FDTD Analysis of Rectangular Microstrip Loop Antennas

Matthew J. Inman; University of Mississippi Atef Z. Elsherbeni; University of Mississippi Charles E. Smith; University of Mississippi Kai-Fong Lee; University of Mississippi Abstract - The characteristics of ordinary loop antennas are well known. This paper explores methods to reduce the physical size of printed rectangular loop antennas by introducing a ground plane into the structure. With the introduction of the ground plane into the antenna structure, the relationship between the dimensions of the modified loop are explored to achieve the design goals. This type of antenna is analyzed using the finite difference time domain (FDTD) method and these results are verified by other numerical simulation packages. Differences between full (unmodified) loop antennas and the modified antennas of the same dimensions are examined. Optimization of bandwidth, gain, directivity, and operational bands are discussed. Reducing the physical size of the antenna allows for more commercial uses in mobile transceiving platforms. By manipulating the dimensions of the modified antenna it is possible to adjust its parameters to achieve maximal operation at a specific band, or within several different bands concurrently. Design examples of this type of antennas for radar applications and personal communication devices are presented. Keywords: FDTD, Microstrip Loop Antennas

I. Introduction

Loop antennas have the advantage of being one of the simplest designs for antennas known. Even when a printed microstrip configuration is used many of its known characteristics still hold, making its response very easy to predict. The loop structure itself holds many advantages over standard dipole type designs. The loop provides good matching for many types of inputs such as transmission lines and coaxial cables. It also has increased directivity not found in many simple designs. However, one of its main disadvantages over other simple antennas is the large surface area it requires. While a dipole design might only require space in one direction, the loop antenna requires a large planar surface area.

One proposed solution to decreasing the size of the

antenna is to insert a reflector strip through the antenna on a plane of symmetry. This reflector strip has the effect of reducing the surface area required for the loop antenna by 50% as seen in Figure 1.

The simulations of this type of antennas were all done

using the FDTD method. Validation is performed using a 3rd party program to verify the results.

II. Loop Structures

The main goal in using a modified loop structure is to reduce the surface area required to construct the antenna while not sacrificing any loss in bandwidth or efficiency. While not a true a ground plane, this reflector strip enables the operation of the modified loop antenna in much the same way as the full loop does. The two parameters that define the loop are designated as W and H, where W is the size of the loop on the side where it is being fed, and H is the size on the adjacent side.

The normal loop structure has a relatively high primary resonance (seen at approximately 10 GHz in Figure 2) as well as a secondary resonance at a lower frequency and a third at a higher frequency. Impedance analysis shows that at this primary frequency the antenna operates at around 50 Ohms for entire bandwidth providing a good match for a feed system. The other resonances do not quite achieve 50 Ohms, but do come closer to 75 Ohms. This multi-band operation is one of the features of the loop antenna that is to be preserved.

2H

H

W

Normal loop

W

Modified loop

Figure 1. Loop structures.

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In confirming the results for the full loop antenna, the structure was both run using a in-house FDTD simulation program and independently with Ansoft HFSS program. Figure 2. shows the comparison of the return loss between the two programs. They both accurately predict the main resonance, although HFSS does show stronger return loss at the lower frequencies. One possible cause of this is that HFSS warns of convergence problems in this operational area. However, even taking this into account, both simulations show promising results.

When this same antenna is modified with the introduction of the reflector strip, the antenna behaves in much the same way as the full sized loop as seen in Figure 3. This sample antenna has a feeding side length of 33mm and adjacent side length of 12mm, both measured on the outside. The strip has a width of 3mm and is situated on a dielectric substrate with thickness of 3mm and relative dielectric permittivity of 7.2. The reflector strip is placed through the antenna at the halfway point on the adjacent sides from the

voltage source. This keeps the feeding of the antenna constant as the physical size of the antenna is reduced in half. With this addition, the modified loop structure performs quite similar to the original. They both exhibit primary resonance at approximately the same frequency and also have a secondary resonance at a lower frequency. The modified structure lowers the frequency of the resonances slightly, but shows enhanced operation and wider bandwidth at the primary frequency. Examination of the impedance (as seen in Figure 4.) in the operation range shows a near constant 50 Ohms, an important feature for wideband operation. The secondary lower resonance, though while shifted lower, also shows improvements both in return loss, and in impedance operation nearer 50 Ohms. The third higher resonance seems to suffer slightly with the addition of the reflector strip.

Figure 2. Full loop return loss comparison.

Figure 4. Modified loop impedance comparison.

III. Feeding side length effects.

Examination of the differences between the full and modified loop shows the possibility of other operating bands that are muted by the current geometry. Changing the physical parameters of the antenna (height and length) can have a multiple effect on the operation of the antenna. In adjusting these lengths both the operating frequencies and the return loss at these frequencies can be changed. By adjusting the height and width parameters of the physical structure while keeping the primary operating frequency the same, it might be possible to control the amount of bandwidth within each operating range. Alternatively, if fewer bands were required, it is possible to simply widen the return loss of two adjacent resonances to the point where it becomes one large band.

Figure 3. Modified loop return loss comparison.

The results in Figure 5. show that when the width is reduced to 27mm, the number of resonant frequencies increases. This increase in frequencies is logical due to the smaller antenna size; however, we see a good effect on the return loss with this change. The two major resonances drop near 25 dB, but, interestingly enough, the higher one is created by the merging of two other resonances creating a

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band stretching from 8.5 to 12 GHz. The impedance plot of this configuration in Figure 6, shows that at both these bands the antenna operates near 50 Ohms. At the higher resonance, the impedance never varies higher than 80 Ohms anywhere in the band. This shows the reasonable level of return loss for both of these resonances.

Figure 7. 39mm modified loop return loss.

Figure 8. 39mm modified loop impedance plot.

Figure 5. 27mm modified loop return loss.

When the size of the loop is expanded on the feeding size from 33mm up to 39mm, a similar result occurs as shown in Figure 7. While the operating frequencies are shifted lower, the same type of multiple resonant bands are shown. In this case the resonance that had occurred at 10 GHz has shifted down to 8 GHz and combined with a previously weak higher one appearing at 11.5 GHz. This creates a very strong band from approximately 8 GHz up to 12 GHz, a bandwidth of 40% around the center of 10 GHz. In addition, as with the 27mm case, the resonances all have good return loss as they approach or exceed 25dB. The impedance plot is presented in Figure 8, which shows that at these frequencies the antenna operates very closely to 50 Ohms. While in the full operation band the impedance magnitude oscillates around the 75 Ohm mark.

IV. Adjacent side length effects

Figure 6. 27mm modified loop impedance plot. As with the feeding side, manipulating the adjacent side length can have a significant effect upon the operation of the modified loop antenna. Changing this length will change the overall operating frequencies of the antenna as the effective perimeter length is varied. However, as this length gets very small, the operation of the antenna goes from a loop to a folded dipole and this mode should be considered when examining this configuration. When this condition of a small length on the adjacent side is considered, the operation compared to the original case is very close. The transformation into a folded dipole has some known advantages when considering parameters such as directivity. Examining a case when this length has been reduced, bringing the length from the inside of the reflector to the inside of the opposite side from 6mm down to 3mm, the effects on the return loss suffer slightly, as shown in Figure 9.

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Surprising is how little the variation in the resonant frequencies is observed. The variation above the major operating frequency at 10 GHz shows a slightly higher return loss not going higher than 10dB until after 13 GHz. The impedance plot in Figure 10, shows the antenna operating near 50 Ohms at the resonant spikes and around 75 Ohms in the rest of the range. The reduced return loss at the lower operating frequency at 5 GHz can be attributed to the very

low impedance of approximately 25 Ohms shown in the plot.

Figure 10. 3mm side length impedance plot.

Figure 9. 3mm side length return loss.

Figure 11. 9mm modified loop return loss.

As the length of the adjacent side is increased from 6mm to 9mm the effects are more as noticeable seen in Figure 11. The return loss at 10 GHz drops slightly down to 20 dB but a new resonance becomes apparent around 8 GHz, as well as one around 13 GHz. While the characteristics are not quite as good as other configurations, this design does show four resonant bands. As the impedance plot in Figure 12 shows, each of the operating frequencies has an impedance between 50 and 75 Ohms, with the more efficient ones nearer to 50 Ohms as expected.

V. Conclusion

Rectangular loop antennas by itself has good controllable operating parameters. Its major deficiency is the large amount of space required. This problem is even more evident when used in a printed strip configuration. When a reflector strip in run through the loop in order to reduce the space required for the antenna, the result is a new antenna that operates comparably to the original. This configuration has shown promise for operation well in multiple bands as well as with excellent return loss depending upon the variation of its size. Adjustment for the number of operating bands, level of return loss, and bandwidth are easily achievable by simply modifying the physical parameters such as length of the two sides.

Figure 12. 9mm modified loop impedance plot.

Biographies:

Matthew J. Inman received his BSEE in 2000 from The University of Mississippi, and is currently pursuing graduate studies in electromagnetics.

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Atef Z. Elsherbeni received an honor B.Sc. degree in Electronics and Communications, an honor B.Sc. degree in Applied Physics, and a M.Eng. degree in Electrical Engineering, all from Cairo University, Cairo, Egypt, in 1976, 1979, and 1982, respectively, and a Ph.D. degree in Electrical Engineering from Manitoba University, Winnipeg, Manitoba, Canada, in 1987. He was a Research Assistant with the Faculty of Engineering at Cairo University from 1976 to 1982, and from 1983 to 1986 at the Electrical Engineering Department, Manitoba University. He was a part time Software and System Design Engineer from March 1980 to December 1982 at the Automated Data System Center, Cairo, Egypt. From January to August 1987, he was a Post_Doctoral Fellow at Manitoba University. Dr. Elsherbeni joined the faculty at the University of Mississippi in August 1987 as an Assistant Professor of Electrical Engineering. He advanced to the rank of Associate Professor on July 1991, and to the rank of Professor on July 1997. He spent his first sabbatical term in 1996 at the Electrical Engineering Department, University of California at Los Angeles (UCLA).

Dr. Elsherbeni received the 2002 IEEE Region 3 Outstanding Engineering Educator Award., the 2001 Applied Computational Electromagnetic Society (ACES) Exemplary Service Award for leadership and contributions as Electronic Publishing managing Editor 1999-2001, the 2001 Researcher/Scholar of the year award in the Department of Electrical Engineering, The University of Mississippi, and the 1996 Outstanding Engineering Educator of the IEEE Memphis Section. His professional interests include scattering and diffraction of electromagnetic waves, numerical techniques, antennas, remote sensing, and computer applications for electromagnetic education. He has published 55 technical journal articles and 10 book chapters on applied electromagnetics, antenna design, and microwave subjects, and presented over 160 papers at professional conferences. Dr. Elsherbeni is a senior member of the Institute of Electrical and Electronics Engineers (IEEE). He is the editor in-chief for the Applied Computational Electromagnetic Society (ACES) Journal and the electronic publishing managing editor of ACES. His honorary memberships include the Electromagnetics Academy and the Scientific Sigma Xi Society. He serves on the editorial board of the Book Series on Progress in Electromagnetic Research, the Electromagnetic Waves and Applications Journal, and the Computer Applications in Engineering Education Journal. He is the Chairman of the Educational Activity Committee for the IEEE Region 3 Section. Dr. Elsherbenis home page can be found at http://www.ee.olemiss.edu/~atef and his email address is Elsherbeni@ieee.org.

Charles E. Smith was born in Clayton, AL, on June 8, 1934. He received the B.E.E., M.S., and Ph.D. degrees from Auburn University, Auburn, AL, in 1959, 1963, and 1968, respectively. While pursuing his advanced degrees from 1959 to 1968, he was employed as a Research Assistant with Auburn University Research Foundation. In late 1968, he accepted the position of Assistant Professor of Electrical Engineering with The University of Mississippi, University,

MS, and he advanced to the rank of Associate Professor in 1969. He was appointed Chairman of the Department of Electrical Engineering in 1975, and he is currently Professor and Chair of this department. He has directed and is heavily involved in the development of The University's current circuits, electronics, HF and microwave, computer-aided-design, and digital systems courses and laboratories. His main areas of interest are related to the application of electromagnetic theory to microwave circuits, antennas, measurements, RF and wireless systems, radar, digital and analog electronics, and computer-aided design. His recent research has been on the application of numerical techniques to microstrip transmission lines, antenna measurements in lossy media, measurement of electrical properties of materials, CAD in microwave circuits, radar designing, and data acquisition using network analyzers.

Dr. Smith has published widely in these areas and has over 200 total publications including journal papers, technical reports, book chapters, and paper presentations. He has advised, or co-advised, 46 M.S. thesis and PH.D dissertations, and has received 6 awards for outstanding teaching and scholarship at The University of Mississippi. He is a Life Senior Member of the IEEE and is a member of the IEEE Antennas and Propagation Society, IEEE Microwave Theory and Techniques Society, IEEE Education Society, American Society of Engineering Education, Phi Kappa Phi, Eta Kappa Nu, Tau Beta Pi, and Sigma Xi.

Kai-Fong Lee received the B.Sc. and M.Sc. degrees from Queens University, Kingston, Ontario, Canada, in 1961 and 1963 respectively, and the Ph.D. degree from Cornell University, Ithaca, N. Y. in 1966, all in electrical engineering. He held research appointments at University of California, San Diego (1966-l967), National Center for Atmospheric Research (1968-1969), National Oceanic and Atmospheric Administration (1972-1973), University of California, Los Angeles (summer 1975), and NASA (summers of 1986, 1987). He was Assistant/Associate Professor at The Catholic University of America, Washington, D.C. (1967-1972), Lecturer/Senior Lecturer/Reader at The Chinese University of Hong Kong (1973-1984), and Professor at The University of Akron, OH (1985-1988). He was the Founding Head of the Department of Electronic Engineering, City University of Hong Kong (1984-1985). He served as Chairman and Professor of the Department of Electrical Engineering, The University of Toledo, OH, from 1988-1995. He was Chairman and LaPierre Professor of the Department of Electrical Engineering, University of Missouri-Columbia from 1996 to 2000. He has been Dean of the School of Engineering at The University of Mississippi since January 2001.

Dr. Lees publications include a textbook, Principles of Antenna Theory (Wiley, 1984), an edited book, Advances in Microstrip and Printed Antennas (Wiley, 1997), several book chapters on microstrip antennas, 141 journal articles, and 116 conference papers. He worked on plasma waves and instabilities from 1965 to 1980 and on antennas since 1981. He is a Fellow of the Institute of Electrical and Electronics Engineers (IEEE) and a Fellow of the Institution of Electrical Engineers (IEE).

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