Multi-Mode Pass-Band Planar Antenna

This invention relates in general to antennas, and more particularly, to microstrip antennas.

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.

Communication signals are usually filtered using a band-pass filter or the like to remove unwanted harmonics before being sent to an antenna for transmission. Such filtering adds to the cost and complexity of a product. Planar patch antennas have been proposed that provide some band pass filtering. For example, it is known to selectively shape a radiator patch to provide narrow-band limited filtering. It is 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 a pass-band planar antenna is needed.

FIG. 1 is a top plan view of a planar pass-band antenna, in accordance with the present invention.

FIG. 2 is a cross-sectional view of the antenna of FIG. 1.

FIG. 3 is a graph showing experimental results of an antenna made in accordance with the present invention.

The present invention 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 a planar pass-band antenna 100, in accordance with the present invention. FIG. 2 is a cross-sectional view of the antenna 100. Referring to FIGS. 1 and 2, the antenna 100 includes 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, alumina substrate is used as the dielectric material, 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.

In the exemplary embodiment, the radiating structure 110 includes three separate planarly disposed patch radiators 112, 114, 116 that resonate, when properly excited by a feed signal. The patch radiators 112, 114, 116 are preferably rectangular in geometry, having a length measured in a direction of wave propagation 150, which is referred to herein as the "resonating length," and a width measured perpendicular to the direction of wave propagation 150. 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 112 is formed using a wide elongated planar microstrip printed at the air-dielectric interface 125 of the grounded dielectric substrate 120. Two secondary radiators 114, 116 are formed from narrow elongated planar microstrips printed at the air-dielectric interface 125 parallel to, and on opposing sides of the primary radiator 112. Preferably, the narrow patch radiators 114, 116 have respective widths that differ from that of the wide patch radiator 112 by at least 50 percent. The patch radiators 112, 114, 116 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 112, 114, 116 are placed such that there is a strong electromagnetic coupling between them. The difference in width between the primary patch radiator 112 and the secondary patch radiators 114, 116, provide for distinct resonating modes with different phase velocities, and thus different resonance frequencies.

In the preferred embodiment, the microstrip line 130 traverses under one of the narrow patch radiators 114, and the wide patch radiator 112, and terminates at or near another of the narrow patch radiators 116. The microstrip line 130 provides a signal that simultaneously excites the fundamental resonating modes of the radiating structure 110.

Adjacent resonating structures 112, 114, 116 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. 3 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 f.sub.0 and 1.04 f.sub.0, where f.sub.0 is the center frequency of the pass-band. For frequencies in the range of 0.96 f.sub.0 to 0.97 f.sub.0 there is a sharp drop off in gain. Similarly, for frequencies in the range of 1.03 f.sub.0 to 1.04 f.sub.0, 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 f.sub.0 to 1.03 f.sub.0, resulting in substantial gain. Thus, for example, one cut-off frequency could be selected at or below 0.97 f.sub.0, and another cut-off frequency could be selected at or above 1.03 f.sub.0, 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 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 .lambda..sup.2, where .lambda. 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 .lambda..sub.0 /60, where .lambda..sub.0 is the free space wavelength. Such thickness is substantially less than that typically obtained for prior art antennas having a similar bandwidth.