WO1999062137A1

WO1999062137A1 - Multifunction compact planar antenna with planar graded index superstrate lens

Info

Publication number: WO1999062137A1
Application number: PCT/US1999/011591
Authority: WO
Inventors: Linda P. B. Katehi; Kamal Sarabandi
Original assignee: The Regents Of The University Of Michigan
Priority date: 1998-05-26
Filing date: 1999-05-26
Publication date: 1999-12-02
Also published as: AU4817899A

Abstract

The present invention relates to the use of a planar graded index superstrate lens in combination with a planar antenna to reduce the antenna size without sacrificing its radiation efficiency. By combining this type of planar superstrate lens with slot-type antennas, it is possible to achieve multifunction, broadband, compact, planar antennas for a variety of mobile wireless communications applications.

Description

MULTIFUNCTION COMPACT PLANAR ANTENNA WITH PLANAR GRADED INDEX SUPERSTRATE LENS

FIELD OF THE INVENTION The present invention relates to a multifunction, broadband, compact, planar vehicular antenna having small, planar, low-loss, and very high index of refraction dielectric lenses.

BACKGROUND OF THE INVENTION Current mobile wireless communications include navigation (global positioning system (GPS) ) , cellular telephone, personal communications services (PCS) such as pager systems, and convenience features. Connection to a ground station or satellite is accomplished through antennas, which must be broadband yet compact in size and small in weight. In addition, the antennas must be low in cost. The cost of implementing multiple antennas to address multiple communication needs becomes prohibitive due to increasing cost . Current methodology is not able to provide solutions to these demands due to a lack of design rules and the complex nature of the interactions involved .

The design of multifunction antennas for military and commercial mobile distribution networks tends to pose major challenges with regard to the antenna size, radiation efficiency, fabrication costs and the like. Broadband, high-performance, low-cost, high data rate systems will be a major technological challenge for the approaching "communication" era of the year 2000 and beyond. The ability to receive and transmit vast amounts of information in video, audio or data form from any ground, satellite, mobile or airborne location to a large number of users requires more than circuit optimization methods applied to existing technology. Current military wireless communications include radio (HF/UHF/VHF) , in addition to cellular telephone, GPS and PCS. The systems operate over a wide frequency spectrum and require some form of antenna or antenna array system to perform each wireless service. Thus, the antennas used for connection to a ground station or satellite must be broadband, compact in size and conformal for improving survivability, and yet must be low in cost. Current technology faces substantial barriers in developing a multifunction antenna, which can service a variety of systems . External antennas exhibit large size which leads to compromised vehicle performance because of unnecessary aerodynamic drag, reduced vehicle security and survivability, and high cost. Interior antennas are sensitive to EMC, increase the electromagnetic radiation in the operator's environment, and exhibit low performance, thus resulting in even higher electromagnetic power requirements. Besides military applications, commercial vehicle communications have additional requirements including styling, very low cost, protection against vandalism, minimal electromagnetic radiation in the operator's environment and ability to accommodate convenience features and other planned utilities. Planar antenna technology could address many of these issues but imposes major modifications on the vehicle structure, which may be incompatible with current U.S. vehicle safety standards. Furthermore, low efficiency demonstrated by conventional planar antennas requires higher power levels, which increase the cost of generation and adversely affect EMC issues.

Planar antennas of microstrip or aperture type have been used for a variety of applications primarily due to simplicity, conformability, low manufacturing cost and enormous availability of design and analysis software. In the past, it has been known to use heterogeneous wafers which resulted in hybrid integration and high development cost. As frequency increases, however, this approach becomes increasingly difficult and costs are driven intolerably high. Compact circuit designs are typically achieved in high index materials, which is in direct conflict with the desired low index substrates imposed by antenna performance requirements . It is widely known that planar antenna designs on high index materials show significantly degraded performance due to the pronounced excitation of surface waves . The electromagnetic wave is reflected by dielectric interfaces and is eventually trapped in the substrate in the form of surface waves. These waves carry a large amount of electromagnetic power along the interface and significantly reduce the radiated power. The power carried by the excited surface waves is a function of the substrate characteristics and increases with increased index of refraction. Consequently, an antenna printed on a high- index material has low efficiency, narrow bandwidth, degraded radiation patterns and undesired coupling between the various elements in array configurations. Several experimental approaches have been put forth to resolve the excitation of substrate modes in these types of materials either by use of a substrate-superstrate configuration, physical substrate alterations, or by use of a spherical lens placed on the antenna substrate. In all cases, the radiation efficiency is increased and antenna patterns are improved considerably as a result of the elimination of the surface wave propagation. However, all implementations have either resulted in non-monolithic designs or have been characterized by large volume and intolerably high cost .

SUMMARY OF THE INVENTION It is desirable in the present invention to provide a superstrate dielectric material lens, which has the capability of reducing antenna size without sacrificing efficiency and thereby allows for compactness and many consolidation options. Furthermore, it is desirable in the present invention to achieve lensing properties by producing simultaneous permittivity grades in either or both radial and axial directions . The resulting graded index superstrate lens according to the present invention provides high radiation efficiency through proper matching with the free space and suppression of substrate surface waves. In addition, the present invention reduces the overall antenna size due to the presence of high permittivity regions in the vicinity of components, which would otherwise have normally large dimensions .

Antennas according to the present invention are preferably of aperture type and are printed on small, planar, low- loss, and very high effective index of refraction dielectric superstrate lenses. According to the present invention, the lens has a graded index of refraction in either or both the radial and axial directions. One approach is to metallize one surface of the superstrate lens to provide antenna and coupling circuit shapes. This process is known to those skilled in the art as disclosed in U.S. Pat. No. 5,268,332 and U.S. Pat. No. 4,448,805, which are incorporated by reference herein. Due to the lensing action of the superstrate, surface waves associated with traditional planar antennas printed on high index materials are suppressed leading to antenna efficiencies approaching between 80% to 90%. By way of example and not limitation, a series of chemically engineered low-loss variable permittivity composites have been formed from a moldable, thermally formed polymer at General Motors Research and Development Center. Suitable composite materials are known to those skilled in the art of design and fabrication of composite materials as disclosed in issued U.S. Pat. Nos . 5,693,429; 5,635,434; 5,635,433; 5,486,491; 5,154,973; 5,601,748; 5,512,196; and 5,497,129, which are incorporated by reference herein. It has been demonstrated that these materials can be formed with permittivity ranging from 2 to 100 with very low loss tangents at frequencies less than two gigahertz (< 2 GHz) . These materials may be either thermally molded or machined to net shape. These materials, or suitable substitutes, may be used to produce predetermined index profiles in the composite materials for use as a planar lens for a multifunction, broadband compact antenna.

The present invention provides a class of cost-effective, efficient and compact mobile antennas printed on small, planar, low- loss, and very high index of refraction superstrate.

Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:

Figures 1A and IB are simplified schematic views of an annular slot antenna printed on a planar superstrate lens according to the present invention;

Figure 2 is a schematic cross section view of a graded index superstrate lens on a slot antenna according to the present invention, where darker areas of the lens region represent higher index of refraction;

Figure 3 is a schematic view of a dual polarized annular slot antenna and its microstrip feed structure suitable for circular polarization;

Figures 4, 5, and 6 are theoretical structures and a graph illustrating expected radiation patterns of two radially and axially graded lens antennas measured at 1.5 GHz for the present invention illustrated in Figures 1A and IB; and

Figures 7A, 7B, 7C, and 7D are theoretical graphs illustrating expected front-to-back ratio versus superstrate lens radius and thickness for the present invention illustrated in Figures 1A and IB.

DESCRIPTION OF THE PREFERRED EMBODIMENT Planar antennas 10 printed on high index substrates according to the present invention provide an integrated monolithic circuit layout to enhance antenna performance .

According to the present invention, an appropriate low- loss dielectric material is used for fabrication of a planar superstrate lens 12, which has the capability of reducing the required antenna size, eliminating surface waves for increased performance, increasing bandwidth and providing antenna patterns, which are very specific mobile wireless functions. Conventional lenses have been used in antenna applications for performance improvement with specific emphasis on efficiency and gain. These spherical lenses are typically machined from silicon (Si) or other materials and are placed on top of the planar antenna with the focal plane at the phase center of the antenna or array. In almost all cases the lens carries a matching layer on the surface in order to minimize unwanted reflections. Ray optics analysis shows that the spherical surface of the lens is responsible for the elimination of the surface waves while at the same time columnizes the pattern and increases efficiency by focusing all the rays which are initiating at the antenna layer in the same direction. One of the requirements of the spherical lens is that the field radiated by the antenna reaches the surface of the lens as a plane wave requiring that the diameter of the lens has to be of the order of a few wavelengths in the material. This is the constraint, which results in large lens size and non-conformal shapes. For the development of mobile antennas, lens columnization through the use of a spherical lens is undesirable due to the need for a broad radiation pattern for successful off-the-horizon reception. At the same time, the ability to eliminate surface waves and reduce surface standing waves in the lens region is critical for high efficiency. According to the present invention, a planar lens 12 formed from graded materials can be formed in a small size and provide antenna efficiency by eliminating undesired substrate modes. As shown in Figure 1, the lens 12 according to the present invention will have a radially graded profile which will allow for the elimination of surface waves as shown in the physical ray optics illustrated in Figure 2. The present invention provides a common-aperture, multi-function, conformal, mobile antenna 10 with small size, high-efficiency (>85%) , wide bandwidth, ability to receive/transmit linear and circular polarization, and the ability to operate with a variety of personal communication systems.

Preferably, the antenna 10 is of planar type with or without a metallized surface on the synthesized superstrate. The planar geometry of the antenna 10 provides conformability, polarization characteristic and broad bandwidth. The antenna 10 according to the present invention is planar, conformal and has high radiation efficiency requiring less operating power by directing most of the energy to one side of the antenna 10. In addition, the antenna 10 according to the present invention is smaller than conventional counterparts and due to planar technology can integrate several antennas into one aperture and prevent the cluttering of protruding antennas. Furthermore, the antenna 10 according to the present invention is wide-band, i.e. capable of transmitting or receiving a wide range of frequencies and can be used for diverse communications services. The antenna 10 is based on printed slot antenna technology combined with topology and graded-index planar lenses 12. The slot design provides the conformity, wide-bandwidth, and multi-functionality. The lens design gives rise to the small size, high efficiency and unidirectionality. The desired radiation efficiency, size and shape of the antenna structure impose stringent constraints on optimization of the antenna 10 according to the present invention.

According to the present invention, it is necessary to identify radiating structures that meet the polarization, bandwidth, and radiation pattern requirements for each specific application the antenna 10 is designed for, and identification of the optimum design parameters, such as the dimension of the radiating elements, dielectric constant of the substrates and dielectric profile of the superstrate lens 12 are obtained to maximize radiation efficiency and at the same time minimize the dimensions of the antenna structure. The aesthetic features desired limit the choice of antenna configuration to planar structures. The planar antenna structure provides ease of manufacturing using printed circuit technology. For purposes of illustration of the present invention, two specific examples of multi-functional compact antenna (MCA) applications are described using GPS and cellular phone antennas. Antennas for personal communications are often electrically small. The frequency of operation is at 825 MHz or 890 MHz with vertical polarization. An appropriate antenna 10 for this type of application should have an omni-directional radiation pattern in the azimuthal plane. However, its radiation pattern in the vertical plane can be non-uniform as long as the three-dB beamwidth of the vertical-plane pattern is more than 60 in elevation. To be compatible with the planar configuration requirement, an annular slot antenna 10 provides radiation characteristics identical with those of a complementary wire loop with electric and magnetic fields interchanged. At the resonant frequency the perimeter of the loop is one wavelength. Here the wavelength is measured within the high dielectric medium of the superstrate lens 12 and therefore the physical size of the loop is much smaller than the free-space wavelength. Another attractive feature of a slot antenna 10 according to the present invention is that the ground plane 16 can be shared among all apertures 14 of the antennas, thereby allowing further reduction in size. For example, a GPS antenna to be described next can be placed within the annular slot 14. The dielectric lens 12 according to the present invention is overlaid on the slot antenna 10. A full-wave numerical analysis can be carried out to characterize the electrical and radiation performance of the resulting antenna according to the present invention. GPS satellites transmit their coded ranging information at two L-band frequencies (L_x = 1.227 GHz and L₂ = 1.575 GHz) each having about 20 MHz bandwidth. GPS receivers should be capable of operating at one or both frequencies. Accurate GPS units use information on both channels to characterize the electrical phase delay that the waves experience as the waves propagate through the ionosphere. To reduce the effect of the ionosphere on propagation and to make the signal reception independent of the antenna orientation, the GPS signals are transmitted with circular polarization. The receiving antenna of a GPS system should provide near hemispherical coverage as satellite reception occurs from near horizon radiation. Most single channel GPS receivers operating at L_x use off-center-feed microstrip patch antenna with a typical coverage of 160 degrees. To design a general purpose compact GPS antenna, a bandwidth of the order of 350 MHz (or 25%) is needed. High bandwidth performance cannot be achieved from resonant planar antennas such as a microstrip patch. The present invention resorts to a slot aperture type antenna, where construction of broadband antennas with high frequency independent configurations are possible. The ring slot antenna 10 illustrated in Figure 3 with two orthogonal feed 18, for example, provides circular polarization, broad radiation patterns, and has a wide bandwidth. Figure 3 is a schematic view of a dual polarized annular slot antenna 10 with lens 12, ring slot 14 in ground plate 16 and microstrip feed parts 18 with 90 degrees phase shifter 20 suitable for circular polarization. Aperture antennas typically radiate on both sides of the ground plane. The present invention provides a planar dielectric lens 12 with tapered profile to direct the radiation into the desired half-space.

Dimensions of the slots 14 and the dielectric superstrate lens 12 together with its permittivity profile can be used as free parameters to achieve the desired performance. According to the present invention, it is believed that optimization of these parameters will result in a highly non-linear and complex relationship between parameters. In these situations, gradient-based optimization methods usually converge to a weak local extreme, thus a more efficient optimization process is incorporated into the present invention. Stochastic algorithms such as simulated annealing and genetic algorithms offer an alternative for the traditional gradient based optimization methods where the dimension of parameter space is large and/or the objective function is non-differentiable .

The performance of the antenna circuits can be measured in terms of scattering parameters versus frequency using a 0.5 to 2.5 GHz measurement station including an HP vector network analyzer and antenna mount and calibration fixtures. By way of example and not limitation, it is believed that a graded index superstrate lens having a gradient index of refraction as indicated in the plan and cross-sectional views attached as Figure 1A, IB and Figure 2 can be produced using materials and methods known to those skilled in the art as disclosed in issued U.S. Patent Nos . 5,693,429; 5,635,434; 5,635,433; 5,601,748; 5,512,196; 5,497,129; 5,486,491; 5,268,332; 5,154,973; and 4,448,805, which are incorporated by reference herein in their entirety. Figures 1A and IB illustrate a simplified schematic view of an antenna 10 according to the present invention having a ground plate 16 and slot ring 14 printed on a planar superstrate lens 12. Figure 2 illustrates a schematic cross sectional view of a graded index superstrate lens 12 on a slot antenna 10 with slot 14, where darker areas (heavier stippling) of the lens region represent higher index of refraction. The ground plate 16 and lens 12 are sandwiched between matching layers L. It is believed that measurements of the performance of such a lens would result in the performance characteristics similar to those illustrated in the graphs of radiation pattern and front-to-back ratio parameter theoretically developed and illustrated in Figures 4, 5, 7A, 7B, 7C and 7D.

Figure 4 is an axially graded lens antenna 10 according to the present invention, where a first region 22 has an index of refraction equal to 8 extending along an axial distance equal to 0.124 times the wave length (WL) from the ground plate 16 and aperture 14 and a second region 24 has an index of refraction equal to 4 extending along an axial distance equal to 0.1 times the wave length (WL) from the first region 22. The graded lens 12 has a diameter of 0.436 times the wave length (WL) . Figure 5 shows a theoretical graph illustrating the expected radiation pattern of the axially graded lens antenna 10 shown in Figure 4 measured at 1.5 GHz. Figure 5 also shows the theoretical graph illustrating the expected radiation pattern of a radially graded lens antenna 10 according to the present invention as illustrated in Figure 6.

Figure 6 is a radially graded lens antenna 10 according to the present invention, where a first region 22 has an index of refraction equal to 8 extending along a radial distance equal to 0.125 (0.25 diametrical length) times the wave length (WL) , and a second region 24 has an index of refraction equal to 4 extending along a radial distance equal to 0.1 times the wave length (WL) from the first region 22. The graded lens 12 has an overall diameter of 0.45 times the wave length (WL) . Figure 5 shows a theoretical graph illustrating the expected radiation pattern of the radially graded lens antenna 10 shown in Figure 6 measured at 1.5 GHz. Figure 5 is a polar coordinate representation of the radiation pattern having radially extending axis at 0 degrees (line 26) through 180 degrees (line 28) in both the clockwise direction and the counterclockwise direction (in 30 degree increments) . Circular axis are illustrated with the outermost circular reference line 30 at 0 decibels (dB) , the next circular reference line 32 being -20 decibels (dB) , and the inner circular reference line 34 being -40 decibels (dB) . The radiation pattern is illustrated in heavy line 36 superimposed over the polar coordinate axis . Referring now to Figure 7A, a theoretical graph is provided illustrating an expected antenna front-to- back ratio (FBR) versus superstrate lens diameter and thickness. Figure 7A is a theoretical graph illustrating expected antenna front-to-back ratio (FBR) in decibels (dB) along the vertical axis between 0 decibels and 25 decibels versus superstrate lens diameter divided by wave length (WL) inside the lens along the horizontal axis between values of 0.53 and 0.88. The graph illustrates six lines corresponding to height divided by wave length (WL) values between 0.04 and 0.26; namely values of 0.04,

0.084, 0.128, 0.172, 0.216, and 0.26 identified as graph lines 38, 40, 42, 44, 46, and 48 respectively. The theoretical graph of Fig. 7A is for e_r = 2.

Referring now to Figure 7B, a theoretical graph is provided illustrating expected antenna front-to-back ratio (FBR) in decibels (dB) along the vertical axis from 0 decibels (dB) to 25 decibels (dB) versus superstrate lens diameter divided by wave length (WL) inside the lens along the horizontal axis between values of 0.75 and 1.24. The graph illustrates height divided by wave length (WL) inside the lens values between 0.06 and 0.43; namely values of 0.06, 0.122, 0.183, 0.245, 0.307, 0.368, and 0.43 identified as graph lines 50, 52, 54, 56, 58, 60 and 62 respectfully. Figure 7B is for e_r = 4.

Figure 7C illustrates a theoretical graph of expected antenna front-to-back ratio (FBR) in decibels (dB) along the vertical axis from 0 decibels (dB) to 25 decibels (dB) versus superstrate lens diameter divided by wave length (WL) inside the lens for values from 0.91 to 1.52 along the horizontal axis. Figure 7C illustrates height divided by wave length (WL) inside the lens values between 0.08 and 0.53; namely values of 0.08, 0.155,

0.23, 0.305, 0.38, 0.455, and 0.53 identified as graph lines labeled 64, 66, 68, 70, 72, 74, and 76 respectfully. The graph of Figure 7C is for e_r = 6.

Figure 7D illustrates a theoretical graph of expected antenna front-to-back ratio (FBR) in decibels (dB) along the vertical axis from 0 decibels (dB) to 25 decibels (dB) versus superstrate lens diameter divided by wave length (WL) inside the lens values between 1.06 and 1.76 along the horizontal axis. Figure 7D illustrates height divided by wave length (WL) inside the lens values between 0.09 and 0.61; namely values of 0.09, 0.177, 0.263, 0.35, 0.437, 0.523, and 0.61 identified as graph lines 78, 80, 82, 84, 86, 88, and 90 respectfully. Figure 7D is for values corresponding to e_r = 8. For purposes of this invention, the term "high radiation efficiency" shall mean greater than 80%, and preferably greater than 85%. The term "high permittivity" shall mean greater than 8, and preferably greater than 36. The term "small planar lens" shall mean in the range of approximately 1 square inch to 4 square inches, inclusive and preferably in the range of approximately 1.5 square inches. The term "lens tangent loss" shall mean a loss of less than 0.01, and preferably less than 0.001. The term "wide bandwidth per frequency" shall mean a bandwidth in the range of approximately 20% and 100% of the base frequency, inclusive and preferably more than 25% of the base frequency. The term "wide range of frequencies" shall mean a range of frequencies between 100 MHz and 1.5 GHz inclusive.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.

Claims

What is claimed is:

1. An antenna and lens combination comprising: a planar antenna; and a planar, superstrate lens having a varying index of refraction profile in at least one direction, the planar, superstrate lens overlaid in combination with the planar antenna for improving radiation efficiency and decreasing antenna size.

2. The combination of claim 1 further comprising: the lens having an index of refraction profile varying in at least two directions.

3. The combination of claim 1 further comprising: the lens having an index of refraction profile varying along a length, a width and a thickness of the planar, superstrate lens.

4. The combination of claim 1 further comprising: the lens formed of dielectric material.

5. The combination of claim 1 further comprising: the lens having permittivity grades varying in at least one of a radial and axial direction.

6. The combination of claim 5 further comprising: the lens having high permittivity in at least regions in a vicinity of the planar antenna.

7. The combination of claim 1 further comprising: the lens antenna having high radiation efficiency in the range of at least 80%.

8. The combination of claim 1 further comprising: the antenna having an aperture-type configuration printed on the lens; and the superstrate lens formed of a dielectric material and having a small planar area, low-los and very high effective index of refraction.

9. The combination of claim 1 further comprising: the antenna having a planar-type configuration and form with a metallized surface on the superstrate lens formed of a synthesized material .

10. The combination of claim 1 further comprising: the lens integrated with the antenna and conformable to a predetermined non-planar shape.

11. The combination of claim 1 further comprising: the antenna having a printed slot configuration for conformity, wide bandwidth and multi-functionality; and the planar lens having a graded index for small size, high efficiency and unidirectionality .

12. The combination of claim 1 further comprising: the antenna having a printed circuit planar configuration.

13. The combination of claim 1 further comprising: the antenna having a shared common ground plane for all apertures formed therein.

14. The combination of claim 1 further comprising: the antenna having a slot configuration; and the lens formed of dielectric material and overlaid on the slot configuration antenna.

15. The combination of claim 1 further comprising: the planar lens formed of dielectric material having a tapered permittivity profile for directing radiation into a desired half-space with respect to the lens .

16. The combination of claim 1 further comprising: the lens configuration determined as a result of a stochastic algorithm optimization procedure.

17. The combination of claim 1 further comprising: the lens configuration determined as a result of a process in which a sequence of values is drawn from a corresponding sequence of jointly distributed random variables .

18. The combination of claim 1 further comprising: the planar antenna having a patch antenna configuration.

19. The combination of claim 1 further comprising: the planar antenna having an aperture type configuration.