The invention relates to an antenna structure that has a broad bandwidth (i.e., the ratio of the highest frequency (fhigh) in the bandwidth to the lowest frequency (flow) in the bandwidth is at least 3:1) and a low-profile.
There is a need for a broadband or broad bandwidth, low-profile antenna. The bandwidth of an antenna is typically defined as the difference between the low frequency (flow) and high frequency (fhigh) at which the power output of the antenna is within 3 dB of the maximum power output of the antenna. In a broadband or broad bandwidth antenna, the ratio of fhigh to flow is greater than 3:1. The profile of an antenna that includes a ground plane and one or more radiators that are all located to one side of the ground plane refers to a plot of the shortest distance between each point associated with the radiators and the ground plane. For a broadband antenna to be considered to be low-profile, the maximum distance in the profile must be less than λ/2 at flow.
An example of a broadband, high-profile antenna can be found in currently known synthetic-aperture radars. The antenna in such a radar is a single-beam antenna that is capable of broadband performance, mounted to a moving platform (e.g., an aircraft), and used to obtain high spatial resolution images of a target region. To elaborate, the antenna is used to transmit pulses of radio signals of varying wavelengths within the bandwidth and receive echo waveforms that are coherently detected and subsequently processed to obtain a high spatial resolution image of the target region. Currently, the known single-beam, broadband antennas that are used in such radar systems have a high profile (i.e., greater than λ/2 at flow) that is disadvantageous. For example, a high profile limits the application of the antenna to moving platforms that have the space to accommodate the profile of the antenna. A high profile can also adversely affect the operation of the moving platform with which antenna is associated. For example, if the antenna is associated with an aircraft, the high profile may require that a significant portion of the antenna project beyond the normal “skin” of the aircraft and to an extent that adversely affects the aerodynamics of the aircraft.
Many types of antennas are known to exhibit a broad bandwidth and a high profile. Among these antennas are: quad-ridge horn, log periodic dipole array, Vivaldi, and log conical spiral antennas. Also known are antennas that have a narrow bandwidth and a low-profile. These antennas include dipole, slot, microstrip patch, and planar inverted-F antennas. However, an antenna structure that has both a broad bandwidth (greater than 3:1) and a low-profile (height less than λ/2 at flow) has proven to be elusive.
The invention is directed to a broadband, low-profile antenna structure that includes a compound radiator and a ground plane located adjacent to the radiator. The compound radiator includes a dipole radiator portion and a Vivaldi radiator portion that is electrically connected to the dipole radiator portion. In operation, the dipole radiator portion operates in the lower end of the bandwidth and, when operating in the lower end of the bandwidth, is fed by the Vivaldi radiator portion (i.e., the Vivaldi radiator portion transmits/receives electrical signals to/from the dipole radiator portion). The Vivaldi radiator portion operates in the upper end of the bandwidth and receives/transmits electrical signal from/to a radio via whatever signal transmission structure extends between the Vivaldi radiator portion and the radio. The low-profile is attributable to the distance between any point associated with the dipole radiator portion or the Vivaldi radiator portion (i.e., a radiator point) and the point on the ground plane that is nearest to any such a radiator point being less than λ/2 at flow. Further, the antenna structure is capable of being adapted to achieve profiles that are substantially less than λ/2 at flow. For example, a profile as low as 0.15λ at flow has been achieved.
In one embodiment, the compound radiator is comprised of a pair of dipole sub-radiators and a pair of Vivaldi sub-radiators. Each of the dipole sub-radiators extends from an inner terminal end to an outer terminal end. The inner terminal ends of the dipole sub-radiators are separated from one another by a first distance and the outer terminal ends of the dipole sub-radiators are separated from one another by a second distance that is greater than the first distance. The second distance is approximately λ/2 at flow. In another embodiment, this second distance is reduced so as to be less than λ/2 at flow and greater than or equal to λ/4 at flow. If the footprint of the antenna structure is defined by the dipole radiator portion and not the ground plane, which can theoretically have an infinite extent, this embodiment of the antenna structure can be used to realize a reduced footprint for the antenna structure.
In a further embodiment, the compound radiator is comprised of first and second dipole sub-radiators and first and second Vivaldi sub-radiators. The first dipole sub-radiator and first Vivaldi sub-radiator are embodied in a single piece of electrically conductive material. For example, the first dipole sub-radiator and first Vivaldi sub-radiator can be formed from a single piece of copper that has been appropriately shaped to realize a dipole structure and a Vivaldi structure. In a further embodiment, the first and second dipole sub-radiators, first and second Vivaldi sub-radiators and ground plane are all embodied in a single piece of electrically conductive material.
With reference to
The compound radiator 22 is comprised of a dipole radiator 26 and a Vivaldi radiator 28 that is electrically connected to the dipole radiator. In operation, the compound radiator 22 is capable of broadband operation, i.e., capable of operating over a bandwidth with at least a 3:1 ratio. More specifically, the dipole radiator 26 of the compound radiator 22 operates in the lower end of the bandwidth and the Vivaldi radiator 28 of the compound radiator 22 operates in the upper end of the bandwidth. Further, it should be appreciated that when the dipole radiator 26 is operating in the lower end of the bandwidth, the Vivaldi radiator 28 functions as feed for the dipole radiator 26, (i.e., transmits/receives electrical signals to/from the dipole radiator). The compound radiator 22 and ground plane 24 present a low-profile, i.e., the maximum distance between any point on the compound radiator and the nearest point on the ground plane to the point of interest on the compound radiator is less than λ/2 at flow.
The dipole radiator 26 is comprised of first and second dipole sub-radiators 30A, 30B that are each planar and substantially co-planar with one another. The first dipole sub-radiator 30A extends from an inner terminal end 32A to an outer terminal end 34A. Likewise, the second dipole sub-radiator 30B extends from and inner terminal end 32B to an outer terminal end 34B. The inner terminal ends 32A, 32B are separated by a distance 36 that is related to a transition region in the bandwidth at which the operation of the antenna moves between dipole and Vivaldi modes of operation. The outer terminal ends 34A, 34B are separated by a distance 38 that is related to the bandwidth of the antenna and is substantially equal to λ/2 at flow. Further, while each of the illustrated dipole sub-radiators 30A, 30B has a planar rectangular shape, dipole sub-radiators with different shapes are feasible. For instance, a dipole sub-radiator in the form of: (a) a wire, (b) a planar triangle, or (c) a planar isosceles trapezoid, as well as other shapes know to those skilled in the art, are each feasible.
The Vivaldi radiator 28 is comprised of first and second Vivaldi sub-radiators 40A, 40B. The first Vivaldi sub-radiator 40A extends from an inner terminal end 42A to an outer terminal end 44A. Likewise, the second Vivaldi sub-radiator 40B extends from and inner terminal end 42B to an outer terminal end 44B. The inner terminal ends 42A, 42B are separated by a distance 46 that presents a desired impedance over the bandwidth of the antenna and is also related to fhigh. The outer terminal ends 34A, 34B are separated by a distance 48 that, in the illustrated embodiment, is substantially equal to the distance 36 between the inner terminal ends 32A, 32B of the dipole radiator 26. With reference to
The dipole radiator 26 and the Vivaldi radiator 28 are electrically connected to one another such that the Vivaldi radiator 28 can act as a feed for the dipole radiator 26 when the dipole radiator 26 is active in the lower end of the bandwidth. More specifically, (a) the outer terminal end 44A of the first Vivaldi sub-radiator 40A is electrically connected to the inner terminal end 32A of the first dipole sub-radiator 30A and (b) the outer terminal end 44B of the second Vivaldi sub-radiator 40B is electrically connected to the inner terminal end 32B of the second dipole sub-radiator 30B. In the illustrated embodiment, these connections are achieved by embodying: (a) the first dipole sub-radiator 30A and the first Vivaldi sub-radiator 40A in the same piece of electrically conductive material (e.g., copper) and (b) the second dipole sub-radiator 30B and the second Vivaldi sub-radiator 40B in the same piece of electrically conductive material. As such, (a) the outer terminal end 44A of the first Vivaldi sub-radiator 40A is substantially coextensive with the inner terminal end 32A of the first dipole sub-radiator 30A and (b) the outer terminal end 44B of the second Vivaldi sub-radiator 40B is substantially coextensive with the inner terminal end 32B of the second dipole sub-radiator 30B.
The ground plane 24 is comprised of first and second ground sub-planes 50A, 50B that are each planar and substantially co-planar with one another. The first and second ground sub-planes 50A, 50B respectively underlie the first and second dipole sub-radiators 30A, 30B and operate to reflect the electromagnetic signal produced dipole radiator 26 back towards the dipole radiator 26.
With reference to
In designing the antenna 20 for broadband operation, the curves 62 and 64 must overlap to an extent that avoids a drop in gain at, about, or near a transition frequency (ftrans) (the frequency at which the curves 62 and 64 intersect) of a magnitude that renders the antenna a narrowband antenna rather than a broadband antenna. For purposes of illustration, a drop in gain 72 is shown in
The profile of antenna 20 is less than λ/2 at flow. However, it should be appreciated that the portion of the profile attributable to the dipole radiator portion can fall within a range substantially under the λ/2 at flow limit based upon the type of dipole radiator employed. With reference to
With reference to
With reference to
As shown in
A number of variations in the components of the antenna structure can be made, provided the broadband characteristic attributable to dipole and Vivaldi modes of operation and low-profile characteristic of the antenna are maintained.
When the dipole radiator is comprised of two, planar sub-radiators and the antenna includes a single, planar ground plane, among the possible variations in the dipole radiator structure are: (a) the sub-radiators can be disposed parallel to the planar ground plane but disposed at different distances from the planar ground plane; (b) one or both of the sub-radiators can be disposed at an angle to the ground plane, and (c) the sub-radiators can be different lengths, i.e., the distance between the inner and outer terminal ends of one sub-radiator can be different than the distance between the inner and outer terminal ends of the other sub-radiator.
When the dipole radiator is comprised of two, planar sub-radiators and the antenna includes a ground plane comprised of two, ground sub-planes where one ground sub-plane is associated with one planar sub-radiator and the other ground sub-plane is associated with the other sub-radiator, among the possible variations in the dipole radiator structure are: (a) the ground sub-planes can be coplanar and the sub-radiators disposed parallel to their associated ground sub-planes but at different distances from their associated ground sub-planes, (b) the ground sub-planes can be disposed parallel to one another but not coplanar with one another and the sub-radiators can be disposed parallel to their associated ground sub-planes and at the same distance from their associated ground planes; (c) the ground sub-planes can be disposed parallel to one another but not coplanar with one another and the sub-radiators can be disposed parallel to their associated ground sub-planes but at different distances from their associated ground planes; (d) the ground sub-planes can be non-parallel to one another and the sub-radiators disposed parallel to their respective ground planes and at the same distance from their respective ground planes, and (e) the ground sub-planes can be non-parallel to one another and the sub-radiators disposed parallel to their respective ground planes but at different distances from their respective ground planes. In each of the foregoing cases, one or both of the sub-radiators can be disposed non-parallel to the their associated ground sub-plane.
When the dipole radiator is comprised of two sub-radiators and the antenna includes a single ground plane, a dipole mode of operation is feasible with one or both of the sub-radiators being non-planar (e.g., bent or curved) and/or the ground plane being non-planar.
A number of variations to the Vivaldi radiator are also feasible, provided the broadband characteristic attributable to dipole and Vivaldi modes of operation and low-profile characteristic of the antenna are maintained. In the illustrated embodiment, a line extending between the inner terminal ends 42A, 42B of the first and second Vivaldi sub-radiators 40A, 40B is substantially parallel to a line between the outer terminal ends 44A, 44B of the first and second Vivaldi sub-radiators 40A, 40B. Further, the first and second Vivaldi sub-radiators are substantially mirror images of one another, i.e., the Vivaldi radiator is symmetric with each of the sub-radiators conforming to the same plane curve that provides a Vivaldi mode of operation. Among the possible variations to the Vivaldi radiator is an asymmetric Vivaldi radiator. One characteristic of a type of one type of asymmetric Vivaldi radiator is that the two lines extending respectively between the inner terminal ends and outer terminal ends are oblique to one another, i.e., neither parallel or perpendicular to one another. Characteristic of another type of asymmetric Vivaldi radiator is that one of the Vivaldi sub-radiators conforms to one type of plane curve and the other Vivaldi sub-radiator conforms to a different type of Vivaldi plane curve. For example, one Vivaldi sub-radiator could conform to an exponential plane curve and the other Vivaldi sub-radiator could conform to a polynomial plane curve. A Vivaldi sub-radiator can also conform to two or more types of Vivaldi plane curves and be symmetric or asymmetric with the other Vivaldi sub-radiator. For example, a Vivaldi sub-radiator could conform to a Klopfenstein plane curve over a portion of the sub-radiator and a hyperbolic plane curve over a separate portion of the sub-radiator.
The foregoing description of the invention is intended to explain the best mode known of practicing the invention and to enable others skilled in the art to utilize the invention in various embodiments and with the various modifications required by their particular applications or uses of the invention.
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