Integrating antenna modeling codes in Web-based visualization environments

z zyxw Integrating Antenna Modeling Codes in Web-Based Visualization Environments zyxwv zyxwvuts Antonis I. Kostaridis, Christos G. Biniaris, Andy J. Marsh, Hristos T. Anastassiu, and Dimitra 1. Kaklamani Department of Electrical and Computer Engineering National Technical University of Athens 9. lroon Polytechniou Str., GR-15780 Zografos. Athens, Greece Tel: +30210 7722277; Fax: +30210 7723557; E-mail: dkaklam@cc.ece.ntua.gr Abstract The development of an interactive online asynchronous environment for advanced antenna modeling is presented. Electrcmagnetic (EM) codes are coupled with the World Wide Web (WWW), and the Virtual Reality Modeling Language (VRML) is used for the creation of a flexible visual pre- and post-processor. The adoption of object-oriented methodologies, in conjuno tion with the employment of platform independent standards, contributes to the flexibility of the developed application. It is generic enough so that various numerical codes for antenna modeling can be easily integrated under a common multi-disciplinary environment. A case study regarding the modeling of conformal microstrip arrays is presented. Keywords: Visualization; modeling; antenna radiation patterns; conformal antennas; microstrip antennas; microstrip arrays; client-server systems; object oriented programming; WWW; Java servlets; VRML; antenna modeling 1. Introduction R esearch in antenna modeling has been greatly favored by ecent advances in computational electromagnetics. The improvement of the diverse numerical techniques and their hybrids, in conjunction with the availability of cheap processing power, has given rise to the simulation of more realistic scenarios. The increase in complexity of the simulated scenarios has also led to the need for development of sophisticated visualization tools that permit the users to conceptualize all the involved parameters in an efficient way. For example, the ability to visualize two- and three-dimensional radiation patterns is a key to the understanding of basic antenna principles. Additionally, antenna analysis and design software packages can he made accessible through standard browsers, due to the unprecedented opportunities offered by current Web technologies, removing otien-complicated interfaces. Several issues on this topic have been addressed in the literature [l-61. Our objective is to introduce the potential possibilities of using the WWW and associated technologies as the interface environment and the communication mechanism to integrate EM codes. By coupling Web technology with visualization languages that offer interactivity and are suited to a Web environment - such as the Vimal Reality Modeling Language (VRML) we can achieve the development of realistic visualization applications,. which provide a number of unique features over traditional visualization techniques. ~ In this context, we present the development of an interactive, online, asynchronous application for antenna modeling. The preprocessing and post-processing phases, along with the numerical IEEEAntennas and Propagation Magazine. Vol. 45, No. 4, August 2003 analysis, are integrated under a unified environment that revolves around a Web server. This environment utilizes server-side components that provide the necessary functionality for the Web integration, and the VRML to visualize two- and three-dimensional objects. The system acts as a front-end to external numerical codes. Preliminary work on the aforementioned application is described in [7]. As a case study, we present the use of the interactive environment for the modeling of conformal microstrip antenna arrays, using the Modified Method of Auxiliary Sources (MMAS) [8-121. zyxwvuts In Section 2 we propose a generic client-server architecture, and in Section 3 we describe the enabling Web technologies. Section 4 focuses on the implementation of components residing on the Web server that provide the core functionality, realizing the Web integration ofthe EM codes. Finally, the prototype interactive environment for our case study is demonstrated in Section 5 . 2. The Client-Server Architecture The main component of the proposed architecture is a Web server where the service'logic is implemented (Figure I). To access the remote facilities, a prospective user makes an HlTP (Hyper Text Transport Protocol) request to the applications-password-guarded home page. He or she enters a password and posts it. At the other side, an HTTP-request-handler component checks the validity of the password, and returns the page containing a fill-in form for the input parameters of the application. The user then completes the form and posts it. This request is handled by another server component, which is responsible for wrapping the EM code, zyxwvuts 11 providing the necessary execution environment, and activating the native executable. Since complex simulations require significant CPU resources and may take several minutes or even longer to complete, the client’s Web-browser timeout setting will quite oAen intempt the process. Thus, a job-submission mechanism is mandatory. Namely, the user submits his or her input processing by additionally providing an e-mail address. After posting the values of the input parameters, the user may close the browser, and wait for the simulation to be completed. Upon completion of the job, the system e-mails a message to the user containing the URL of the visualization home page. This page requires a password and an e-mail address in order to access the user-specific directory containing the results of the last simulation. After posting this information, a last component is activated that performs the transformation of the output data into the VRML format, and constructs the post-processor page containing links to the VRML files. zyxwvutsrq zyxwvutsrqp zyxwvutsrq zyxwvutsrqponm zyxwvutsrq zyxwvuts The client-server architecture utilized provides the possibility for multiple users to interact with the system simultaneously. Furthermore, accessing the stored simulated results through the Web enhances collaboration, as engineers can interact with their colleagues and review their work. Moreover, by adopting an objectoriented approach and by standardizing the format of the EM codes’ output data, a generic and extensible framework can be developed so that established engineering analysis and design tools can he embedded into the system with minor additions andor modifications. 3. Web Technologies As has been previously mentioned, the integration of EM codes in a Web environment requires an adequate server-side mechanism. The most common mechanism is provided by the Common Gateway Interface (CGI) scripts [14], written in either the Per/ or C languages. Nevertheless, during the last few years, significant developments toward Java servlets [I51 have been achieved. Servlets are an efficient platform-independent integration technology for server-centric development and seamless deployment of back-end systems and modules. They are pieces of Java source code that add functionality to a Web server in a manner similar to the way applets add functionality to a Web browser. Servlets are designed to support a requestlresponse computational model that is commonly used in Web browsers. Unlike CGI scripts, a servlet does not run in a separate process, and remains in memory between requests. On the other hand, when a server receives a CGI request, it must create a new process, allow this process to start and terminate, and then return the resulting text to the Web browser. As one can imagine, sending many simultaneous CGI requests would very quickly bring a dramatic slowdown in the performance of the processor hosting the Web sewer. On the contrary, when a servlet is invoked as a thread by an instance of the Java Virtual Machine, multiple requests for the same servlet do not result in multiple instances of the Java interpreter running simultaneously. Figure 2. A typical servlet implementation. Under the examined framework, servlets can act as wrappers for the native EM code, with primary responsibilities: namely, the verification of the integrity constraints on input data that a user posts, the activation of the EM code, and, finally, the transformation of the code’s results according to VRML [16]. VRML specifies a file format (.wrl) for describing three-dimensional interactive worlds and objects. It may be used to create three-dimensional representations of complex scenes, such as illustrations, product definition, and virtual-reality (VR) presentations. VRML is capable of representing static and animated objects, and can have hyperlinks to other media, such as sound, movies, and images. Interpreters (browsers) for VRML [17, 181 are widely available for many different platforms, as are authoring tools for the creation of VRML files. 4. Implementing Java Servlets The proposed architecture mainly defines four types of servlets, residing on a servlet-capable Web server. The first one handles the user-authentication stage. This servlet interacts with userprofile data, determining the EM codes that a user is authorized to access and the position where simulation results should he stored. The second servlet handles the application-profile management, guiding the user in the data-input process, and performing the validation according to the selected application. The third servlet acts as a wrapper for the EM code, enabling the allocation, configuration, and remote execution of the selected application. This servlet monitors the simulation process, and is responsible for informing the user after execution has been completed. Finally, the fourth servlet generates the VRML objects that represent the simulation results (e.g., radiation pattems, current distributions, etc.) and zyxwvutsrqp The main feature of a servlet is the capability of dynamic HTML code generation. In addition, being Java source code, the servlets inherit the advantages of the object-oriented and platformindependent Java language, and can access the Application Programming Interfaces (APls), as do any other programs written in Java. Thus, the functionality they can provide is significant. 12 IEEEAntennas and Propagation Magazine, Vol. 45, No. 4, August 2003 zyxwvutsrqponml zyxwvutsrqponmlkji zyxwvutsrqpo P a 5 m d Reqvest Form EM nwlt 3 Graphics1user m c e zyxwvutsrqponmlkjih Figure 1. The proposed architecture. Figure Sc. Visualization of the array: an expanded view of the upper-right target frame in Figure Sa. Figure Sa. Visualization of the results. Figure Sd. Visualization of the array: an expanded view of the lower-left target frame in Figure Sa. zyxwvutsrq Figure Sh. Visualization of the array: an expanded view of the upper-left target frame in Figure Sa. /€€€Antennas and Propagation Magazine, Vol. 45, No. 4. August 2003 Figure Se. Visualization of the array: an expanded view of the lower-right target frame in Figure Sa. 13 zyxwvutsr zyxwvuts zyxwvutsrq zyxwvutsrqpo zyxwvutsrq responds with the post-processing HTML pages containing hyperlinks to these VRML objects. From the developer's perspective, the aforementioned sewlets should extend the HttpServlet class of Sun's Servlet API. The respective compiled classes have to be placed in appropriate directories inside the Web server's installation path [19, 201. Each servlet interacts using two objects, HttpServletRequest and HttpServletResponse [15]. A new request invokes the servlet doPost ( ) method, which should be overridden so that parameters of the request can he accessed and processed according to the given sewlet call. The example code in Figure 2 presents a typical implementation of a servlet class, and some indicative methods that some of the servlets should implement. Within the doPost method, the getparameter ( 1 method gets the sewlet's expected argument(s) and processes them. As a response to the client's request, the example doPost method uses a Writer from the HttpServletRespOnSe object to return the HTML data. Before accessing the Writer, the example sets the contenttype header. At the end of the doPost method and after the response has been set, the writer is closed. lnstnuteof Communicationsand Computer Systems - MICIOW~M~ and Fibel Optics Laboratory - Virtvol A m e m Derign Simulotw Figure 4a. The introductory pre-processing Web page. zyxwvutsrqpon Concerning the method that calls the EM code, it must be pointed out that Java provides two options for executing an external native code from the context of a Java program. The first one is provided through the Java Native Interface ( N I ) API, which gives a Java program the possibility to call native methods [15]. The second option - which is simpler and more adequate for hatchtype simulations - is provided through the use of the exec method ofthe Runtime class o f the standard Java API. As it can be seen from the code fragment of Figure 3, a new Process object must be instantiated that is used to execute a specific command. When Java forks a new process, it redirects stdin, stdout, and stderr. For this reason, in order to communicate with the program one must make use of the methods getInputStream(), getOutputstream0, and getErrorStream0 of the Process object. This provides full control of the new executing process. It is important, though, to underline that Java applications that use the exec method lose their portability. However, it is possible to identify the underlying operating system from a Properties object, and to adapt the command to be executed I' ProcessEM_code: Figure 3. A code fragment for the exeeution of an external numerical program from fava. 14 Figure 4b. The introductory pre-processing Web page for entering geometrical parameters. zyxw Figure 4c. The introductory pre-processing Web page for entering MMAS parameters. IEEE Antennas and Propagation Magazine, Vol. 45. No.4, August 2003 zyxwvutsrq Figure 6. A 2 x 3 antenna-array geometry Figure 8. Combining the antenna geometry and the radiation pattern. Figure 9a. A sample output VRML file. Figure 9b. A sample output VRML file. zyxwvutsrqp zyxwvutsrqpo zyxwvutsr Figure 9c. A sample output VRML file. IEEEAnfennas and Propagation Magazine. Vol. 45, No. 4, August 2003 Figure 9d. A sample output VRML file. 15 zyxwvutsrqpo zyxwvutsrqpo zyxwv zyxwv zyxwvutsr zyxwvutsrq zyxwvuts accordingly. The code in Figure 3 presents an example of how the execution differs on Windows 98 and 2000 platforms. 5. A Case Study: Modeling Conformal Microstrip Antenna Arrays 5.1 The Interactive Environment In our case study, Java servlets interface with a code wrinen in FORTRAN90 to provide a Web-based front-end. This code analyzes conformal patch arrays with microstrip excitation, printed on a cylindrical surface. The numerical technique employed is the Modified Method of Auxiliary Sources (MMAS) [8-12]. The main concept of MMAS is given in Subsection 5.2. a m y of patch antennas, fed by microstrip lines. Each antenna consists of a radiating element (conducting patch), mounted on a dielectric substrate. MMAS was introduced to improve the ability of MAS to treat relatively flat three-dimensional structures. Unlike standard MAS, where the analysis is based on elementary source fields [I 112, 23-24], in (MMAS), the building blocks of the methodology are the source current and charge densities on the auxiliary surfaces. Both charges and currents lie at the nodes or midpoints of a canonical grid, which serves as a discrete model of the auxiliary surface where derivatives can be approximated by finite differences. Numerical calculations have demonstrated that this modification, apan from improving the accuracy of the technique, also significantly reduces the computational effort, when applied to geometries of small thickness. Applying MMAS, and assuming an exp(-/ot) The process of performing a simulation is initiated by a client request that includes the interaction of several dynamically generated HTML pages controlled by servlets. More specifically, the user interface at the pre-processing phase comprises a passwordprotected HTML page, and two pages containing fill-in forms, where all the parameters of the simulation are defined (see Figure 4). In the first page, the user inserts the geometrical parameters of the structure under investigation. At the end of the page, he or she is requested to type his or her e-mail address, in order to enable the asynchronous communication. The form is then posted, and the triggered servlet creates the file containing the geometrical parameters and the VRML representation of the conformal antenna array geometry. : If a parameter value is invalid or missing, the servlet returns a page describing the error(s). Otherwise, it returns a second page, where the user must fill or select from drop-down lists the EM parameters and the parameters related to the implementation of MMAS, such as the number of auxiliary sources, the amplitudes and phases of the array elements, etc. This page also contains a link to the VRML file of the geometry, so that the user can inspect it and make modifications, if needed, by pressing the back button of his or her browser. The user automatically receives an e-mail upon completion of the job, notifying him or her of the URL of the page containing the links of the results. The lack of a standardized command-line email system in Windows 9YNT has been overcome by using the JavaMail API [ZI]and the JavaBeans Activation Framework API time dependence, the electric field, E(r) , due to a surface current density J(r') on the auxilialy surface ,,s , is given in terms of the vector potential, A, and the scalar potential, 0 , as E(r) = - j o A ( r ) - V @ ( r ) , (1) where together with the current continuation law - j o p ( r ' ) = Vs* J (1'). (3) In Equations (l)-(3), J is the unknown surface current density, and p is the unknown surface charge density, both being due to the auxiliary sources. Moreover, p and E are the medium permeability and permittivity, respectively, and o is the frequency. To approximate the divergence in Equation (3) in a discrete way, an auxiliary source grid is utilized, the planar projection of which is shown in Figure 7. Following a particular discretization pattern, PI. The post-processor Web page is shown in Figure 5 . It consists of one inspector frame, on the left, and four target frames, on the right. From the inspector kame, the user can select one or more hyperlinks of the VRML files to he visualized on a target frame. In this way, easy comparison of radiation pattems and independent manipulation of the different plots are achieved. 5.2 The EM Solver zyxwvu zyxwvutsr The EM solver developed implements MMAS [S-121 to ana- 16 U' IEEE Antennas and Propagation Magazine, Vol. 45. No. 4, August 2003 zyxwvuts zyxwvutsrqp the charge density is sampled at the nodes of the grid, whereas the current density is sampled at its midpoints. If the subscripts in J denote a vector component along a given direction, the discrete approximation of Equation (3) at a point (mAu’,nAv’)of the grid is written as (for simplicity, Au’ and AV’ are suppressed in all arguments) -iw(m,n) objective of the authors’ current activities is to utilize the Web resources for parallelidistributed computing, and to demonstrate the capabilities of the interaction between advanced EM solvers, remote computing networking, and advanced visualization techniques, including stereoscopic imaging and immersive VR. At present, the results in VRML format can be used to illustrate a threedimensional far-field model. The VRML 2.0 format could also be used to introduce a temporal dimension and WWW-supported facilities. For example, audio could also be used to enhance the presentation of results by using sonification. 7. Acknowledgements The authors would like to thank the EC DGIII for the supportive funding of this work, within the MADS Project EP 28363 under the ESPRIT Program. implying a dependency between charge and current samples. Equation (4) infers that it is advantageous to consider the currentdensity samples as primary unknowns that exclusively define the degrees of freedom in the problem, and the charge-density samples as secondary (dependent) unknowns. Thus, the position of the “auxiliary sources” in MMAS is identical to the sampling location of the current densities (grid midpoints). Moreover, at each sampling site, a single component of the current density is defined (either along U‘ or v’ ,but never along both directions), 8. References zyxwvutsrq zyxwvutsrqpo zyxwvutsrqpo zyxwv By expanding the auxiliary source currents in terms of weighted basis functions, the unknown weights are determined as a solution to a linear system of equations, constructed through enforcement of the boundary conditions on the physical surfaces of the structure. More mathematical details about MMAS are given in 1131. 5.3 Post Processing The results of the analysis include characteristic antenna parameters of the antenna array, which may be either single-Valmued, such as the input impedance, or functions of the co-ordinates, such as the antenna near field at a specified distance, and the far field. Furthermore, it is advantageous to the code user to visualize the far-field pattern superimposed on the actual antenna geometry. In this way, the user can have a global perception of how the geometrical features of the antenna affect the radiation pattern. As shown in Figure 8, the geometry (leA-hand side) and far-field pattern (right-hand side) which can be gyrated independently are combined onto a single screen. Each one of them is presented in its own three-dimensional coordinate space, with a pointer depicting an angle of 6 and (. The user can manipulate the pointers by adjusting the slide-bar values for 8 and @ .The user can also visualize each plot in a separate window by clicking on it. A list of sample outputs, consisting of two- and three-dimensional patterns, is depicted in Figure 9. ~ ~ I. E. S. Fisher and E. Michielssen, “Mathematica Assisted WebBased Antenna Education,” IEEE Transactions on Education, 41, 4, November 1998, pp. 353-368. 2. D. P. Economou, A. Marsh, and D. I. Kaklamani, “P(MoM/MAS): An Interactive Environment, Coupling WWW Technology and Parallel Processing, to Solve Large Size Electromagnetic Problems,” IEEE Antennas and Propagation Magazine, 41, 1, February 1999, pp. 130-137. 3. A. Marsh, D. 1. Kaklamani, H. T. Anastassiu, and P. Shuhitidze, “The P(MMAS) Interactive Environment: An Intemet Service Coupling Web Technology and HPCN to Analyze Conformal Microstrip Arrays,” Proceedings of the I 0th Mediterranean Elecrrotechnical Conference (MeleCon ‘2000), Cyprus, May 19-21, 2000,pp.411-414. 4. C. Bajaj and S. Cutchin, “Web Based Collaboration Visualization of Distributed and Parallel Simulation,” Proceedings of the I999 IEEE Symposium on ParaNel Visualization and Graphics, 1999, San Francisco, California, United States, pp. 47-54. 5. R. Muralidhar, S. Kaur, and M. Parashar, “Architecture for Web-Based Interaction and Steering,” Proceedings of Euro-Par 2000, August 2000, pp. 1332-1339. 7. A. I. Kostaridis, C. G. Biniaris, A. Marsh, H. T. Anastassiu, and D. I. Kaklamani, “Advanced Visualization Techniques in Antenna Modelling Using VRML and Web Technologies,” List of Abstracts of the 8th International Conference on Advances in Communications and Control (COMCON S’, Crete, Greece July 25-29, 2001, p. 16. 6. Future Work This paper concentrates on using the WWW and the associated technologies as a remote service to solve EM problems. The IEEEAnlennas and Propagation Magazine, Vol. 45, No. 4, August 2003 zyxwv zyxw zyxw 6 . R. D. Kriz, R. T. Levensalor, and S. D. Parikh, “Interactive Scientific Visual Data Analysis Using Java, PV-Wave, and IMSL,” Visualization Development Environments 2000 Proceedings, Princeton, New Jersey, April 27-28, 2000. 8. P. Shubitidze, D. I. Kaklamani, and H. T. Anastassiu, “Modified Method of Auxiliary Sources Applied to the Analysis of Planar and Cylindrically Shaped Microstrip Antennas,” Proceedings of the Intu‘mational Conference on Electromagnetics in Advanced 17 zyxwvutsrqpo zyxwvut zyxwvutsr zyxwvutsrq zyxwvutsrqp zy zyxwvutsrq zyxwvutsrqp Applications (lCEAA99), Torino, Italy, September 13-11, 1999, pp. 375-378. 14. The Common Gateway Interface, http://hoohoo.ncsa.uiuc.edu/ cgi/overview.html. 9. D. L. Kaklamani, H. T. Anastassiu, and P. Shuhitidze, “Analysis of Conformal Patch Arrays via a Modified Method of Auxiliary Sources (MMAS),” Abstract Book of the Millennium Conference on Antennas and Propagation, Davos, Switzerland, 9-14 April 2000, p. 292, and Proceedings of the Millennium Conference on Antennas and Propagation (AP2OOO), Davos, Switzerland, April 914,2000. 15. SUN Microsystems Java Tutorial, http:/ljava.sun.com/docs/ booksltutoriall. 16. VRML 2.0, http:/l~w.vrml.org/VRML2.0/FINAL. 17. Cosmo Software, http:l/www.cai.comlcosmoihtml/player.htm. 18. OpenVRML, http://openvrml.sourceforge.neti. 10. H. T. Anastassiu, A. I. Kostaridis, C. G. Biniaris, and D. I. Kaklamani, “Reduced Complexity Analysis of Microsnip Patch Arrays, Conformally Mounted to a Cylindrical Conducting Surface,” 2001 IEEE International Symposium on Antennas and Propagation and USNCAJRSI National Radio Science Meeting, Boston, Massachusetts, July 8-13, 2001. 11. D. I. Kaklamani, “The Method of Auxiliary Sources in Computational Electromagnetics,” Proceedings of the European Congress on Computational Methods in Applied Sciences and Engineering (ECCOMAS ZOOO), Barcelona, Spain, September 11-14, 2000. 12. D. I. Kaklamani and H. T. Anastassiu, “Aspects of the Method of Auxiliary Sources (MAS) in Computational Electromagnetics,” IEEE Antennas and Propagation Magazine, 44, 3, June 2002, pp. 48-64. 13. F. Shuhitidge, H. T. Anastassiu, and D. 1. Kaklamani, “An Improved Accuracy Version of the Method of Auxiliary Sources for Computational Electromagnetics,” submitted to IEEE Transactions on Antennas and Propagation. 19. The Apache Software Foundation, http:llwww.apache.org/. 20. The vqServer Home Page, http://www.vqsoft.comJvq/server/. 21. SUN Microsystems JavaMail API, http://java.sun.com/ productsljavamaill. 22. SUN Microsystems Java Beans, http:/ljava.sun.comeans, 23. P. J. Papakanellos, D. I. Kaklamani, and C. N. Capsalis, “Analysis of an Infinite Current Source Above a Semi-Infinite Lossy Ground Using Fictitious Current Auxiliary Sources in Conjunction with Complex Image Theory Techniques,” IEEE Transactions on Antennas andPropagation, 49, October 2001, pp. 14911503. 24. H. T. Anastassiu, D. I. Kaklamani, D. P. Econornou, and 0. Breinbjerg, “Electromagnetic Scattering Analysis of Coated Conductors with Edges Using the Method of Auxiliary Sources (MAS) in Conjunction with the Standard Impedance Boundaty Condition (SIBC),” IEEE Transactions on Antennas and Propagation, 50, January 2002, pp. 59-66. $? I Changes of Address Information regarding subscription addresses is managed by IEEE headquarters. It is not maintained, nor can it he changed, by any member of the Magazine staff. If you are a member of the IEEE, your subscription is sent to the address in your IEEE member record. Your record can he updated via the Web a1 http:llwww.ieee.orglmembersbip/coa.html. This can also he done by contacting IEEE headquarters: Member Address Records, IEEE Headquarters, 445 Hoes Lane, Piscataway NJ 08855-1331 USA; Tel: +I (908) 981-0060 or +I (800) 678-4333; Fax: +I (908) 9819667; E-mail: address. change@ieee.org. If you are an institutional or other non-member subscriber, contact IEEE Customer Service at the above address, telephone, and fax numbers; E-mail: customer.service@ieee.org. Do not send requests to any member of the Magazine Staff. zyxwvuts tEEE Antennas and Propagation Magazine, Vol. 45, No. 4, August 2003