Patent Grant
11581663
The present invention relates to antennas, and more particularly, to a dual S-band and Ka-band, circularly polarized high gain antenna (HGA).
A large diameter, dual frequency, circularly polarized reflector antenna with an as low as possible depth was required. Thus, the reflector F/D ratio (the focal length divided by the reflector diameter) required had to be close to 0.25, as that provides the overall minimum antenna depth, with the prime reflector focus near the aperture plane. The consequence is a relatively large primary reflector depth requirement, about equal to the overall antenna depth.
The current antenna invention is inspired by prior art, in particular the smaller Lunar Reconnaissance Orbiter (LRO) HGA, where the low frequency feed antenna is placed at the primary reflector feed, and the high frequency system includes a secondary reflector with the high frequency feed antenna placed at one of the foci of the secondary reflector. The secondary reflector, which is reflective over the high frequency band, is essentially transparent to the low frequency band, by means of a frequency selective surface (FSS) that also acts as a radome for the low frequency feed antenna.
The low F/D ratio required a low frequency feed antenna that has a relatively wide beam with low cross-polarization covering the full hemisphere, to fully illuminate the primary reflector. Current state of the art, such a s S-band feed antennas of the LRO HGA and the Global Precipitation Measurement Mission (GPM) HGA do not have wide enough beams. Although the latter has the desired low cross-polarization over a wide angular range, a feed antenna with a higher front to back ratio (i.e. the amount of radiation energy radiated forward versus the energy radiated backward) than either of these prior art could provide, was also desired.
The LRO antenna's secondary reflector was relatively small and of low curvature depth, which simplified fabrication of the FSS, as it could be approximated with two conical sections with minimal deviation from the desired curvature. For the current invention, a larger secondary reflector with a deeper profile was required, which could not be divided into just a few conical segments without significantly larger deviations from the ideal curvature.
The LRO HGA high frequency feed antenna employed a relatively expensive, corrugated horn antenna, with very few vendors prepared to take on the challenge of fabricating it, and who required long lead times. Other easier to fabricate prior art horn antennas include the Potter horn, but it has a narrower bandwidth, and typically higher sidelobes and cross-polarization.
Therefore, as the current antenna invention cannot be based directly on any prior art without introducing new modifications and inventions, there is a need for an alternative high gain antenna.
Certain embodiments of the present invention may provide solutions to the problems and needs in the art that have not yet been fully identified, appreciated, or solved by current antenna technologies. For example, some embodiments of the present invention pertain to a high gain antenna.
In an embodiment, an apparatus for space and terrestrial communication applications includes a Ka-band horn combined with a S-band cross-polarization cup. The S-band cross-polarization cup is placed around a neck of the Ka-band horn in a form of a collar.
In order that the advantages of certain embodiments of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. While it should be understood that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which:
Some embodiments generally pertain to a high gain dual S- and Ka-band, circularly polarized antenna for space and terrestrial communication applications. For purposes of simplicity, the high gain dual S- and Ka-band, circularly polarized antenna will be referred to as “antenna”. The antenna may include an integrated prime-fed S-band and Cassegrain-based Ka-band reflector system. The Cassegrain primary and secondary reflectors are designed and shaped for optimal Ka-band gain, while a frequency selective surface on the secondary reflector provides reflectivity at Ka-band. The secondary reflector and frequency selective surface may act as a dielectric radome for the S-band feed antenna.
In some embodiments, the antenna is suited for lunar missions and beyond, or in long range terrestrial communications, where the large distances and high data rates require an antenna with as much high gain as possible within given size constraints. Some embodiments include an improved S-band feed antenna and cross-polarization compensation, improved Ka-band horn, and special shaping of the secondary reflector for easy of fabrication, all of which will be described in more detail below.
Some embodiments were motivated by the communications requirements for the Roman Space Telecope (RST) mission, which may send an infrared space telescope to orbit at the earth-sun L2 Lagrange point 1.5 million km from earth. As noted above, the antenna requires as much high antenna gain as possible, especially at the Ka-band, while meeting size constraints imposed by the launch vehicle, the practical mounting considerations and antenna steering capabilities of the spacecraft. Further, some embodiments were inspired by the lunar reconnaissance orbiter (LRO) and Cassini missions, which similarly employed multi-band high gain communication antennas.
For Ka-band operation, Ka-band feed horn 108 illuminates FSS 104, which in this frequency band is highly reflective. The reflection from FSS 104 (or the secondary reflector) illuminates the primary reflector, which then produces a coherent, narrow Ka-band beam. The primary and secondary reflectors are not paraboloid and hyperboloid surfaces respectively as in a standard Cassegrain system. Instead, these reflectors are shaped to provide an aperture wavefront of uniform phase and amplitude, for optimal aperture efficiency. For even higher efficiency, the secondary reflector shaping also diverts radiation energy away from the central part of the primary reflector that would otherwise have been obstructed by the secondary reflector itself.
Some embodiments, however, improve on the prior art's front to back ratio, i.e. the amount of radiation energy radiated forward versus the energy radiated backward. A significantly higher front to back ratio is achieved in these embodiments by the introduction of a circumferential choke slot in the cylindrical side of the antenna, without increasing the antenna footprint. The antenna beam width is also increased by reducing the diameter of the outer circular wall diameter. The frequency bandwidth over which the improved front to back ratio is effective, is increased by the use of notches in the choke wall.
Normally, a corrugated horn has been used to achieve the lower required sidelobes. A corrugated horn also has the added advantage of a wider frequency band of operation than a standard Potter horn, but for some applications, the narrower Potter horn bandwidth suffices. The smooth interior walled Potter horn is simpler and less expensive to fabricate than a corrugated horn. Therefore, the modified Potter horn embodiment with its low side-lobe performance is desirable for relatively narrow band applications.
Some embodiments utilize a S-band cross-polarization cup to reduce the antenna cross-polarization in the S-band. This is achieved by cancelling reflections back to the S-band feed antenna that reflects off the central part of the reflector, the Ka-band horn and its support structure. Without the S-band cross-polarization cup, interaction of these reflections with the S-band feed results in increased S-band cross-polarization. This is especially relevant if the feed antenna has a simple reactive feed structure as opposed to a dual polarization feed mechanism that would have allowed the reflections to be absorbed into a terminating load instead of being re-radiated.
A cross-polarization cancellation cup has been used in the LRO and GPM high gain antennas, and in other embodiments. Some embodiments may combine the cross-polarization cup with the Ka-band horn geometry. See, for example,
The curvature of cup 405 is used for mechanical strength reasons. The operation of cup 405 is solely governed by its diameter and position with respect to the horn aperture. The curvature, and the lip along the edge, of cup 405 greatly increases the stiffness compared to a flat annular disk with the same diameter and mass.
Primary Reflector and FSS Shaping
The typical shaping algorithms that modifies the standard Cassegrain reflector system for optimized gain, leads to smoothly curved reflector surfaces. The fabrication of a FSS that conforms to the required curved secondary surface is very problematic. Some embodiments overcome this problem by breaking the FSS up into multiple conical sections that approaches the ideal optimized shape. See
Going into more detail, as shown in
The reflected phase and amplitude response of an FSS in general varies somewhat depending on the polarization and the wave's angle of incidence. Each point on the FSS in this case experience different but predictable incident wave angles. Therefore, the FSS's resonant components are sized and shaped accordingly to provide a response with minimal phase and amplitude errors for both polarizations over the whole surface.
In some embodiments, S-band cross polarization cup 605 is designed experimentally, or by way of simulations, by monitoring the boresight radiated cross-polarization in a transmit mode setup. If the S-band feed's circular polarization is created by 3 or more phased internal feed points, then a computationally more efficient technique in a simulation environment is by monitoring the S-band feed's active S11 reflection coefficient at one of the internal feed points in the presence of the primary reflector assembly. The reflected energy at the internal feed points leads to zero combined energy exiting the feed port, but re-combines as re-radiated cross-polarization.
Since the internal feed points are phased to provide circular polarization, the reflected energy arrives out of phase at the input port, and is once more reflected back to the feed points. It arrives with three times the amount of the original phasing—which is exactly the phase configuration for the opposite handed circular polarization, and is therefore radiated as cross-polarized energy. In the graph, “free-space” curve 705 represents the internal port reflection S11, and hence the fraction of radiated cross-polarization energy when the antenna is placed in a free-space environment. Similarly, “without cup” curve 710 represents the cross-polarized radiated energy in the presence of the high gain antenna assembly, when a “cross-polarization” cup is absent. The “with cup” curve 715 represents the cross-polarized radiated energy in the presence of the high gain antenna assembly, but with an optimized “cross-polarization” cup installed. Therefore, graph 700 shows that the cross-polarization cup in the HGA environment restores the radiated cross-polarization energy of the S-band feed antenna similar to the low levels it exhibits in a free space environment.
These embodiments may be used to exploit feed antennas, i.e., the Ka-band horn and/or the S-band feed antenna, where robust mechanical properties and low electrical losses are premium such as antennas for spacecraft or vehicles, where high vibration environment require these properties. These antennas may also be scaled to operate at frequency bands other than Ka-band or S-band. The high gain antenna system can be commercially applied in terrestrial situations, where high data rate communications are required between line-of-site points such as in the Telecommunications industry.
It will be readily understood that the components of various embodiments of the present invention, as generally described and illustrated in the figures herein, may be arranged and designed in a wide variety of different configurations with variations on the materials indicated. Thus, the detailed description of the embodiments of the present invention, as represented in the attached figures, is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention.
The features, structures, or characteristics of the invention described throughout this specification may be combined in any suitable manner in one or more embodiments. For example, reference throughout this specification to “certain embodiments,” “some embodiments,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in certain embodiments,” “in some embodiment,” “in other embodiments,” or similar language throughout this specification do not necessarily all refer to the same group of embodiments and the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
It should be noted that reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.
Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.
One having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order, and/or with hardware elements in configurations which are different than those which are disclosed. Therefore, although the invention has been described based upon these preferred embodiments, it would be apparent to those of skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims.
The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefore.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3438041 | Holtum, Jr. | Apr 1969 | A |
| 5373302 | Wu | Dec 1994 | A |
| 6252559 | Donn | Jun 2001 | B1 |
| 20130050048 | Green et al. | Feb 2013 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 63074190 | Sep 2020 | US |
