiation properties of a planar antenna on a photonic crystal substrate, J. Opt. Soc. A., pp 404, 1993.

CONCLUSIONS

[10] M. Plihal and A.A. Maradudin, Photonic band structure of two-dimensional systems: the triangular lattice, Phys. Rev. B, vol 44, pp 571.

A procedure to design a low-profile high impedance microstrip antenna integrated on PBG substrate is reported. Through numerical simulations and experimental analysis it was possible to compare a conventional antenna with the PBG antenna. The latter presented a superior performance and efficiency, with higher directivity (1.6dB), a better axial ratio and lower VSWR for the 1.176GHz L5 working frequency when compared to the conventional antenna. The improved performance is mainly due to the reduction of surface waves on PBG substrates. A reduction on the coupling level among nearby systems is also achieved by the reduced back radiation pattern. The circular polarization was obtained by applying two probe feeds to the antenna 90 degrees out of phase using a quarter wave transformer. At the early stages of the antenna design, engineers can strongly benefit from using numerical simulations, as presented in this paper, in order to optimize the geometries so the desired parameters may be achieved with confidence. REFERENCES [1] F. Ferrero et al., Dual-band circularly polarized microstrip antenna for satellite applications, IEEE Antennas Wireless Propag. Lett., no. 4, pp. 13–15, 2005. [2] F. Yang and Y. Rahmat-Samii, Curl antenna over electromagnetic band-gap surfaces: a low profiled design for CP application, Proc. IEEE Antennas and Propagation Society Int. Symp., vol. 3, 2001, pp.372– 375.

[11] A.C. Ludwig, Wire grid modeling of surfaces, IEEE Trans. Anten. and Prop., vol 34, pp 1045-1048, 1987.