Satellite broadcast "receive only" television signals are very weak and require the use of a large antenna with a large collecting area in order to receive a useful signal. It is common for the large collecting area to constitute a paraboloidal reflector dish. The signal collected and reflected by the paraboloidal dish is focused by the reflector surface to a point in front of the dish. The distance of the focal point of any particular dish from the dish is dependent upon the curvature of the reflecting surface of the dish, which is usually paraboloidal.
The signal reflected from the dish is normally detected by a device referred to as a prime focus feed antenna. As will be understood by those skilled in the relevant art, the prime focus feed antenna is located as precisely close to the focal point of the reflector as is possible. The principal function of all prime focus feed antennas is to provide uniform illumination of the paraboloidal reflector surface of the dish without any spillage of energy beyond the outer rim of the reflector surface.
Many different prime focus feed antennas have been used heretofore for such purposes. These include feed antennas such as open waveguides, conical or pyramidal horns, dipoles, slotted waveguide arrays, helix antennas, dielectric rod antennas, microstrip antennas, corrugated circular waveguides and conical horns. These devices provide well documented varying levels of overall performance achieved. While the overall function of receiving and collecting the reflected signal as efficiently as possible is essentially the same for all of these types of antennas, the physical principles and the way by which these different antennas function to produce the desired radiation beam or pattern so as to efficiently illuminate the paraboloidal reflector are not nearly the same and differ widely.
Downlink waveguide equipment is presently characterized by the use of waveguide antennas of the type which are sometimes known as scalar feedhorns. Scalar feedhorns generally consist of a waveguide, the radiating aperture of which is surrounded by one or more concentric grooves. The grooves may be peripheral corrugations formed within the radiating aperture of the waveguide or they may be concentrically placed around the outside of the aperture. The nature, number and placement of such corrugations depend upon the particular requirements of the system in which the waveguide is to be used. Until fairly recently, TVRO satellite signals have been transmitted principally in the operating frequency band of from 3.7 to 4.2 GHz, an operating band referred to by persons in the field as the C-band. C-band waveguide antennas are positioned at the focal point of a suitable paraboloidal reflector dish and such antennas have had to demonstrate superior performance characteristics for reception of TVRO signals at C-band. This has been due, for the most part, to the relatively low power at which C-band signals are transmitted from the orbiting satellites used to transmit such television information. The most commonly used scalar rings for C-band TVRO communications are those shown in U.S. Pat. No. Des. 272,910 to Taggart et al., owned by the assignee of the present invention.
In the past few years, some TVRO satellite channels have, for many reasons, also been transmitted at frequencies within the range of from 10.95 to 12.75 GHz, a frequency band referred to by persons in the field as the Ku-band. Thus, some satellite television stations are transmitted in the C-band range, while others are transmitted in the Ku-band frequency range. Accordingly, it had become desirable prior to 1986 for TVRO earth stations to have system components capable of receiving and processing both C-band and Ku-band signals simultaneously without the components used to receive at one frequency interfering with the efficiency of the signal reception at the other frequency.
Prior types of dual frequency feed assembly used heretofore have consisted of C-band and Ku-band waveguides arranged together in a common feed assembly so that at least one of the waveguides is offset from the boresite of the parabolic reflector. Such devices adequately received C-band and Ku-band signals simultaneously but were relatively expensive and occasionally yielded inconsistent reception quality due to offset phase centers of the C-band and Ku-band waveguide apertures.
Accordingly, it has been understood in the TVRO art since at least about 1986 (and in related commercial art long before that) that substantially common phase centers for dual frequency feed assemblies may be achieved by the use of concentric waveguides, the smaller higher frequency waveguide being located coaxially with respect to the larger lower frequency waveguide. There has developed heretofore a proliferation of TVRO and other microwave waveguide junctions consisting of coaxial waveguides for simultaneous reception of multiple frequency ranges.
For example, U.S. Pat. No. 3,864,687 to Walters et al. describes a coaxial horn antenna provided with three cylindrical waveguides 12, 14 and 16 which are progressively sized to provide an inner radiating aperture 18, a concentric intermediate aperture 20 and a concentric outer aperture 22 at the front end of the assembly. The beamwidths of the frequencies propagated within these waveguides are controlled by stepping the forward ends of the horns, with the inner horn projecting furthest. The phase centers of the concentric waveguides are purportedly substantially constant over the band of coverage.
U.S. Pat. No. 3,665,481 to Low et al. discloses a multifrequency feed assembly for use with a single dish reflector. The feed assembly consists of a plurality of coaxial waveguide pipes including a circular inner pipe 16 for receiving the highest frequency signal, an intermediate pipe 18 and an outer pipe 20 for receiving the lowest frequency. The space between the intermediate pipe 18 and the outermost pipe 20 defines a coaxial tracking waveguide 21 containing inwardly projecting probes. Illumination of the dish reflector is effected efficiently by the use of an outer flared horn section 12 for the tracking waveguide and an outer flared horn section 44 for the inner highest frequency waveguide 16. In this arrangement, the innermost and highest frequency waveguide 16 is spaced from all the walls of the surrounding lower frequency waveguide region 21.
U.S. Pat. No. 3,086,203 to Hutchison discloses a waveguide structure for multiple frequencies having an outer circular waveguide 10, a cylindrical core 16 and an inner circular waveguide 22. The cylindrical core and the outer circular waveguide define a coaxial region 18 therebetween. Lower frequency signals are coupled into the coaxial region 18 and are detected therein by probes extending radially into the coaxial space. The coaxial region propagates the coaxial TE11 mode. The inner waveguide 22 propagates signals of a different frequency without interfering with signals in the coaxial region 18. A probe 30 couples signals from the inner waveguide to a receiver 32.
U.S. Pat. Nos. 4,819,005 and 4,821,046 to Wilkes show similar dual frequency microwave feed assemblies for use with a parabolic reflector. Both patents show coaxial circular waveguides where the higher frequency waveguide is disposed in and concentric with the surrounding lower band waveguide. The diameters of the waveguides are adjusted so that the innermost waveguide does not degrade the performance of the lower frequency surrounding waveguide. The preferred frequencies are the C and Ku frequency bands for satellite communications.
U.S. Pat. No. 3,508,277 to Ware et al. discloses the use of two cylindrical waveguides mounted coaxially with respect to each other. Flared horns are provided at the ends of the waveguides for feeding multiple signals to a common load such as a parabolic reflector dish. The patent discloses an inner circular waveguide for transmission of the signals in the upper frequency band and an outer circular waveguide for transmision of the signals in the lower frequency band. Ware et al. deliver the higher frequency signal directly out the back wall of the surrounding lower frequency waveguide.
U.S. Pat. No. 3,325,817 to Ajioka et al. appears to show a dual frequency feed assembly in which a higher frequency pyramidal horn 10 is mounted coaxially within a surrounding lower frequency pyramidal horn 12. The higher frequency horn 10 is centered along the longitudinal axis 14 of the lower frequency horn 12. The signal may be transmitted from (or received by) the higher frequency horn 10 which is spaced from the sidewalls of the surrounding lower frequency waveguide and extends longitudinally through the lower frequency waveguide to deliver the higher frequency signal through the rear wall 24 of the lower frequency waveguide. The presence of the higher frequency waveguide within the lower frequency waveguide along the longitudinal axis of the latter so as to space the former from the sidewalls of the latter provides an uninterrupted signal path for the lower frequency signal, which is detected by a pair of lower frequency probes 26 and 28 located near the rear wall of the lower frequency waveguide 12. The phase centers of the higher and lower frequency feed horns are selected to be as nearly coincident as possible, given the tolerances of the particular reflector systems employed. (Col. 3, lines 7-10).
U.S. Pat. No. 2,425,488 ("'488 patent") to Peterson et al. also discloses the use of a pair of coaxial and concentric pyramidal waveguides for simultaneously receiving signals at different frequencies. The axes of both feed horns coincide. A high frequency pyramidal horn is situated within the interior of a surrounding low frequency pyramidal horn such that the high frequency pyramidal horn is separate or spaced from all of the walls of the low frequency pyramidal horn. The high frequency signal is coupled out laterally through the sidewall of the low frequency waveguide thereby leaving an open waveguide space behind the high frequency waveguide in which a low frequency pick-up probe 24 is located. In the embodiment shown there is added structure in the form of partitions 28 and 29 which provide uniform uninterrupted signal paths for the low frequency signal around the high frequency waveguide. (Col. 2, lines 44-46). The low frequency signal thereby passes around the high frequency feedhorn to the low frequency pick-up probe 24, which is located just in front of the rear wall 25 of the low frequency waveguide. The partitions on the upper and lower sides of the high frequency waveguide are to provide for a smooth electrical path for the low frequency energy to get to the back of the low frequency waveguide for detection by the probe 24. Although the partitions 28,29 physically block part of the open space along the sides of the high frequency waveguide, it would be obvious to use dielectric partitions to support the coaxial high frequency waveguide in an application where the uninterrupted signal path for the lower frequency signal might preferably be annular or coaxial in cross section (i.e., to support a coaxial TE11 mode) such as when circular waveguides are used in place of the pyramidal horns, for certain applications mentioned hereinbelow. By way of example, were the waveguides disclosed in Peterson et al. to be circular in cross section, the space within the low frequency waveguide behind the high frequency waveguide and between the rear wall 25 and the rear point of the higher frequency waveguide would constitute a circular waveguide section and therefore support the dominant TE11 circular waveguide mode common in TVRO applications.
U.S. Pat. No. 4,041,499 ("'499 patent") to Liu shows a dual frequency feed similar to that of Ajioka et al. but which uses coaxial circular waveguides instead of pyramidal horns. Liu et al. disclose a waveguide antenna in which inner and outer waveguides are side-fed by fixed coaxial probes. The inner waveguide is fed with a monopulse signal in the sum or in-phase mode and the outer waveguide is similarly side-fed with a monopulse signal in the difference or out-of-phase mode. In fact, the presence of the circular higher frequency waveguide in Liu et al. defines an uninterrupted signal path for the lower frequency signals in the form of a coaxial transmission line cavity within the surrounding lower frequency waveguide that extends to the rear wall of the lower band assembly. This means that the dominant mode present in the lower band waveguide is the TE11 coaxial waveguide mode.
U.S. Pat. No. 4,785,306 ("'306 patent") to Adams shows a Ku band circular dielectric rod waveguide coaxially mounted in a lower frequency circular waveguide and spaced from all the walls of the surrounding lower frequency waveguide. In this patent, the signal on the coaxial dielectric rod is coupled into a cavity waveguide by bending the dielectric rod at approximately 45 degrees and letting it pass through the side wall of the surrounding lower frequency waveguide. The end of the rod is tapered to provide for efficient launching of the signal into the Ku-band cavity waveguide. The dielectric rod is bent at a 45 degree angle in order to minimize reflections of the C-band signals within the C-band circular waveguide, thus rendering the coaxial Ku-band feed essentially transparent to C-band signals. This patent teaches the use of a Ku-band waveguide coaxially mounted to be spaced from all of the walls of a surrounding lower frequency circular C-band waveguide and the use of a signal transmission means to couple the signal from the coaxially mounted Ku-band waveguide through the side wall of the C-band waveguide. In this arrangement, the C-band signal is detected by a probe situated at the rear of the C-band waveguide in the space behind the coaxial Ku-band waveguide. The C-band signal has an uninterrupted signal path around the Ku-band waveguide to the polarization switch at the back of the C-band waveguide.
An important requirement in a dual frequency feed assembly for frequency re-use satellite systems, and in particular in connection with TVRO systems, is that the system be able to detect signals having different, usually orthogonal, polarizations. One way to meet this objective is to provide components able to switch, upon demand, from one polarization of the incoming signal to the other. For example, this requirement has given rise to the common use in TVRO prime focus feeds of a small rotatable metal probe assembly located at the bottom or back of the waveguide and coupled electrically to the relevant standard rectangular waveguide. Such a probe assembly and feed horn for use at C-band is shown and described in U.S. Pat. No. 4,414,516 to Taylor Howard, owned by the assignee of the present application, although the probe assembly of the '516 Howard patent may be suitably scaled to work at any desirable frequency. The foregoing '306 patent to Adams suggests the use of rotatable probes for the purpose of polarization switching.
U.S. Pat. No. 4,740,795 ("'795 patent") to Seavey (of record in applicant's parent application) discloses a dual frequency coaxial feed assembly for receiving electromagnetic signals at two different frequencies and conveying them to an external signal utilization device. The feed assembly consists of a waveguide for C-band signals having a circular aperture at one end and being closed at the other end. A rotatable dipolar probe is mounted at the closed end of the C-band waveguide for receiving C-band signals entering and propagating within the waveguide from the aperture. The probe, which is within a circular C-band waveguide section, couples the C-band signal to a rectangular waveguide section mounted on the exterior of the C-band waveguide housing. From the rectangular waveguide section, the C-band signal is appropriately amplified and processed. A Ku-band circular waveguide cavity and circular aperture is coaxially and concentrically mounted within the surrounding C-band circular waveguide. This structure enables simultaneous reception of both the C-band and Ku-band frequency ranges. The Ku-band waveguide is smaller in diameter than the surrounding C-band waveguide and it is spaced from all of the working walls of the C-band waveguide. The Ku-band signal is coupled out of the Ku-band circular waveguide by a rotatable dipolar probe element which is supported by dielectric means within the cavity. The Ku signal is coupled through a suitable transmission means to a rectangular waveguide section mounted on the exterior of the C-band waveguide casting. The rotatable dipolar probes are connected to rotate together on a common axis within their respective waveguides. In this Seavey patent, the C-band cavity consists of two portions: a coaxial annular portion surrounding the Ku-band waveguide and a circular waveguide portion behind the Ku-band waveguide in which the rotatable C-band probe is located. The two portions are electrically interconnected by four coaxial lines so that C-band signals incident at the C-band aperture have an uninterrupted signal path through the coaxial C-band cavity around the Ku-band waveguide and into the circular C-band cavity containing the C-band probe detector.
U.S. Pat. Nos. 4,903,037 ("'037 patent") and 5,107,274 to Mitchell et al. (of record in applicant's parent application) and International Application No. PCT/US90/04356 (WO 91/02390) to Blachley (of record in applicant's patent application) describe essentially the dual frequency feed assembly of Seavey in which a pair of circular waveguides 14 and 16 are coaxially mounted such that the smaller higher frequency waveguide 16 is within the larger lower frequency waveguide 14. The waveguides are designed to operate simultaneously in the C and Ku-band frequency ranges. The larger C-band waveguide 14 contains antenna probe 33 to detect the C-band signals and the smaller Ku-band waveguide 16 contains antenna probe 20 to detect the Ku-band signals. Each antenna probe 33 and 20 is coupled to a respective waveguide section 41 and 31 to couple the signals to an external amplifier. In contrast to Seavey, Mitchell et al. and Blachley mount their Ku-band waveguide by means including a coaxial line for coupling the Ku-band signal to the exterior of the C-band waveguide casting. Mitchell et al. and Blachley also utilize a cumbersome harp structure to rotate the entire Ku-band assembly, which includes the probe fixed therein, for polarization switching.
Mitchell et al. and Blachley describe their central purpose as being to avoid degredation of the C-band signals by making the Ku-band cavity substantially "transparent" to C-band. This is seen to be accomplished in two ways: (1) by adjusting the length of the Ku-band waveguide assembly, and (2) by empirically establishing an optimum axial position for the Ku-band assembly within and spaced rearwardly or inwardly from the C-band aperture plane. With respect to the first technique, the '037 patent discloses that the Ku-band cylinder or assembly is approximately 1.6 inches long. (Col. 4, lines 57-59). This length is approximately one-half wavelength of the propagating C-band signal within the surrounding C-band waveguide. With a length near 1.6 inches the Ku assembly operates on the fundamental principal of a "halfwave plug" provided that it is positioned somewhat inwardly of the C-band aperture, as shown in FIGS. 2 and 5-7 of the '037 patent. Under these circumstances, it is well known in the relevant art that, as a halfwave plug, the Ku-band assembly is rendered essentially invisible, or reflection free, by a familiar consequence of two equal, but oppositely phased, reflections. One reflection arises within the C-band cavity at the input side of the assembly and the other substantially equal and cancelling reflection arises at the output side, 1.6 inches further down the C-band cavity. The two reflections essentially cancel each other thereby rendering the Ku-band assembly of 1.6 inches in length essentially invisible to the C-band signals within the waveguide. As shown by Mitchell et al. in FIGS. 8 and 9 of the '037 patent, this placement of the Ku-band assembly inwardly of or "behind" (Col. 4, line 13) the C-band aperture opening has produced an enhancement of the C-band performance relative to such performance in the absence of the Ku assembly. (Col. 4, lines 31-38).
In the structure disclosed by Mitchell et al., moreover, the signal from the Ku-band circular waveguide is coupled to a coaxial transmission line that passes substantially radially or laterally outward from the Ku-band waveguide and through the side wall of the C-band waveguide. In this respect, Mitchell et al. and Adams ('306 patent) disclose well known equivalent structures for the purpose of coupling signal away from the Ku-band waveguide. Mitchell et al. use a radially extending coaxial line and Adams uses a nearly radially extending dielectric rod. As in the '306 patent to Adams, the existence within the Mitchell et al. C-band cavity of the laterally extending Ku-band transmission line prevents one from drawing a line with a pencil completely around the outside of the higher frequency feed horn assembly without being interrupted by the transmission line. Accordingly, Mitchell et al., and Adams disclose coaxial feed assemblies in which the higher frequency feed is spaced from the walls of the surrounding lower frequency waveguide, except for the connecting link represented by the transmission line carrying the higher signal to the exterior of the feed assembly.
Seavey ('795) does not differ in substance from these structures. Seavey happens to use a waveguide cavity, as does Peterson et al. ('488 patent) to convey the detected Ku-band signal to the exterior of the lower frequency waveguide. Seavey simply selected, as a matter of choice, a different partition arrangement for mounting the Ku-band (higher frequency) waveguide within and spaced from the working walls of the C-band (lower frequency) waveguide. In all these structures the mounting means for the Ku-band waveguide is essentially invisible to the C-band signal.
For waveguide assemblies of the type disclosed by Mitchell et al., Seavey ('795) Adams or Peterson et al., moreover, the dominant C-band signal mode is reflected from the rear wall of the C-band circular waveguide to produce a standing wave configuration within the waveguide. The pick-up probe within the waveguide is located at a standing wave maximum to provide efficient coupling or excitation of the mode with or by the probe. This reflection from the rear wall and the location of the probe within the C-band waveguide do not affect the radiation pattern established by the feed assembly.
Finally, Mitchell et al. mount the Ku-band signal launch box on the scalar rings making the illumination characteristics of the feed unadjustable. Accordingly, the mechanism disclosed by Mitchell et al. lacks a means for lowering the noise temperature of the device in response to varying installation parameters.
Ideally, a feed assembly should have a radiation pattern such that it radiates toward the parabolic reflector to illuminate the entire surface with little or no spillage, or loss of radiation around the sides of the reflector. As has been demonstrated by the art disclosed above, the most common choice of a feed assembly for a parabolic reflector is the waveguide. Antennas, L.V. Blake, September 1991, Munro Publishing Company, pp. 264-265.
The radiation pattern of the waveguide is established by the physical size of the aperture opening of the waveguide and the electric field established at that opening. For example, in waveguides of the type utilized by Mitchell et al., the beam of radiation used to illuminate the paraboloidal reflector is dependent upon the electric field established in the aperture of the waveguide by the TE11 mode that is incident on the waveguide aperture. Thus the surface area defined by the aperture rim is in fact the most important working wall of the waveguide. Energy of the appropriate frequency undergoes a transition upon being incident at that surface or wall of the waveguide.
In such waveguide feeds the TE11 mode can be excited by a variety of different means such as by a probe of the type disclosed by Howard or by coupling signal energy into the circular waveguide by means of another waveguide using slot coupling, as in Ware et al., or other means. Such different means for exciting the TE11 mode can be located at any arbitrary distance from the aperture and these means and where they are located have no effect on the radiation pattern produced by the circular waveguide aperture with an incident TE11 mode. It is only the physical size of circular aperture and the electric field established at the aperture by the incident TE11 mode that determines the radiation pattern of a waveguide assembly.
Contrary to the functioning of waveguides, a dipole antenna does not have a radiation pattern focused by a physical aperture and is known to be ill-equipped to the task of illuminating a reflector dish with a focused radiation pattern, as is desirable at the TVRO frequencies of interest. Sometimes a double-dipole endfire array may be used. Blake, supra. But these have been found useful only at significantly lower frequencies than Ku-band and have not been known to work in dual frequency feed assemblies. S. Silver, Microwave Antenna Theory and Design, Vol. 12 of MIT Radiation Laboratory Series, McGraw-Hill, New York, 1949. The dipole array of the present invention therefore cannot be said to have been known or suggested in the art known heretofore.
Another of the important requirements in a feed assembly for a dual frequency, frequency re-use satellite system is that the feed assembly have low cross-polarization characteristics. This is important because of the dual polarized nature of TVRO satellite signals and the relatively closely spaced broadcast satellites in orbit around the earth. Cross-polarization is undesirable because of the possible existence of co-channel cross-talk caused by substantial interference between signals at the same frequency but having orthogonal polarization characteristics.
Whether an antenna assembly has suitable low cross polarization feed characteristics is a complicated function of the particular antenna with which one is concerned. In general it is necessary to have substantially equal E and H plane patterns, i.e., rotational symmetry, in order to achieve low cross-polarization characteristics.
It is understood in the relevant art that the circular waveguide, particularly with the proper mode and a corresponding aperture diameter, provides very good rotational symmetry in its radiation pattern and resulting low cross-polarized feed characteristics. In circular waveguide feeds, the cross-polarization characteristics depend on the relative size of the circular aperture in terms of the relevant wavelength, and upon the mixture of modes that one excites in the aperture of that circular waveguide. Accordingly, it has become recognized in the art that in circumstances where cross polarization cannot be tolerated, such as is generally the case in TVRO reception, circular waveguides having an aperture diameter approximately equal to one wavelength of the frequency of interest are especially usefull because such circular waveguides have been shown to exhibit rotationally symmetrical radiation patterns and relatively low cross polarization characteristics. Such circular waveguides have accordingly come into wide use in TVRO applications.
In contrast, the dipole antenna is well understood to have a radiation pattern that is not rotationally symmetrical and therefore exhibits relatively poor cross-polarization characteristics. As stated above, the dipole has been known heretofore to represent a relatively poor choice for use in a TVRO feed assembly.
Where dipoles have been included heretofore in connection with broadcast television reception from satellites they have mostly appeared as dipolar probes within waveguides, as in the foregoing '795 patent to Seavey and (in some embodiments) U.S. Pat. No. 4,504,836 ("'836 patent") to Seavey. As a further example, U.S. Pat. No. 4,862,187 to Hom (of record in applicant's parent application) discloses the use of dipole elements in a feedhorn for a satellite dish. Hom discloses a feedhorn 10 having a metallic housing 12 which defines a throat 18 formed within a metallic cup 20. Dipole antenna elements 38,40 are situated within the cup 20 which constitutes a waveguide. Thus, the radiation pattern and cross polarization characteristics of the assembly of Hom are determined by the waveguide aperture.
Where dipole antennas have been used heretofore as feeds without a waveguide they have not been seen in dual band assemblies nor to function suitably for TVRO. The '836 patent to Seavey discloses the use of a particular dipole antenna for C-band operation in a single band feed. In one embodiment, the dipole 15 is within and near the rear wall of a circular waveguide 12. In such an embodiment, the radiation pattern and cross-polarization characteristics of the feed would be determined essentially by the nature and function of the waveguide. In another embodiment, as shown in FIGS. 7 and 9, the depth of the circular waveguide cavity is reduced (Col. 4, lines 18-19) and the dipole with its elements drooped is placed outside the face of a corrugated ring structure. Without substantial influence of a waveguide, the corrugated ring structure is required to shape the radiation pattern of the feed. However, good cross polarization results from this embodiment are unlikely in light of FIG. 8 which shows cross polarization in the H plane of this embodiment at about -20 dB when it should theoretically be zero. Even in the 45 degree plane, where the cross polarization lobes are usually a maximum, the desired value for frequency re-use systems should be below -30 dB in order to avoid TV picture interference. Seavey fails to provide cross polarization levels in the 45 degree plane and the levels he does provide do not appear suitable. Thus the Seavey '836 patent does not disclose or suggest the use of the dipole array of the present invention, and especially does not suggest the use of a dipole array in a dual band feed.