Planar Antenna With Patch Radiators for Wide Bandwidth

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

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. Pat. No. 5,572,222 issued to Mailandt et al. on Nov. 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. Therefore, a new approach for planar patch antenna with increased bandwidth is needed.

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.

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.

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.

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.