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  1. 1. 1 High Gain Printed Ultra Wideband Antenna Concept Veselin Brankovic, Adalbert Jordan, Djordje Simic, Jens Weber, Jagjit Bal TES Electronic Solutions GmbH; Stuttgart Germany Abstract Ultra wideband (UWB) communication systems are emerging on the short range communication market covering variety of the application scenarios. Fact that UWB Systems are based on utilising wide frequency bands of operation makes specific requirements for antenna solution quite demanding. Antenna dispersion behaviour is critical for the system performance. Printed low cost antennas with the capability to be integrated on the same substrate with the UWB front end are very suitable for different applications. In many specific cases, having asymmetrical data traffic like access point applications, it is highly desired to have high gain printed UWB antenna solution, being able to function as sector antenna with wide azimuth beam width and moderate beam width in elevation. In this paper, high gain UWB printed antenna concept covering complete frequency range from 3-10 GHz is presented. The antenna can be easily integrated with UWB ICs on the same substrate, offering inherently low cost solutions with antenna again in the range of 7-10 dBi. Proposed concept has been verified by simulations and finally confirmed by measurements. Index Terms- Multi band OFDM, IEEE 802.15.4a, Ultra wideband (UWB), Printed Antenna, High Gain Antenna I. INTRODUCTION Ultra wideband (UWB) communication systems [1]-[2] for both High Date Rate Approach (MB-OFDM, HDR) and Low Data Rate Location and Tracking Approach (LDR-LT) have some typical application scenarios requiring high gain antenna solutions. In the case of the HIDR applications, typical scenarios might cover: - Receiving multimedia streams from car dash board to the seats having integrated displays (antenna integrated in the seat) Authors are with the TES Electronic Solutions GmbH, Zettachring 8, Stuttgart Germany, Tel. +49 71172877450, TES Electronic Solutions is a leading global B2B High Tech Company with strong advanced R&D on UW1 Technology development and products deployment. www.tesbv.com -AReceiving multimedia and data streams to the displays in home and business environment (antenna integrated in the display) In the case of the LDR-LT typical application scenarios might be: - Receiving data of mobile tag in location and tracking system as antenna ofthe access point. - Receiving information from wireless sensors by concentrator, having integrated antenna radiating in dedicated space sector In the state of the art printed antenna solutions several different approaches have been reported, mainly providing solutions based on monopole approach, for lower (3-5GHz) , higher (6-10 GHz) and complete UWB band (3-10 GHz) [3]-[4]. SMD solutions have been also promoted for lower and higher UWB bands, usually being sensitive to the implementation of the PCB ground surface, and usually addressing one of the UWB bands. The common approach is that the proposed antenna solutions provide omni directional radiation characteristics, with limited gain capabilities and generally no possibility to influence radiation direction in the simple way. Recently, printed broadband radiation elements have been investigated and deployed in high gain printed antenna solutions [5]-[6]. II. CHALLENGES AND CONCEPT SOLUTION A. Challenges & Requirements The concept solution must be able to cope with following three requirements: - Planar printed radiation elements, with 50% of the operation bandwidth related to the center frequency in the manner that thickness of the dielectric substrates does not essentially influence the performance of the antenna. This may allow to deploy the concept on substrates of different permittivity and thickness and allow easy integration of the antenna with the front end - Feeding network for radiation elements should be implemented as planar printed circuit, being able to support signals transmission using frequency non-selective planar power splitters. - A metallic structure, ideally printed, should be used as reflector to shape the radiation pattern ofthe antenna
  2. 2. 2 B. Radiation Elements In order to provide broadband operation of the elements being able to be easily fed with printed structures, printed circular dipoles have been proposed as radiation elements. In this approach one half ofthe dipole is printed on one side of the substrate and another part of the dipole on the opposite side, being fed by balanced microstrip line like it is shown in Fig. 1. Having half dipoles with circular or ellipsoidal shapes as proposed in this paper, achievable antenna bandwidths have been increased for about 20% compared to approach published in literature [5] . Figure 1 - Circular printed dipole radiation element; the second part of the dipole is on the opposite side of the substrate. Exponentially tapered strip is introduced to allow transformation to unbalanced line and therefore assembly of the connector for tests The robustness to manufacturing tolerances is also increased, meaning that proposed shapes could be easily manufactured by low-cost printed technologies, even for mm-waves applications. Moreover, the proposed antenna shapes allow easier optimisation to meat required input impedances, so mainly one parameter needs to be fine tuned. Measured return loss of the printed antenna on the R04003 substrate of 0.51mm thickens and Er of 3.38 over the entire frequency band from 3 to 10 GHz is better than - 12 dB. C. Feeding network implementationfor multiplied dipoles Implemented dipoles could be easily used in antenna arrays to obtain higher directivity. Due to the nature of the balanced microstrip line, deployment ofthe feeding network on the same substrate where the radiation elements are printed is straight forward. In order to ensure splitting ofthe signals in the wideband manner, instead of using frequency selective quarter wave-transformers, linear tapering of the strips is proposed. This simultaneously addresses more typical problems, like discontinuities, manufacturing tolerance vulnerability and offers matching with satisfactory results in entire band ofinterest. t igure 2 - teeaing network outiookjor two raaiation aipoies D. Antenna implementation with corner reflector Symmetrical mechanical reflectors, built from bended metal plate, may be used for shaping radiation diagram of the printed antenna in azimuth. The substrate with printed dipole structures with feeding networks is placed in axial symmetry plane of the corner reflector through the narrow cut slot, like presented in the Fig. 3a. Slight change of the reflector position in the respect to the radiation elements, which is initially around quarter wavelength on the center operation frequency, combined with the variation of the feeding network lines width and radius of the circular half dipole may be used for final optimization of the antenna characteristics. A~OO Figure 3a -High gain antenna outlook andphoto with bended metallic reflector UWB Antenna Array with "31D" reflector 0 -5 -10 co - 15'a -520 c> -25 -30 -35 -40 - Simulation Measurement 0 1 2 3 4 5 6 7 8 9 10 Frequency [GHz] Figure 3b - Simulated and measured input reflection lossfor high gain antenna with bended metallic reflector with excellent matching between simulation and measurements; the same substrates and size ofthe dipoles as in Fig] are used.
  3. 3. 3 The measured gain is 9.6 dBi at 4 GHz and the reflection coefficient in the frequency range between 3 and 10 GHz is below -10 dB. The dimensions ofthe antenna related to Fig. 3a are: 89 mm, wide, 63 mm long and the height of the reflector is 40 mm. The concept of the corner reflector has several drawbacks like: increased manufacturing cost due to the use of the bended metallic plate, mechanical issues connected with exact placement of the substrate in the axial symmetry plane of the metallic structure, and overall mechanical size, which can be too big for some applications. Also, this type of antenna has more nonlinear phase response since position of its phase centre is not stable with frequency because of reflector position. The graph on the Figure 4 shows the impulse response of the antenna which illustrates that the antenna can be used in systems which have system response longer of Ins, like IR-UWB with 500 MHz bandwidth, compatible with IEEE 802.5.4a standard. The deterioration of the pulse shape for wider channels can make significant influence on the system performance, so applicability of the antenna in such systems is limited. Impulse response 0,8 Figure 4 - Time response ofthe high gain antenna with 3D reflector On the other hand, with the change of the corner angle and the size of the reflector radiation characteristics may be influenced. So the main beam width in azimuth can be varied between 60 and 150 degrees and also maximum gain can be changed for approximately - 2.5 to 3 dB. Of course, there are also corresponding changes in the beam width in elevation direction. III. HIGH GAIN PLANAR ANTENNA CONCEPT In many application scenarios requiring high gain antenna features, three common performance and construction requirements are: a) Half-space radiation pattern in azimuth (180° beam width) b) Moderate beam width in elevation direction (not to wide, not to narrow) ideally about 60 degrees. c) Flat construction of the antenna, without external mechanical reflector, offering easy integration of the antenna in end product, ensuring minimal production costs and time. Achieving those requirements is possible by modification of the corner reflector approach. In the Fig. 5 novel design is presented. Instead ofhaving bended metallic structure as the reflector, metallic surfaces are printed on the same substrate as the dipoles. The basic distance from the centre of dipoles to the central part of the reflector is approximately quarter wavelength of the central frequency. The proposed reflector is "following" the changes of the printed dipoles and feeding network shape, forming two banana-shaped plate reflectors. The antenna radiation pattern is influenced by the reflector geometry and may be optimized by further change of the reflector plate size and shape. The distance of the printed metallic reflector to the feeding line should be more than one strip width in order not to influence the characteristic impedance of the feeding line and to avoid possible discontinuities. Suppressing "odd" mode of propagation in the vicinity of the reflector may be provided in the manufacturing process by "bridging" two reflector halves by bond wires or by other means, ensuring the same potential ofboth reflector metallic surfaces. Proposed concept has one essential feature that the reflector portion of the printed metallic structure is concentrated near to the structure edges and is considerably small compared to the complete printed area. This can be essentially used for placing the RF front end components of the UWB communication system inside the reflector structure. In that case no antenna connector is required and compact high gain UWB solution is obtained. Figure 5 reflector !gain antenna concept withprinted metallic A. Simulation and measurement results Verification of the proposed concept is done by simulation and finally by producing related prototypes. E -0,8
  4. 4. 4 Simulation is performed by using WIPL-D software [7], like it has been done in the case of 3D reflector antenna. Microwave substrate R04003 with Er of 3.38 and thickness of 0.51mm is used, with 18pm copper thickness with hard gold coating. The dimensions of the antenna are: 89 mm times 63 mm, which is the same as the size of 3D reflector antenna board, but in this case, the thickness of the antenna is much smaller (0.5mm, without connector), enabling easy integration in many space-restricted applications. Of course, in some cost sensitive application low-cost substrate materials like FR-4 can be used, with some reduction of the achievable antenna gain due to the losses in the material. In Fig 6 Input reflection loss (simulations and measurements) for the planar high gain antenna is presented, confirming expected performance in complete band of operation better than -10 dB. Measured and simulated results 0 -10 m -20 V - 30 C/ -40 -50 -60 - Simulation Measurement 2 3 4 5 6 7 8 9 10 Frequency [GHz] Figure 6 - Simulated and measured reflection lossfor high gain planar UWB antenna with printed metallic reflector BRadiation pattern at four frequencies Figure 7 -Measured antenna diagram in azimuthfor high gain antenna concept with printed metallic reflector Fig 7 shows measured antenna radiation pattern in azimuth for set of frequencies, showing expecting behavior. 3dB beam width varies from 180 degree to 220 degree for 3 and 10 GHz respectively. From 5 to 7 GHz measured 3-dB beam width is constant and is more as 250 degrees. Figure 8 shows simulated 3D radiation pattern at 4 GHz. 3dB beam width of 250 degrees in predicted, what corresponds very well with the measurements. The gain of the antenna is still smaller as one obtained with previously described antenna with 3D reflector, but it is compensated with wider radiation angle and better phase linearity which results in more ideal impulse response ofthe antenna. (Fig. 9.). 7 35 252-7 2. -7787e thWIu 2A83ee ._m _7. 403 e 1 Figure 8 -Radiation pattern at 4 GHzfor high gain antenna with printed metallic reflector Measured impulse response of the proposed antenna concept may be observed in the Fig. 9. The data is obtained by using frequency domain measurements with two antennas as described in [8]. The measured results look promising, showing the peak of the second response maximum 7 times less in absolute field strength, showing small antenna dispersion in time, what is consequence of good antenna phase linearity. It shows that the antenna can be successfully used for both impulse-based and MB-OFDM UWB systems. Impulse response a) -5- E 1 0,8 0,6 0,4 0,2 0 -0,2 -0,4 -0,6 -0,8 -1 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 Time [ns] Figure 9 - Measured impulse response oftheplanar high gain antenna For demonstration purposes, proposed antenna concept is successfully integrated in the automotive seat to check the typical application scenario on unsymmetrical distribution of the video signals in car entertainment system. The data transmission is performed from the dashboard to the seat, which had a display mounted on the back as illustrated on the Fig. 10. The antenna was placed on the side of the seat above arm-rest position. The same scenario is compared using an omni-directional antenna as a reference; showing expected much higher performance, robustness and -3 GFt -5 GFt -TGFt 9 GFtI-------
  5. 5. 5 resistance toward blocking with human body. The tests have been performed with high data rate WiMedia compliant chip sets from Wisair, being connected to the antenna via SMA connector, using FCC radiation mask in 3-5 GHz range. Figure 10 - Implementation of the high gain antenna for demonstration of video streaming application in a car entertainment system IV. CONCLUSION High gain UWB printed antenna concept operation in the complete frequency band from 3-10 GHz is presented. Coverage in azimuth of 180 degree with measured gain between 7-8 dBi that have been verified by simulation and measurements for the entire band. Proposed low cost single printed antenna concept is able to be used for easy integration of the UWB ICs on the same substrate without cables and connectors. The design is compared to the antenna using same radiating elements and suitable 3D reflector, showing improvements in radiation diagram and impulse response. Proposed antenna is advantageously successfully tested and benchmarked against classic omni directional printed antennas on typical video streaming application. The practical implementation value of the concept is therefore also confirmed. Realisation of the antennas on very low cost substrates is possible with some reduction in achieved overall gain. V. ACKNOWLEDGEMENT The work presented here has been partially performed in scope ofthe European research project PULSERS, which is partly being funded by the Commission of the European Union within the 6th EU research framework. The authors would like also to thank ENSTA- Paris antenna laboratory for kindly supporting antenna radiation pattern measurement activities on their facilities and providing help in measurement data interpretation. VI. REFERENCES [1] Multiband OFDM Alliance, "MultiBand OFDM Physical Layer Proposalfor IEEE 802.15 Task Group 3a", September 2004 [2] Anuj Batra, Jaiganesh Balakrishnan, Anand Dabak, et al. "TI PHY layer proposal", IEEE 802.15- 03/ 141r4, IEEE 802.15.3aPHY layer Working Group, 2003 [3] Targonski, S.D.; Waterhouse, R.B.; Pozar, D.M.; "Design of wide-band aperture-stacked patch microstrip antennas", Antennas and Propagation, IEEE Transactions on Volume 46, Issue 9, Sept. 1998 Page(s): 1245 - 1251 [4] C. Y. Wu, C. L. Tang, and A. C. Chen.;"Compact Surface mount UWB Monopole Antenna for Mobile Applications", Progress In Elecromagnetics Research Symposium 2006, Cambridge, USA, March 26- 29. [5] Nesic, A.; Brankovic, V.; Krupezevic, D.; Ratni, M.; Nesic, D.," Broadband printed high gain antenna with wide angle radiation in azimuth" Antennas and Propagation Society International Symposium, 2001. IEEE Volume 2, 8-13 July 2001 Page(s):468 - 471 vol.2 [6] Nesic, A.; Radnovic, I.; Brankovic, V.; Ultra widebandprinted antenna arrayfor 60 GHzfrequency range", Antennas and Propagation Society International Symposium, 1997. IEEE., 1997 Digest Volume 2, 13-18 July 1997 Page(s): 1272 - 1275 vol.2 [7] WIPL-D High Frequency 3D EM Modeling and Simulation Software,w [8] W. Sorgel, F. Pivit, W. Wiesbeck ,"Comparison of Frequency Domain and Time Domain Measurement Proceduresfor Ultra Wideband Antennas," IEEE EMC Magazine, September 2004, pp3 1-35