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Microwave Journal 1996.9

A Survey of Broadband Microstrip Patch Antennas

Microstrip patch antennas (MPA) have received a lot of attention in the last few years. However, the antennas inherent narrow bandwidth is one of their major drawbacks. This is one of the problems that researchers around the world have been trying to overcome. This paper describes the main techniques produced to increase the bandwidth of patch antennas. Likewise, a survey of the novel designs intended to enhance the bandwidth of patch antennas that have been developed since the last publication concerning MPAs is presented.

David Sanchez-Hernandez Departmento de Communicaciones, Universidad Politécnica de Valencia Valencia, Spain Ian D. Robertson MMIC Research Team, University of London London, UK The microstrip antenna has now reached maturity, wherein only a few mysteries about its behavior are still undiscovered. The invention of such antennas has been attributed to several authors, but it was certainly placed in the 1960s with the first works published by Deschamps, Greig and Engleman, and Lewin, among others. But it was not before 1970 when the research publications started to flow with the appearance of the first design equations. The most important workshop was held at Las Cruces, NM in 1979. It was also at that time when the first books on microstrip antennas were printed. As an example, Bahl and Bhartia or James, Hall and Wood wrote two classical guidebooks that are still in use. An exhaustive handbook of microstrip antennas has been published by James and Hall. One of the major disadvantages of microstrip antennas is their inherent narrow bandwidth. Throughout the years, authors have dedicated their investigations to creating new designs or variations to the original antenna that, to some extent, produce either wider bandwidths or multiple-frequency operation in a single element. However, most of these innovations bear disadvantages related to the size, height or overall volume of the single element, and the improvement in bandwidth suffers usually from a degradation of the other characteristics. It is the purpose of this paper to introduce the general techniques produced to improve the narrow bandwidth characteristic of patch antennas, and to provide a good reference for the variety of broadband elements developed in the last years.

Bandwidth Enhancement Techniques

In MPAs the pattern bandwidth is usually many times larger than the impedance bandwidth and, therefore, the discussion of bandwidth in this paper will concentrate on impedance rather than patterns. For a single element operating at the fundamental lowest mode, the typical bandwidth is from less than one to several percent for thin substrates. With the original patch antenna, three ways of increasing the bandwidth exist. The first technique is simply increasing the thickness of the substrate. However, this technique introduces various problems. A thicker substrate will support surface waves, which will deteriorate the radiation patterns as well as reduce the radiation efficiency. Also, problems with the feeding technique of the antenna appear. Additionally, depending upon the z-direction, higher order modes may arise, introducing further distortions in the pattern and impedance characteristics. The second technique to increase bandwidth is decreasing the relative permittivity, which has an obvious limitation based on size. The third method is by means of a wideband matching network. However, this concept was not feasible until an impedance-matching technique was proposed. This first impedance-matching approach was analytical. The real frequency matching technique and the simplified real frequency technique are improved versions that followed. Yet, the inherent complexity of these techniques is apparent. Seven main techniques have been

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found to enhance the bandwidth of patch antennas, either by obtaining a wider bandwidth or performing a dual-band operation.

Multilayer Structures

The idea of stacking two elements came soon after the original microstrip antenna was proposed. Thus, a stacked circular antenna was constructed using two discs etched on different boards, feeding the lower disc by a coaxial connector through the ground plane, as shown in Figure 1. Two distinct resonances were observed. It was found that the lower resonant frequency was relatively steady over a range of different diameters for the upper conductor, whereas the second resonance was highly dependent on those diameters. However, up until 1979, making a precise design was a difficult task.

Fig. 1 A stacked circular disc antenna.

It was not until 1986 that a radioelectric model was presented to explain both the technique and the influence of the director size upon the quadratic patch bandwidth. Since then, many theoretical studies have been published; from experimental results with bandwidths up to 26 percent of the center frequency f0 (SWR = 2) using either contiguous stacked elements, to a comparison of the coaxial probe and slot-coupling feeds. A study of multilayer MPAs with radiating elements of various geometries was also realized, revealing some interesting features. To obtain a lower level of cross-polar components, it is necessary to avoid the equilateral triangular shape. A larger bandwidth may be achieved by varying the value of the two heights and adjusting. For offset elements, a shift in the OX and OY directions has a significant impact on both input impedance and radiation patterns. More recently, a similar study was also completed where the influence of the parasitic element''s size, shape and position on input impedance was investigated. In the multilayer configuration it was found that, although offsets enable a better adjustment of coupling effects and hence a wider bandwidth, structural asymmetry translates as some beam dispersion in the E-plane. Consequently, two symmetrically offset parasitic elements were needed to avoid pattern asymmetry. One of the main applications for these dual-band multilayer structures is the global positioning systems. However, these structures create problems in the design and manufacturing stages along with a considerable increase in height and, therefore, are not well suited for mass manufacturing.

Parasitic Elements Coupled to the Main Patch

Another bandwidth enhancement technique for microstrip antennas is to incorporate parasitic coplanar metallic strips coupled to the main patch. The idea appeared as early as 1978 with a broadband microstrip resonator antenna, together with a design method. However, this antenna was more like an array and, subsequently, a novel broadband microstrip antenna was introduced using additional resonators that were gap coupled to the radiating edges of the main patch. When the resonant lengths of the two coupled patches were different, two separate loops appeared in the input impedance locus, and the gap widths S1 and S2 could be changed to yield a broader bandwidth; up to 5.1 times that of a single rectangular patch antenna. The microstrip antenna with nonradiating edges and additional resonators gave as much as four times the bandwidth of the rectangular patch antenna, whereas for the four-edge gap-coupled multiple resonator the figure was 6.7 times the bandwidth. Different versions of these antennas have also been studied, such as triangular resonators, one-parasitic patch22 or a broadband gap-coupled microstrip antenna. Another example is the lm/4 with a short circuit along the edge, which was capacitive coupled with the radiating edge of a driven patch. This geometry avoids the interferometer action of the two edge sources and the coupling between them. This variation achieved 5.35 times the bandwidth of a single element. However, the significant enlargement in size is obvious, and could be a big handicap for phased-array applications. Although the bandwidth is increased substantially, the variations in the radiation pattern with

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frequency make the proposed antenna configuration unsuited for the applications where such variations cannot be tolerated.

Log-periodic and Quasi-log-periodic Structures

Based on the log-periodic idea, a new design method for a wideband array was proposed in 1981.24 In this approach, the resonators are directly fed by microstrip lines in a one-layer structure. In this structure, the low and high frequency limits of the bandwidth are set by the largest and smallest dimensions of the structure, respectively. The quasi-log-periodic antenna was achieved by arraying different narrow bandwidth radiators, each having its own frequency band of operation. The main advantages are the absence of an array effect in the E-plane and the fact that the antenna can be designed for a specified degree of matching by the proper choice of spacing between the resonant frequencies, namely, the log-periodic expansion factor. But a considerable part of the input power reaches the end of the feed line, which was not terminated in an attempt to maintain the high efficiency. The effect of the open circuit was not studied, and other options could improve the efficiency better than the open circuit. Application of the log-periodic principles to an array involves several inherent problems, with the constant substrate thickness as the most apparent difficulty. In this case, the substrate thickness was kept constant, but the traveling-wave effect was not considered in the network analysis. If more than a few elements are used, the feed end will become too long for some elements and the performance will deteriorate. Yet, the increase in size may not justify what this antenna can offer, and the radiation patterns also vary strongly with frequency. The measured bandwidth for an SWR of £ 2.6 was 22 percent, which is an improvement of approximately 10 times. A nine-element completely log-periodic structure was published with a 30 percent bandwidth for an SWR of £ 2.2 and 70 percent efficiency. In this case, the feed line was terminated with a matched load in order to prevent reflections degrading the radiation patterns at the edges of the frequency band. Yet, other studies for a log-periodic array of narrow rectangular microstrip elements indicate that bandwidth at least as great as 50 percent can be achieved. However, this technique suffers from nearly all the disadvantages of the quasi-periodic technique. When this antenna is compared with the stacked-resonator techniques, a bandwidth one and a half times wider than that of multilayer structures, with efficiencies significantly higher than the values obtained with the microstrip spiral, is found. Planar log-spiral antennas have also been developed. This kind of antenna was successfully used in a broadband, low noise superconductor-insulator-superconductor receiver for submillimeter astronomy.

Tuning Stubs and Loads

The reactively loaded patch technique is perhaps the most common method to enhance the bandwidth of microstrip antennas. A practical method for the simultaneous tuning of both the resonant frequency and reflection coefficient of microstrip antennas is the use of two tuning stubs in a coaxially fed patch. It was discovered that the match generally degrades for increasing stub length, which limits the practical tuning range. This problem is overcome by using two tuning stubs positioned on opposite edges of the patch, in line with the coaxial feed point. The patch is then tuned in an iterative manner by systematic trimming of either of the stubs. Thin stubs allow very sensitive tuning over a limited range, while wide stubs increase the range, but with less sensitivity. This technique allows tuning to a specific frequency and reflection coefficient in a few iterations, with the disadvantage that the tuning is only possible from a lower to a higher frequency owing to the destructive trimming technique. The technique does not compensate for variations in the resonant frequency due to temperature dependence of the substrate dielectric constant, and it may not be practical when a radome is used. In the circular patch, stubs have also been applied to produce dual-frequency operation. As the length of a single narrow strip was increased to 3le/4, the element was well matched simultaneously at two different frequencies. However, the radiation patterns at these two frequencies were quite dissimilar and good pattern performance was obtained only when two strips were employed. Again, the overall size is increased. A similar effect can be achieved by

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loading the patch with an adjustable-length short-circuited coaxial stub,31 as shown in Figure 2. The spacing between the bands can be controlled by increasing the characteristic impedance of the loading stub, by increasing its length or by changing the inset of the load position. However, only one stub makes the impedances at the centers of the two bands of operation identical, and the increase in height is considerable.

Fig. 2 A reactively loaded microstrip antenna.

Diode Use

The use of varactor diodes to perform dual-frequency operation is another wideband technique. Two diodes are positioned symmetrically in the patch to minimize the cross-polarization effects, and the relationship between the power and the bias voltage level of the varactor diodes represents a way of tuning the structure. In a previous experiment, a tuning range of 20 percent was achieved with a 10 V bias. The flaws of this technique are the dependence of the resonant frequencies on the position of the diodes and, hence, the lack of versatility, along with difficulties in the manufacturing process and nonlinearity problems in high power applications. Similarly, the effects of an optically controlled PIN diode were incorporated into a model. The parasitic element, shown in Figure 3, increased the gain and performed a dual-frequency operation, and did not disturb the radiation patterns. Yet, the complexity of this technique, although compatible with MMIC structures, is apparent.

Fig. 3 A tunable antenna using an opyically controlled PIN diode.

Shorting Pins

An interesting study of microstrip antennas with frequency agility and polarization diversity using shorting pins was published. By changing the number and location of the posts, the operating frequency can be tuned over a 1.5-to-1 range, and the polarization can be changed from horizontal to vertical, or right-hand or lefthand circular. Tuning ranges in excess of 50 percent are achieved by adding more posts. The radiation patterns are not changed significantly by the shorting posts. One-, two- and multiplepost configurations have been studied. This geometry could be used in MMIC applications where size is important and via holes are produced easily for the shorting pins. However, the design is complicated in MIC applications by the added components, and their precise position is also important. For high frequencies, the patch size is small and it becomes difficult to accommodate the diodes or pins underneath it. These complications multiply when an array based on these elements is designed.

An Adjustable Air Gap between the Substrate and the Ground Plane

A theory on microstrip antennas with an air gap to tune the resonant frequency of the patch antenna was developed. The structure is made of two layers, including the substrate of thickness h and an air region of thickness D. The effective permittivity is evidently reduced, tending toward the free space value e0 as the air thickness increases. This concept was also applied to stacked patches, performing a tunable arrangement with two stacked discs. In this case, the upper air gap has the effect of altering the resonant frequency of the upper resonance, while the lower air gap has more complicated impacts. The air gap does not affect the radiation fields significantly. A variation of this concept, including a lossless matching network and therefore increased bandwidth, has also been introduced. It was discovered that the air gap width cannot be altered arbitrarily, and that there are three regions in which a trade-off between good pattern characteristics and wider bandwidth was somehow different. For D less than 0.14 l0, the patterns show good broadside features, but between 0.14 l0 and 0.31 l0 the bandwidth was reduced drastically and the patterns showed a dip at broadside. For values of D greater than 0.31 l0, the pattern returned to normal and a high gain was achieved

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with a narrow bandwidth. This idea of an air gap can also be used to obtain a dual-band antenna. These antennas can be used in vehicle satellite communications antenna systems, where at least eight percent bandwidth is needed. The disadvantages are evident; the width of the air gap D has to be changed mechanically, and electronic tuning appears to be a rather troublesome idea. Also, the antenna is thicker than other novel designs with the same characteristics.

Aperture-coupled Parallel Resonators and Slotted Patches

A broadband dual-lobed aperture-coupled element employing a full-wavelength-long patch was envisaged. A butterfly-shaped aperture was used to increase the coupling between the patch and the feed network. Yet, the main concept of this technique was significantly improved recently by several authors almost simultaneously. The novel idea was to introduce two slots in the radiating edges of the patch, as shown in Figure 4. These slots, which are etched close to the radiating edges, do not change the first resonance frequency and radiation pattern of the patch significantly while introducing another resonance with similar radiating properties that is strongly dominated by the slot length. When the patch is loaded by the two slots, minor perturbations occur on the TM100 mode because the slots are located on its current minima. On the other hand, the same loading slots interact strongly with the TM300 mode, transforming its current distribution onto that of the dominant mode and expecting similar radiation patterns. The experiments revealed that the dual-frequency operation was possible with two slots or only one slot printed on the patch, and curves of the fh/f1 ratio vs. slot length and slot position are given.41 Dual-frequency operation can also be achieved by employing dual slots beneath a single antenna. However, this variation will introduce the complexity inherent to dual-fed antennas.

Fig. 4 A slotted patch.

The dual-band patch antenna with a spur-line filter technique is another example. In this structure, shown in Figure 5, the energy stored by the resonant structure was determined mainly by the odd mode with fields confined to the vicinity of the conductors and concentrated in the gap between the strips, while the even modes tend to fringe away from the strips. The even mode is more affected by close objects than the odd mode. Hence, it can be concluded that the filter is disturbed only by objects close to the gap between the strips. The coupled lines are situated in the radiating edge of the patch opposite the feed point, forcing the signal to be rejected close enough to the resonant frequency of the filter to obtain a new cavity at that frequency. The measured SWR of the structure showed an overall 7.37 percent bandwidth.

Fig. 5 A dual-band patch antenna with a spur-line filter.

Specially Shaped Patches

A novel circular patch antenna was created. A sectoral slot of the circular patch was removed and shunted by a conducting strip. The schematic design is shown in Figure 6. The slots act as a perturbing element and modify the current distribution of the patch surface. A bandwidth of 1.9 percent was achieved with an SWR of £ 2.

Fig. 6 A modified circular patch schematic.

The bow-tie antenna, shown in Figure 7, is a planar antenna with inherent broadband impedance. This antenna has been used in superconducting tunnel junction and Schottky diode mixers in the frequency range of 94 to 466 GHz, and in linear imaging arrays, plasma diagnostic systems and radio astronomy. Additionally, a logperiodic version of this antenna has also been built and tested. In

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this case, a centralized peak in both E- and H-planes at broadside was found, which makes the log-periodic bow-tie antenna more suitable for substrate-lens receiver systems, resulting in improved coupling efficiencies.

Fig. 7 The bow-tie antenna.

Considerable research has been performed on the folded dipole radiator above a ground plane, finding a 50 to 200 percent bandwidth improvement when compared to patch antennas. The geometry of the improved folded dipole is shown in Figure 8. The experimental data on this dipole showed a 5.5 percent total bandwidth under an SWR of 2. The trapezoidal dipole is another example of a broadband antenna. A broadband two-dimensional array was constructed with 128 trapezoidal dipoles.The highest sidelobe level was below 10 dB, and the SWR results were one octave in frequency (8 to 16 GHz) for an SWR of £ 2.

Fig. 8 The dual-folded dipole.

A new hexagonally shaped MPA was demonstrated experimentally to exhibit greater bandwidth and increased gain over an equivalent rectangular radiator. Only small changes in the far-field radiation pattern were found for the hexagonal patch. The patch is formed by adding triangles along the sides of a rectangular patch, where the extension from the original rectangle is described in terms of a, the apex distance, measured from the side of the original rectangle. It was found that the bandwidth depends upon the apex distance. The increase in the bandwidth with increasing apex distance was believed to be due to an increase in radiation from the sides of the patch. Hence, the quality factor is decreased and cavity losses are increased. A six percent bandwidth was found for a 4 mm apex distance. A novel ring-patch antenna for dual-band operation was proposed. The antenna, shown in Figure 9, consists of two patches with different resonant frequencies. The key to this novel design was the thick, short cylinder at its center, in contrast with the thin, short pin commonly used for patch antennas. The patterns were found to have almost the same response as that of the dipole, with a rotationally symmetric pattern in the horizontal plane that could be applied for a conical beam antenna.

Fig. 9 A ring patch antenna for dual-frequency operation.

Other Techniques

The dichroic antenna, shown in Figure 10, is another interesting method to achieve dual-band operation. Two antennas are superimposed based on the invisibility of the uppermost antenna, constructed from a frequency-selective conducting surface (FSCS). A ripple in the radiation patterns and a deviation of approximately 2 dB in the maximum sidelobe are usually the consequences of the superimposition. Feed connections to the FSCS antenna can also be a significant problem. This FSCS technique has commonly been used in satellite systems. The Cassini deep-space program, recently initiated to explore the Saturnian planet system, is a clear example. In the spacecraft, scheduled for launch in 1997, a multifunction multifrequency reflector antenna system is envisaged. The single-reflector antenna will have a Ka-band cassegrain feed for telecommunication and radio-science experiments, several Ku-band offset linear array feeds, an X-band cassegrain feed for deep-space telecommunications and an S-band focal feed for radio-science experiments. Thus, to accommodate this multifrequency operation, a subreflector with frequency-selective surface capability has been designed.

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Fig. 10 The dichroic antenna.

A peculiar technique is the wedge-shaped microstrip antenna, shown in Figure 11. Unlike ordinary patches, the width-to-height ratio is not constant, and eeff depends on that ratio. To calculate the resonant frequency, the analysis was made equivalent to that of the cut-off wavelength of a parallepipoid waveguide of equal crosssectional dimensions. A considerable improvement in bandwidth was achieved and the resonant frequency varied as the angle of the wedge-shaped antenna changed. Yet, manufacturing complications are predicted easily.

Fig. 11 A wedge-shaped patch antenna.

The Huygens MPA, shown in Figure 12, is an example of combining two elements that have complementary impedances to produce near-constant impedance and unidirectional patterns over a wide band. However, in microstrip technology two different modes of the same structure can be used instead of two separate elements. The microstrip feed line is then located below the ground plane and tapered to provide the necessary impedance at the feedpoint in order to produce the desired excitation of the two modes relative to one another. The impedance response of the antenna was indeed good. When both modes were excited simultaneously, the input SWR was below 2 at all frequencies from 500 to 1165 MHz, corresponding to an impedance bandwidth greater than 2.3. Thus, this antenna is useful for wideband applications where a directional antenna is needed. In fact, two patterns with beams in opposite directions can be obtained simultaneously from the same structure, and the antenna responds to both electric and magnetic fields, which could be useful in diversity receiving systems.

Fig. 12 The Huygens MPA

Another example of an interesting effort was the insertion of metallic strips on a rectangular patch antenna. The structure consists of two layers with low and high dielectric constants. Since the layers are very thin, only the dominant mode exists everywhere in the cavity, with the exception of the metallic strips. The rapidly decaying evanescent modes are confined within a small region near the edge of the inner strips, which separate the region of high dielectric constant from that of low dielectric constant. These strips are allocated symmetrically to ensure symmetric radiation patterns. This arrangement results in two types of field excitation. The fields of lower resonance are highly excited in the high dielectric region and decay exponentially in the low dielectric region. On the other hand, the fields of the higher order mode are strong in the low dielectric region. One problem appears quickly: Since the high dielectric region occupies a smaller volume than the low dielectric material, the lowest order mode results in less radiation efficiency. An experimental study of a new class of broadband microstrip antenna was completed. A pair of microstrip lines loaded with patches and having slightly different dimensions and spacing were developed. The structure is excited from the back using a 50 W coaxial cable and terminated by a matched load at the other end. An SWR of £ 2 was obtained in the frequency range from 8 to 11 GHz, which means a 40 percent bandwidth. Additionally, 19 dB cross-polarization levels were found in the X-band. The disadvantage is that the maximum length required for this antenna was 5 lmax, and the spacing between the radiating rows of patches should be 1.5 lmax.

Conclusion

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This paper presents a survey of the different structures used to accomplish dual-frequency or wideband operation in either a single-element patch antenna or a multi-element scheme. However, a closer study reveals that some of these examples create problems in the design or manufacturing stage, along with an increase in size or a degradation in any of the other characteristics. By introducing slots in the patch, the dual-frequency operation can be achieved in a single-element patch antenna. This dual-frequency operation is not accomplished at the expense of any other feature such as cross polarization or distorted radiation patterns. An extra improvement of four percent in bandwidth can be achieved with slotted patches, which means an overall bandwidth of approximately 7.5 percent. This improvement is much less than the 20 percent possible with multilayer structures or the 50 percent possible with log-periodic arrangements, but no deterioration is observed on any of the other characteristics, such as size, height or radiation patterns. However, in these designs, discrepancies between simulated and measured results have been found. It appears that the differences can be attributed to inaccuracies in the construction process. The theory involved is not completely accurate and some approximations have been taken for granted. However, future research may show that these conjectures might not be so trivial.

Acknowledgment

This work was supported by the EEC under the Training and Mobility of Researchers Programme (IV Framework) and the Engineering and Physical Sciences Research Council.

References

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49. R.C. Compton et al., "Bow-tie Antennas on a Dielectric Half Space: Theory and Experiment," IEEE Transactions on Antennas & Propagation, Vol. 35, No. 6, 1987, pp. 622ñ631. 50. P.H. Siegel, "A Planar Logic-periodic Mixtenna for Millimeter and Submillimeter Wavelengths," 1986 IEEE Microwave Theory and Techniques International Symposium Digest, pp. 649ñ652. 51. G. Dubost, Flat Radiating Dipoles and Applications to Arrays, John Wiley & Sons, Research Studies Press, pp. 24ñ28. 52. A. Nesic et al., "Broadband Two-dimensional Printed Antenna Array," 20th European Microwave Conference Proceedings, 1990, pp. 252ñ256. 53. N.M. Martin and D.W. Griffin, "A New Approach to Microstrip Antenna Bandwidth Determination and Its Application to a Novel Hexagonally Shaped Patch," 1982 IEEE Antennas & Propagation International Symposium Digest, pp. 175ñ178. 54. M. Kuribayashi and N. Goto, "A Dual-frequency Microstrip Antenna," IECE Japan National Convention, 1982, No. 643. 55. R. Mizzoni, "The Cassini High Gain Antenna (HGA): A Survey on Electrical Requirements, Design and Performance," IEE Seminar on Spacecraft Antennas, London, May 1994, pp. 6/1ñ10. 56. R.K. Mishra and S.S. Pattnail, "Resonant Frequency of Wedge-shaped Microstrip Antenna," Electronic Letters, Vol. 26, No. 13, 1990, pp. 912ñ913. 57. C.S. Lee et al., "Dual-frequency Microstrip Antenna with Inserted Strips," 1993 IEEE Antennas & Propagation International Symposium Digest, pp. 1486ñ1489. 58. P.E. Mayes et al., "A Huygens Microstrip Patch Antenna," 1987 IEEE Antennas & Propagation International Symposium Digest, pp. 932ñ935. 59. W.R. Dong and D.L. Sengupta, "An Experiment Study of a New Class of Broadband Microstrip Antennas," 1982 IEEE Antennas & Propagation International Symposium Digest, pp. 289ñ291.

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