Patent Application
20090096704
The invention relates generally to a method and apparatus for shaping a signal pattern. In one embodiment, the invention relates to wideband antenna with a Non-Cutoff Frequency Selective Surface ground plane.
Positioning and navigation systems can require an antenna that has high-accuracy and operates over multiple frequencies.
Positioning and navigation systems can also require elimination of multipath signal reflections.
Axial-ratio is one measure of multipath signal rejection capability for Right Hand Cross Polarized (RHCP) antennas, such as GPS antennas. Multipath signals are primarily Left Hand Cross Polarized (LHCP) reflection signals from objects located within a close proximity to the antenna. Current high accuracy GPS antennas feature an axial-ratio bandwidth that is too narrow to cover frequencies between 1.15 and 1.60 GHz
Current antenna systems can eliminate multipath signal reflections and achieve sufficient antenna performance for two bands, L1 and L2, using for example, GPS antennas equipped with choke ring ground planes. The choke ring ground plane efficiently mitigates multipath signal reflections at L1 and L2 by eliminating propagation of surface wave on the ground plane and thereby suppressing undesired multipath signals at low elevation angles. The choke ring ground plane enhances antenna performance by reducing back lobe and side lobe radiation that also improves multipath signal reflection mitigation.
Plane waves and surface waves that travel on a finite sized non-corrugated metal ground plane radiate causing ground plane edge diffraction, thus increasing back lobe and side lobe radiation. A choke ring is a corrugated surface having deep metal concentric rings. Corrugated surfaces do not support propagation of plane waves. Consequently, choke rings to do not support propagation of plane waves. Moreover, for a choke ring to ensure the absence of propagation of surface waves the corrugation depth (i.e. concentric rings) d, must be λ/4≦d≦λ/2, for each frequency of operation (operation at cutoff). The absence of propagation of surface waves eliminates the antenna back lobes and side lobes, thus preventing reception of multipath signals at low elevation angles. For a dual-frequency, L1 and L2, GPS antenna to operate with surface wave cutoff, the corrugation depth is typically between 61 mm≦d≦95 mm and the diameter of the choke ring is typically approximately 360 mm.
Current high accuracy antennas cannot support frequencies over the entire range of 1.15 to 1.60 GHz.
In one aspect, the invention features an antenna having a feeding element capable of receiving dual-polarized wideband electromagnetic signals and a Non-Cutoff Frequency Selective Surface ground plane. The Non-Cutoff Frequency Selective Surface ground plane has a metal plate with a plurality of corrugations, such as concentric rings, each corrugation having a predetermined height and a predetermined spacing from adjacent corrugations to cause a line-of-sight signal and a surface wave signal to cancel.
In some embodiments, the Non-Cutoff Frequency Selective Surface causes multipath signal rejection for a multipath signal with a low or negative elevation angle. In some embodiments, the antenna receives the electromagnetic signals within a bandwidth of 1.15 GHz to 1.60 GHz. In some embodiments, the Non-Cutoff Frequency Selective Surface ground plane is a choke ring.
In some embodiments, the corrugation depth range is less than λ/4. In some embodiments, the edges of the choke ring are rolled. In some embodiments, the feeding element is a droopy turnstile bowtie. In some embodiments, the droopy turnstile bowtie has a droop angle between 30 and 45 degrees. In some embodiments, the Non-Cutoff Frequency Selective Surface causes elimination of edge diffraction.
In another aspect, the invention is a signal pattern shaping method. The method involves controlling phase of a surface wave propagating on a surface of a Non-Cutoff Frequency Selective Surface ground plane having a geometry that tunes the surface waves phase to be a multiple of π relative to phase of a line-of-sight signal. The method also involves canceling a low elevation signal that is the composition of a surface wave and a line-of-sight signal having a phase difference tuned to be a multiple of π.
In some embodiments, the method involves rejecting a multipath signal having a low or negative elevation angle. In some embodiments, the method involves receiving signals within a bandwidth of 1.15 GHz to 1.60 GHz. In some embodiments, the method involves receiving signals with a droopy turnstile bowtie. In some embodiments, the droopy turnstile bowtie has a droop angle between 30 and 45 degrees.
In some embodiments, the Non-Cutoff Frequency Selective Surface ground plane includes geometry of a choke ring. In some embodiments, the corrugation depth is less than λ/4. In some embodiments, the edges of the choke ring are rolled. In some embodiments, the method involves elimination of edge diffraction.
The corrugation depth can be in the range of d<16 to 25 mm to achieve the surface wave and line-of-sight signal cancellation in the band 1.15 to 1.60 GHz. In some embodiments, the number of corrugations is the rounded ratio of EQN. 1
Nc=[(G/2)−R](g+t) 1
where G is the diameter of the flat ground plane 330, R is the radius of the cavity, and g+t is the corrugation period.
In some embodiments, the corrugations are a conical shape, a frustro-conical shape, a circular shape or an oval shape. In one embodiment, the Non-Cutoff FSS ground plane has the geometry of a choke ring with rolled edges. In some embodiments, the Non-Cutoff ground plane is aluminum, brass or stainless steel.
While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
This application claims the benefit of and priority to U.S. Provisional Patent Application No. 60/973,025, filed Sep. 17, 2007, the entire disclosure of which is incorporated herein by reference.
The technology described herein was developed with funding provided by the National Science Foundation, contract number DMI-0450524, PSI-7225-010. The federal government may have rights in the technology.
| Number | Date | Country | |
|---|---|---|---|
| 60973025 | Sep 2007 | US |
