The subject matter described herein relates generally to antennas and, more particularly, to antennas that are capable of full duplex operation.
Simultaneous transmit, and receive (STAR) refers to the ability of a radio frequency (RF) circuit, device, or system to transmit and receive at the same time, in the same frequency band, with adequate performance in the receiver. Such capability is desired for applications such as, for example, cognitive radio and full-duplex communications systems. In a conventional approach, RF transmit and receive operations within a particular frequency band are performed at different times. This is because transmit energy from a transmit antenna will typically leak into the front end of a collocated receiver and overdrive the receiver if transmit and receive operations are performed concurrently. This transmitter leakage can mask the desired receive signals, thus making it difficult or impossible to detect, demodulate, and decode the signals. For STAR operation to be possible, therefore, a certain minimum level of isolation must be main tamed between a transmit antenna and a receive antenna.
Antennas have been designed in the past that are capable of supporting STAR operation. However, such antennas have invariably been of relatively low bandwidth. There is a need for antennas that are capable of simultaneous transmit and receive operation over wider bandwidths.
Antenna structures and methods are described herein that are capable of simultaneous transmit and receive (STAR) operation, with high isolation between transmit and receive antennas, over relatively wide bandwidths. In at least one embodiment, a STAR antenna includes a ring array having an even number of transverse electromagnetic (TEM) horn elements spaced at equal angular intervals in a circular configuration. Opposing elements in the ring array are driven 180 degrees out of phase. In at least one implementation, the ring array includes 8 elements and the phasing of the elements is 0, 45, 90, 135, 180 225, 270, and 315 degrees, respectively, around the ring. The TEM horn elements in the array include capacitive feeds. The antenna also includes another antenna element that is centrally located with respect to the ring. The central element may include, for example, a mono-cone or bi-cone element. During operation, the ring array may operate as a transmit antenna and the central element may operate as a receive antenna. In an alternate approach, the central element may operate as the transmit antenna and the ring array may operate as the receive antenna.
In some embodiments, the elements of the ring array may include solid-metal TEM horn elements. As described above, these TEM horn elements may be capacitively fed. It was found that capacitive feeds could improve bandwidth considerably in the underlying antenna architecture, when using the corresponding excitation scheme, by improving the low frequency impedance match of the ring array elements. Because opposing elements in the ring array are excited 180 degrees out of phase, signals transmitted by opposing elements will substantially cancel at the location of the central element (i.e., zero sum interference from opposing elements). In at least one embodiment, the ring array is located between two metallic reflector structures with the elements of the ring oriented so that their corresponding directional beams point radially outward from the ring. The central element may be mounted above one of the two reflector structures, is a central location. The elements of the ring array may be fed from below by coaxial cables that couple to a capacitor associated with each corresponding horn. A coaxial feed may also extend up through the center of the ring array to feed the central element at the top of the antenna.
In accordance with the concepts, systems, circuits, and techniques described herein, an antenna system for simultaneous transmit and receive (STAR) comprises a ring array having an even number (N) of TEM horn elements arranged in a circular configuration on a horizontal plane, where each of the TEM horn antennas includes a capacitive feed, and a center element substantially centered on a vertical axis that extends through a center point of the ring array. The center element may include either a monocone element or a bicone element.
In one embodiment, the antenna system may further include circuitry for exciting the TEM horn elements of the ring array such that opposing elements in the ring array are phased at a 180 degree phase difference and adjacent elements in the ring array are phased at a 360/N phase difference.
In one embodiment, the ring array is a transmit antenna and the center element is a receive antenna.
In one embodiment, the ring array is a receive antenna and the center element is a transmit antenna.
In one embodiment, the ring array includes at least one solid-metal TEM horn element.
In one embodiment, all elements of the ring array include solid-metal TEM horn elements.
In one embodiment, the antenna system achieves at least 40 dB of isolation between a transmit antenna and a receive antenna over at least a 6:1 bandwidth.
In one embodiment, the TEM horn elements of the ring array are miniaturized by exploiting mutual coupling between the elements of the ring array.
In one embodiment a largest dimension of each of the TEM horn elements of the ring array is approximately 0.17 wavelengths at the lowest operational frequency of the antenna system.
In one embodiment, the antenna system further comprises upper and lower reflectors each having a truncated cone shape, wherein the ring array is disposed within a region between the upper and lower reflectors.
In one embodiment, the TEM horns of the ring array are mounted within a depression in an upper surface of the lower reflector; and the upper and lower reflectors are spaced from one another using dielectric spacers.
In one embodiment, the center element is mounted above the upper reflector and is fed by a coaxial feed extending through fee center of the ring array.
In one embodiment, the TEM horns of the ring array are oriented so that directional beams associated with the horns are directed radially outward from the ring array.
In one embodiment, the ring array includes a first TEM horn element having a substantially vertical metallic surface at an end thereof closest to the center point of the ring array, wherein the capacitive feed associated with the first TEM horn element includes a parallel plate capacitor disposed against the substantially vertical metallic surface.
In one embodiment the parallel plate capacitor includes a circuit board having metallization on at least one surface thereof.
In one embodiment, the capacitive feed associated with the first TEM horn element further includes a coaxial transmission line having a center conductor conductively coupled to metallization on the circuit board.
The foregoing features may be more folly understood from the following description of the drawings in which:
A design goal for the antenna 10 of
Because opposing elements in the array are fed in anti-phase (i.e., 180 degrees out-of-phase), corresponding radiated fields cancel at the central element, resulting in high isolation between the ring array and the central element. In some embodiments, circuitry may be provided within the antenna that is capable of maintaining the excitation scheme 30 of
As described previously, mutual coupling between adjacent elements in the ring array may be exploited to reduce the size of the TEM horn element 40 (i.e., the element and the array may be miniaturized). This is evident in the curves provided in
Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.
The present application claims the benefit of U.S. Provisional Patent Application No. 61/908,476 filed on Nov. 25, 2013, winch is hereby incorporated by reference herein in its entirety.
This invention was made with Government support under Contract No. FA8721-05-C-0002 awarded by the U.S. Air Force. The Government has certain rights in the invention.
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Number | Date | Country | |
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20150145741 A1 | May 2015 | US |
Number | Date | Country | |
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61908476 | Nov 2013 | US |