WO1998056067A1

WO1998056067A1 - Planar antenna with patch radiators for wide bandwidth and pass band function

Info

Publication number: WO1998056067A1
Application number: PCT/US1998/011734
Authority: WO
Inventors: Antonio Faraone; Quirino Balzano; Oscar Garay
Original assignee: Motorola Inc.
Priority date: 1997-06-06
Filing date: 1998-06-05
Publication date: 1998-12-10
Also published as: GB9902395D0; CN1231071A; GB2331186A; AU8060398A; DE19880947T1; KR20000068078A

Abstract

A microstrip antenna (100) achieves wider bandwidth by using an asymmetric radiating structure (110). The radiating structure (110) supports at least two resonating modes, which are preferably a differential and a common resonating mode. A feed system (130, 135) is coupled to the radiating structure (110) to excite the respective resonating modes at different frequencies to provide a radiating band for communication signals. Preferably, the antenna (100) includes patch radiators (112, 114) of substantially different widths, and a buried microstrip line (130) that simultaneously feeds the patch radiators (112, 114). In one embodiment, the antenna (400) has a multi-mode resonating structure (410) that includes three electromagnetically coupled resonators (412, 414, 416) carried by a dielectric substrate (420). A feed system (430, 435), electromagnetically coupled to the multi-mode resonating structure (410), excites three resonating modes that operate together to produce a pass-band.

Description

PLANAR ANTENNA WITH PATCH RADIATORS FOR WIDE BANDWIDTH AND PASS BAND FUNCTION

TECHNICAL FIELD This invention relates in general to antennas, and more particularly, to planar antennas using patch radiators.

BACKGROUND OF THE INVENTION Planar, microstrip antennas have characteristics often sought for portable communication devices, including advantages in cost, efficiency, size, and weight. However, such antennas generally have a narrow bandwidth which limits applications. Several approaches have been proposed in the art in an effort to widen the bandwidth of such structures. One such approach is described in U.S. Patent Number 5, 572, 222 issued to Mailandt et al. on November 5, 1996, for a Microstrip Patch Antenna Array. Here, a microstrip patch antenna is constructed using an array of spaced-apart patch radiators which are fed by an electromagnetically coupled microstrip line. Generally, with such structures, electromagnetic coupling between radiators is negligible, as it is regarded as a second-order undesired effect. Mailandt's structure is contemplated for use in fixed communication devices. For portable communication devices, size and weight considerations are paramount and such structures may not be suitable. Many other prior art approaches have similar drawbacks. Current trends demand a reduction in size, weight, and cost for portable communication devices. Planar patch antennas could provide a part of the solution if bandwidth concerns are addressed without a significant compromise in size and weight. Moreover, these antennas can provide additional advantages in terms of directivity and efficiency. It is also desirable to provide band pass behavior, with strong rejection of undesired side-band noise, in a cost effective manner. Planar patch antennas could provide a part of the solution if bandwidth concerns are addressed, and more effective band-pass filtering provided. Therefore, a new approach for planar patch antenna with increased bandwidth and band pass behavior is needed. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a patch antenna, in accordance with the present invention.

FIG. 2 is a cross-sectional view of the patch antenna of FIG. 1, in accordance with the present invention.

FIG. 3 is a top plan view of a patch antenna configuration that uses circular polarization, in accordance with the present invention.

FIG. 4 is a top plan view of a planar pass-band antenna, in accordance with the present invention. FIG. 5 is a cross-sectional view of the antenna of FIG. 4.

FIG. 6 is a graph showing experimental results of an antenna made in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention provides for a patch antenna, preferably of planar construction, that achieves a wide bandwidth using an asymmetric radiating structure. The radiating structure supports at least two resonating modes, which are preferably differential and common resonating modes. A feed system is coupled to the radiating structure to excite the respective resonating modes at different frequencies to provide a radiating band for communication signals. In the preferred embodiment, the radiating structure includes a grounded dielectric substrate that carries resonating structures, such as patch radiators, which have substantial electromagnetic coupling. The resonating structures are simultaneously fed to excite differential and common resonating modes which operate with a substantially similar effective dielectric constant. A common resonating mode exists for electromagnetically coupled resonating structures when current simultaneously travels on each resonating structure in substantially the same direction. A differential resonating mode exists for electromagnetically coupled resonating structures when current simultaneously travels on each resonating structure in a substantially opposite direction. The combination of the differential and common resonating modes produces a wide radiating band. The present invention also provides for an antenna, preferably of planar construction, that achieves a wide bandwidth and band-pass filtering using a resonating structure that has a particular geometry and arrangement of elements. The resonating structure supports at least three resonating modes that operate together to produce a pass-band, i.e., a continuous radiating band delimited by substantial radiated field cancellation at spaced apart cut-off frequencies. A feed system is coupled to the radiating structure to excite the resonating modes to provide a radiating band for communication signals, and to produce opposing currents that cause a destructive superposition of radiated fields at the cut-off frequencies. In the preferred embodiment, the antenna includes a grounded dielectric substrate that carries a resonating structure formed from three patch radiators of different dimensions that have substantial electromagnetic coupling. The patch radiators are preferably simultaneously fed by an electromagnetically coupled microstrip line. FIG. 1 is a top plan view of planar patch antenna 100, in accordance with the present invention. FIG. 2 is a cross-sectional view of the planar patch antenna 100. Referring to FIGs. 1 and 2, the planar patch antenna 100 comprises a grounded dielectric substrate 120, a radiating structure 110 carried or supported by the substrate 120, and a feed system 130, 135. The dielectric substrate 120 is formed by a layer of dielectric material 122, and a layer of conductive material 124 that functions as a ground plane. In the preferred embodiment, the dielectric material used is alumina substrate which has a dielectric constant of approximately ten (10). The feed system 130, 135 includes a buried microstrip line 130, disposed between the ground plane 124 and the radiating structure 110. A coaxial feed 135 is coupled to the microstrip line 130 to provide a conduit for communication signals.

The radiating structure 110 includes two patch radiators 112, 114 that form resonating structures, when excited by a feed signal. The patch radiators 112, 114 are preferably rectangular in geometry, having a length measured in a direction of wave propagation 150 (herein referred to as "resonating length"), and a width measured perpendicular to the resonating length. According to the present invention, the resonating structures form an asymmetric geometrical structure in which complementary resonating modes, such as differential and common modes, are presented within a particular operating frequency band. In the preferred embodiment, a primary radiator 112 is formed using a wide planar microstrip printed at the air-dielectric interface 125 of the grounded dielectric substrate 120. A secondary radiator 114 is formed from a narrow planar microstrip running parallel to the primary radiator. Preferably, the patch radiators have respective widths that differ by at least 50 percent. In the preferred embodiment, the narrower patch radiator has a width of at most 30 percent of that of the wider patch radiator. The patch radiators may also have a difference in resonating length for tuning purposes. The dimensions and placement of the patch radiator are significant aspects of the present invention. The patch radiators are placed such that there is a strong electromagnetic coupling between them. The asymmetric structure, i.e., the difference in width between the patch radiators, provide for distinct resonating modes with different phase velocities, and thus different resonant frequencies. The resonating structures 112, 114 are dimensioned to have distinct resonating modes at frequencies that are close together, preferably within ten percent of each other. The result is an enhancement to the overall operational bandwidth for the antenna. The microstrip feed is positioned to apply a different excitation to each patch radiator. The overall excitation can be seen as a superposition of a differential mode excitation and a common mode excitation. The presence of the wide patch radiator produces a greater confinement of the electromagnetic energy within the substrate, both for the common and differential modes supported by the radiating structure. This results in differential and common resonating modes operating with a substantially similar effective dielectric constant, preferably within ten percent of each other. The substantial difference in width between radiators provides for asymmetry in the radiating structure and for the generation of the differential and common resonating modes that are used to effect a wide continuous radiating band. In operation, the microstrip line 130 provides a signal that simultaneously excites the differential and common resonating modes of the radiating structure, with maximum excitation occurring at their respective resonating frequencies. In the preferred embodiment, the microstrip line 130 traverses under the narrow patch radiator and terminates at or near the wide patch radiator. This particular asymmetry produces a dominance in radiation of the greater current flowing on the wide radiator.

Thus, the present invention provides for an antenna with a radiating structure that supports at least two distinct radiating modes, such as differential and common radiating modes. A feed system is coupled to the radiating structure and excites the radiating modes at different frequencies to provide a radiating band for signal transmission. The feed system is preferably a microstrip line that simultaneously excites the distinct resonating modes within the resonating structures. FIG. 3 is a top plan view of a second embodiment of a planar patch antenna 300 having circular polarization, in accordance with the present invention. Here, three patch radiators 312, 314, 316 form a radiating structure that is disposed on a grounded dielectric substrate 320, and two microstrip lines 332, 334 provide orthogonal time quadrature feeds to the patch radiators 312, 314, 316. As before, the patch radiators combine to form an asymmetrical geometrical structure that generates distinct resonating modes with a substantially similar effective dielectric constant. A first narrow patch radiator 314 is situated proximate to a wide patch radiator 312 such that there is substantial electromagnetic coupling therebetween. Both radiators 312, 314 are fed by a buried microstrip line that traverses under the narrow patch radiator 314 and terminates under the wide patch radiator 312. A second narrow patch radiator 316 is situated proximate to the wide patch radiator but oriented orthogonal to the first narrow patch radiator. Another microstrip line 334 traverses the narrow patch radiator 316 and terminates under the wide patch radiator 312.

FIG. 4 is a top plan view of a planar pass-band antenna 400, in accordance with the present invention. FIG. 5 is a cross-sectional view of the antenna 400. Referring to FIGs. 4 and 5, the antenna 400 includes a grounded dielectric substrate 420, a radiating structure 410 carried or supported by the substrate 420, and a feed system 430, 435. The dielectric substrate 420 is formed by a layer of dielectric material 422, and a layer of conductive material 424 that functions as a ground plane. In the preferred embodiment, alumina substrate is used as the dielectric material, which has a dielectric constant of approximately ten (10). The feed system 430, 435 includes a buried microstrip line 430, disposed between the ground plane 424 and the radiating structure 410. A coaxial feed 435 is coupled to the microstrip line 430 to provide a conduit for communication signals.

In the exemplary embodiment, the radiating structure 410 includes three separate planarly disposed patch radiators 412, 414, 416 that resonate, when properly excited by a feed signal. The patch radiators 412, 414, 416 are preferably rectangular in geometry, having a length measured in a direction of wave propagation 450, which is referred to herein as the "resonating length," and a width measured perpendicular to the direction of wave propagation 450. The patch radiators form a multi-mode resonating structure in which three fundamental resonating modes are presented within a particular operating frequency band. A primary radiator 412 is formed using a wide elongated planar microstrip printed at the air-dielectric interface 425 of the grounded dielectric substrate 420. Two secondary radiators

414, 416 are formed from narrow elongated planar microstrips printed at the air-dielectric interface 425 parallel to, and on opposing sides of the primary radiator 412. Preferably, the narrow patch radiators 414, 416 have respective widths that differ from that of the wide patch radiator 412 by at least 50 percent. The patch radiators 412, 414, 416 may also have differences in length, measured in the direction of wave propagation, for tuning purposes. The dimensions and placement of the patch radiator are significant aspects of the present invention. The patch radiators 412, 414, 416 are placed such that there is a strong electromagnetic coupling between them. The difference in width between the primary patch radiator 412 and the secondary patch radiators 414, 416, provide for distinct resonating modes with different phase velocities, and thus different resonance frequencies.

In the preferred embodiment, the microstrip line 430 traverses under one of the narrow patch radiators 414, and the wide patch radiator 412, and terminates at or near another of the narrow patch radiators 416. The microstrip line 430 provides a signal that simultaneously excites the fundamental resonating modes of the radiating structure 410.

Adjacent resonating structures 412, 414, 416 are dimensioned to have distinct fundamental resonating modes at frequencies that are close together, preferably within ten percent of each other. The result is an enhancement to the overall operational bandwidth for the antenna. The microstrip feed is positioned to apply a different excitation to at least two of the patch radiators at or about two frequencies that delimit the pass- band. These two frequencies are referred to herein as "cut-off frequencies." The overall excitation creates a superposition of the three resonating modes which operate together to produce a pass band delimited by the cut-off frequencies. Between the cut-off frequencies, the excitation of the resonating modes results in a substantially constructive superposition of radiated fields from the various radiators. At the cut-off frequencies, the excitation of the resonating modes results in opposing currents in at least two radiators. The opposing current causes a substantially destructive superposition of radiated fields.

FIG. 6 shows a graph comparing gain versus normalized frequency for one embodiment of a pass-band antenna made in accordance with the present invention. It can be seen that a wide pass- band exists between frequencies 0.96 f₀ and 1.04 f₀, where f₀ is the center frequency of the pass-band. For frequencies in the range of 0.96 f₀ to 0.97 f₀ there is a sharp drop off in gain. Similarly, for frequencies in the range of 1.03 f₀ to 1.04 f₀, there is a sharp drop off in gain. This drop off in gain results from a destructive superimposition of resonating modes. Meanwhile, a constructive superimposition of resonating modes exists for frequencies ranging from 0.97 f₀ to 1.03 f₀, resulting in substantial gain. Thus, for example, one cut-off frequency could be selected at or below 0.97 f₀, and another cut-off frequency could be selected at or above 1.03 f₀, depending on desired minimum gain for the radiating band. The present invention provides for an antenna with a radiating structure that supports at least three fundamental resonating modes. A feed system is coupled to the radiating structure and excites the resonating modes at different frequencies to provide a radiating band. The differences between radiation fields at different portions of the radiating structure at the cut-off frequencies causes the field cancellation that delimits the pass-band. In the preferred embodiment, these differences are created by opposing radiator currents on electromagnetically coupled patch radiators generated at the cut-off frequencies. The combination of narrow and wide patch radiators, and the microstrip feed provide for a wide radiating band having a substantially sharp drop in gain versus frequency at or about the cut-off frequencies.

The principles of the present invention may be used to form a variety of antenna structures of varying configurations that yield a substantial improvement in operational bandwidth. For example, the relative positioning of wide and narrow patch radiators may be interchanged to form other useful configurations. By utilizing an asymmetrical geometry that presents differential and common resonating modes to expand bandwidth, planar patch antennas can be incorporated in portable communication devices to yield reductions in size, weight, and cost, and improvements in directivity and efficiency.

The principles of the present invention may also be used to form a variety of antenna structures of varying configuration that yield a substantial improvement in operational bandwidth, while providing for band-pass filtering. For example, the relative positioning of wide and narrow patch radiators may be interchanged to form other useful configurations. The antenna described achieves its wide-band and filtering characteristics in a small package, which makes it suitable for use in portable communication devices that must satisfy tight constraints in size, weight, and costs. For example, in the preferred embodiment, the surface area occupied by the radiating structure is approximately 0.25 λ², where λ is the wavelength of the fundamental guided mode that would be supported by a microstrip line having the same width of the main radiator. Moreover, for the dielectric material of the preferred embodiment, an antenna of appropriate bandwidth can be constructed with an overall thickness of less than λ₀ / 60, where λ₀ is the free space wavelength. Such thickness is substantially less than that typically obtained for prior art antennas having a similar bandwidth. What is claimed is:

Claims

1. An antenna, comprising a radiating structure that supports at least two distinct radiating modes, and a feed system coupled to the radiating structure that excites the at least two distinct radiating modes at different frequencies to provide a radiating band for signal transmission.

2. The antenna of claim 1, wherein: the radiating structure comprises: a grounded dielectric substrate; first and second resonating structures carried by the dielectric substrate and having a substantial electromagnetic coupling therebetween, each of the first and second resonating structures having a geometry selected to have, in combination, first and second distinct resonating modes operating with a substantially similar effective dielectric constant; and the feed system is coupled to the first and second resonating structures and is operable to provide a signal to simultaneously excite the first and second distinct resonating modes.

3. The antenna of claim 2, wherein the first and second resonating structures comprise first and second patch radiators, respectively, each having a direction of wave propagation, the first and second patch radiators having a substantial difference in width measured in a direction perpendicular to the direction of wave propagation.

4. The antenna of claim 3, wherein the first and second patch radiators are simultaneously fed by the buried microstrip line.

5. The antenna of claim 2, further comprising a third resonating structure carried by the dielectric substrate and electromagnetically coupled to the second resonating structure, wherein the feed system comprises orthogonal time quadrature feeds that are coupled to the first, second, and third resonating structures.

6. An antenna operable in a operating frequency band delimited by first and second frequencies, comprising: a grounded dielectric substrate; three resonating structures that are supported by the substrate, and that are electromagnetically coupled to each other to form a radiating structure operable to generate three resonating modes; a feed system coupled to the three resonating structures, which feed system is operable to provide a signal to simultaneously excite three resonating modes to produce opposing currents on at least two of the three resonating structures at first and second frequencies, the opposing currents causing a destructive superposition of radiated fields.

7. The antenna of claim 6, wherein the three resonator structures comprise a first, second, and third patch radiators disposed in sequence in a particular direction, such that the first and third patch radiators are disposed on opposing sides of the second patch radiator, the second patch radiator having a width, measured in the particular direction, substantially greater than that of the first and third patch radiators.

8. A pass-band antenna comprising a grounded dielectric substrate carrying three resonator structures that are electromagnetically coupled to each other, and that are simultaneously fed to excite three resonating modes that operate together to produce a continuous radiating band delimited by substantial radiated field cancellation at first and second frequencies.

9. The pass-band antenna of claim 8, wherein the three resonator structures comprise three patch radiators that are arranged and fed to produce opposing currents on at least two of the three patch radiators at the first and second frequencies, the opposing currents causing substantial radiated field cancellation.

10. A planar antenna operable in a operating frequency band defined by first and second frequencies, comprising: a grounded dielectric substrate; a first, second, and third microstrip patches, having electromagnetic coupling therebetween, and disposed sequentially on the substrate in a particular direction, the first, second, and third microstrip patches having first, second, and third widths, respectively, measured in the particular direction, the first and third widths being at most 30 percent of the second width; and a microstrip line, embedded within the substrate and electromagnetically coupled to the first, second, and third microstrip patches, the microstrip line providing a feed to simultaneously excite first, second, and third resonating modes that produce current flowing in opposite direction on at least two of the first, second, and third patch resonating structures, at first and second frequencies.