15 Dec 2011

Modelling antennas – cutting out the clashes (Page 2 of 2)

There are a number of key antenna elements that need to be calculated.

These are items that are of particular importance for the overall perfromance of the antenna in its given environment and applications.

Important antenna parameters which should be calculated using electromagnetic software are input impedance, voltage standing wave ratio, VSWR, return loss, radiation pattern, gain, and directivity.

Three dimensional simulations of the electromagnetic systems in general and antennas in particular provides users with a wide variety of analysis options. This includes the ability to create contour plots and graphs of near field quantities both in time and frequency domains.

Far zone fields and directivity plots in the frequency domain can be calculated as can the Z,Y,S matrices of a high frequency circuit in the frequency domain.

Antenna Z, Y, S Matrix

Figure 2 Z, Y, S Matrix

To get the maximum benefit of performing simulation using electromagnetic software, the user needs first to construct a geometric model of the physical system. This can be done either by using the built in geometric modeller of the electromagnetic software or, alternatively, the user can import files directly from many of the most popular commercial CAD packages. The physical properties (such as boundary conditions, materials, sources, etc.) are then assigned. Then, the boundary box, the near and far field requirements, and the time and cell sizes (for time domain solvers) are specified to obtain the numerical solution.

Antenna size

Another challenge in the antenna design is to optimise the antenna size and performance whilst reducing the total size and weight of the communication equipment alongside optimizing the impedance matching and the radiation efficiency.

Printed antennas can be considered as a good solution for reducing the antenna size due to their lightweight, simple structure and ease of mass production. In the design of a printed antenna, the radiator shapes and dimensions plus the materials used can be optimized to achieve the optimal antenna behaviour.

The applications of printed antennas have extended to the ultra-wideband (UWB) technology after the Federal Communication Commission, FCC released permission for commercial use of the frequency band (3.1 to 10.6GHz) in 2002 [4]. Generally, UWB communication antennas require low voltage standing wave ratio (VSWR<2), constant phase centre, constant group delay and constant gain over entire operating frequency band [5].

UWB Antenna

Figure 3 UWB Antenna

In the design of a printed UWB antenna, the radiator and the ground plane shapes, as well as the feeding structure, can be optimised based on electromagnetic software and a parametric study to achieve a good broad impedance bandwidth. In addition, time domain analysis of the UWB antenna plays an important role to determine pulse distortion of the received signal. However, existing Wireless Local Area Network, WLAN, IEEE 802.11a systems operating in the frequency band (5.15 – 5.825GHz) can cause interference with UWB systems. Therefore, a band stop filter that rejects this limited bandwidth would be required in UWB RF front-ends to reduce the inference between UWB systems and these systems.

Therefore, many antennas with band rejection characteristic have been researched with the utilisation of advantages of composing more simply RF front-ends. Different configurations for band notching can be designed using parametric study of electromagnetic software. An example of modelling the UWB antenna of [6] using the CHRONOS time domain solver is described in Figures 2(a) and (b) which shows the field visualisation on the antenna surface and the return loss versus the frequency from DC to 15 GHz.

Eventually the antenna engineer needs to compare results of the simulation with the results of the measurements to verify and confirm his design and scientific conclusions.

References:

[1] C.R.Paul, Introduction to Electromagnetic Compatibility, John Wiley& Sons, Inc., 1992.

[2] Special Issue on “Lightning”, IEEE Trans.Electromagn. Compat., Vol. 40, Nov. 1998.

[3] A.Taflove, Computational Electrodynamics The Finite-Difference Time- Domain Method, Artech House,Inc.,1995.

[4] New Public Safety Applications and Broadband Internet Access among Uses Envisioned by FCC Authorization of Ultra-Wideband Technology-FCC News Release 2002.

[5] N. H. M. Sobli and H. E. Abd-El-Raouf, “Design of A Compact Printed Band- Notched Antenna for Ultrawideband Communications,” Progress In Electromagnetic Research (PIER) M, 2008, vol. 3, pp. 57–78.

[6] Seok H. Choi, Jong K. Park, Sun K. Kim and Jae Y. Park, “A new ultra-wideband antenna for UWB applications,” Microwave and Optical Technology Letters, Vol. 40, No. 5, March 2004.

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About the author

Hany E. Abd El-Raouf received the BS degree in electronics and communication systems from Helwan University, Cairo and an MS and PhD in electrical engineering from Al-Azhar University, Cairo. In 1989 he joined the Microwave Engineering Department, Electronic Research Institute, Cairo, as a Research Assistant and later became Assistant Professor. He moved to the Electrical Engineering Department, Pennsylvania State University, University Park 2001 where he was a Postdoctoral Fellow and Visiting Assistant Professor. In 2005 he became an Associate Professor in the Department of Electrical & Computer Engineering, Faculty of Engineering at the International Islamic University Malaysia. Currently, he is a Research and Development (R&D) Engineer at Enginia Research Inc.

Integrated Engineering Software is a leading developer of hybrid simulation tools for electromagnetic, thermal and structural design analysis. Founded in 1984, IES provides a complete line of fully integrated 2 and 3 dimensional simulation software. Easy to use, IES's products allow engineers and scientists to design, simulate and optimise complex devices and systems via computer simulation, thereby reducing costs and risks associated with physical prototyping and avoiding costly mistakes during manufacturing. The software allows companies to reduce design time and costs, spend less money on expensive prototypes, improve product performance, decrease time to market and ultimately increase profitability. IES pioneered the creation of the BEM field solver 25 years ago, and today, no one else in the market offers such a variety of field solvers with the same software packages.

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