Abstract:
A high-isolation broadband polarization diverse circular waveguide feed apparatus capable of supporting any arbitrary linear, right-hand circular, left-hand circular or elliptically polarized electromagnetic wave with desirable performance over a broad range of frequencies and small size is disclosed. The waveguide feed employs the combination of a symmetrical shaped conical frustrum waveguide and circular waveguide segments together with a novel arrangement of orthogonal and nonplanar electric field probes and radio frequency impedance posts to achieve broad bandwidth, low cross-polarization when operating in arbitrary linear mode, and high-isolation for rejection of undesired cross-polarization components when operating in circular or elliptical polarization mode. Details of a 10.95-12.7 gigahertz embodiment of the waveguide feed including dimensions are provided. This apparatus is an elegant, simple, compact, and cost effective design that is applicable to a broad family of microwave antennas, but in particular those required to meet minimal radome swept volume requirements.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to microwave radio frequency waveguide feed systems, and more particularly to a high-isolation, broadband, and polarization diverse circular waveguide feed for reception of Direct Broadcast Satellite (DBS) television and Internet satellite downlink services that operate worldwide. 
     The widespread demand for high-quality video, audio, and data communications via satellite has resulted in the need for additional bandwidth and better cross polarization rejection as well as reduced interference from noise or adjacent frequency operation. As a result, satellite broadcast systems are operating over broader and higher frequency ranges and implementing sophisticated methods to reduce interference and improve the intelligibility of communication signals that limit their operating capability. However, the radio frequency apparatus that operate at higher frequencies and with broader bandwidth require considerable design attention and often result in multiple and complicated waveguide feeds in order to account for electric and magnetic field behavior that exists inside the microwave waveguides that propagate their signals. 
     Also, in order to maintain reliable communication, transmit and receive systems must possess polarization compatibility, which is that property of a radiated wave of an antenna that describes the shape and orientation of the electric field vector as a function of time. Polarization compatibility further complicates the waveguide feed design because electromagnetic energy may be transmitted in arbitrary linear, right-hand circular, left-hand circular, or elliptical polarization. 
     It is well known in the art that square waveguides produce mode patterns that allow high efficiency injection or removal of energy for linear polarized electromagnetic waves using probe coupling, which results in orthogonal linear polarizations of high-isolation needed to reduce noise and unwanted adjacent frequency interference. Satellite systems, however, typically operate with circular polarization, which propagates well in circular waveguides, but generates undesirable cross polarization components and poor isolation when using orthogonal probe coupling methods in planar orientation. To minimize cross polarization components that result in a circular waveguide from the two orthogonal polarizations that comprise the circular polarized wave, elaborate conversion methods are employed to transform circular polarized electromagnetic waves. Polarity converters and filters are methods used to condition the circular polarized wave, but have the disadvantage of being difficult to design and possessing high cost and large size. 
     The present invention is a microwave feed assembly of simple, elegant, and scalable design that incorporates the desirable characteristics of broadband operation, polarization diversity, high-isolation between the orthogonal linear polarizations, low insertion losses, small size, and applicability to a broad family of antennas. 
     SUMMARY OF INVENTION 
     The present invention relates to a high-isolation, broadband, and polarization diverse circular waveguide feed for microwave frequency antennas. In one aspect of the invention, the waveguide feed supports transmission or reception of any arbitrary linear, right-hand circular, left-hand circular, or elliptical polarized microwave signal while achieving desirable performance over a wide range of frequencies with small size. In another aspect of the invention, the waveguide feed incorporates high cross-polarization rejection of unwanted TE 11  Mode components when operating in arbitrary linear mode. In yet another aspect of the invention, the waveguide feed employs high probe-to-probe isolation for rejection of undesired cross-polarization when operating in circular or elliptical polarization mode. A waveguide feed assembly is disclosed, which comprises a combination of symmetrical shaped conical frustrum waveguide and circular waveguide segments together with a novel arrangement of orthogonal and nonplanar electric field probes and radio frequency impedance posts to achieve high-isolation, broad bandwidth, and polarization diversity. 
     It is an object of the present invention to provide a microwave waveguide feed system that can transmit or receive arbitrary linear, right-hand circular, left-hand circular, or elliptically polarized electromagnetic waves. 
     It is another object of the present invention to provide a microwave waveguide feed system that will support operation over a broad range of frequencies. 
     It is yet another object of the present invention to provide a microwave waveguide feed system with cross polarization rejection greater than 20 dB. 
     It is yet another object of the present invention to provide a microwave waveguide feed system with probe-to-probe isolation greater than 30 dB when rejecting undesired linear cross polarization of the two orthogonal linear polarizations that comprise circular or elliptical polarized electromagnetic waves. 
     It is a feature of the present invention to provide a waveguide assembly that is polarization diverse for operation with arbitrary linear, right-hand circular, left-hand circular, or elliptically polarized electromagnetic waves. 
     It is yet another feature of the present invention to provide a compact, reliable, and simple to manufacture waveguide assembly that uses common materials and is suitable for reflector type antennas used to meet minimal radome swept volume applications by reducing the axial length of the waveguide assembly. 
     It is an advantage of the present invention to provide a waveguide assembly that is low cost, rugged, and applicable to a broad family of microwave antennas. 
     It is another advantage of the present invention to provide a microwave waveguide feed that can operate as a stand-alone microwave antenna system. 
     It is yet another advantage of the present invention to provide a waveguide assembly that incorporates design characteristics that are scalable to any frequency of microwave operation. 
     These and other objects, features, and advantages are disclosed in the specification, figures, and claims of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the high-isolation broadband polarization diverse circular waveguide feed constructed in accordance with the preferred embodiments of the present. 
     FIG. 2A is cutaway view of the high-isolation broadband polarization diverse circular waveguide feed in FIG. 1 having an exemplary view of component orientation and layout. 
     FIG. 2B is a side cross-section view of the high-isolation broadband polarization diverse circular waveguide feed in FIG. 1 having an exemplary view of component orientation and layout. 
     FIG. 2C is a top cross-section view of the high-isolation broadband polarization diverse circular waveguide feed in FIG. 1 having a exemplary view of component orientation and layout. 
     FIG. 2D is a front view of the high-isolation broadband polarization diverse circular waveguide feed in FIG. 1 having an exemplary view of component orientation and layout. 
     FIG. 3A is an example illustration of dual linear polarization decomposition and electromagnetic signal extraction methodology for an embodiment of the FIG. 1 waveguide feed. 
     FIG. 3B is a first example illustration of circular polarization wave decomposition and electromagnetic signal extraction methodology for a first embodiment of the FIG. 1 waveguide feed. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings wherein like numerals refer to like matter throughout, FIG. 1 shows a perspective view of the high-isolation broadband polarization diverse waveguide feed assembly  140  that incorporates the teachings of the present invention. The embodiment of FIG. 1 will be described with reference to operating ranges from 10.95 GHZ to 12.7 GHZ, X and Ku band, and for communication signals that are transmitted or received in arbitrary linear, right-hand circular, left-hand circular, or elliptical polarization. It is to be understood, however, that the invention is suitable for any broad frequency range and arbitrarily polarized electromagnetic wave transmit or receive system for which waveguides may be selected to meet the criteria described in detail herein. 
     In FIG. 1, the microwave energy of the desired frequency range is shown to propagate through the circular waveguide along the direction of the dofted line  135  in a conventional manner. Circular waveguide section  125  is provided to form an aperture for receiving or transmitting electromagnetic energy of a desired frequency range and is selected to have length and diameter sufficient to meet desired radiation properties of gain, beam width, crosspolarization or the like. Symmetrically shaped tapering conical frustrum waveguide section  115  is provided as a means to transition from circular waveguide section  125  to circular waveguide section  110 , sustain propagation of electromagnetic energy of the desired frequency range, while providing a low impedance path for higher order modes, which become evanescent within the taper region, and is selected to have a larger diameter sufficient to dispose concentrically with the radiation aperture provided by circular waveguide section  125 . The smaller diameter and length of tapered waveguide section  115  are chosen to optimize attenuation of higher-order modes without reaching the waveguide cutoff frequency of the dominant mode of the desired frequency range. Circular waveguide section  110  provides a coupling means to minimize attenuation of the propagated electromagnetic microwave energy of the desired frequency range while providing a transition means for injection or removal of electromagnetic energy from the waveguide, and is selected to have a diameter to dispose concentrically with waveguide section  115  and length to support propagation of electromagnetic waves of the desired frequency range. Circular waveguide termination wall  120  is provided as a means to contain electromagnetic energy within the waveguide, present a low impedance reference plane for electromagnetic energy of the desired frequency range, and is selected to have a diameter sufficient to dispose concentrically with circular wave-guide section  110 . The intersecting waveguide elements  125 ,  115 ,  110 , and  120  may be fabricated in integral unitary relationship from a single piece of metal, casting, or by fusible metals or methods, with material of sufficient conductivity for the frequency of operation and sufficient strength for intended purpose by those persons skilled in the microwave art. For operating ranges between 10.95 GHz to 12.7 GHz, X and Ku band, cylindrically shaped waveguide section  125  is approximately 0.745 lD×1.0 inches, conical frustrum waveguide section  115  is about 0.5 inches in length tapering roughly 3.38° radially from 0.745ID-0.686ID, and cylindrically shaped waveguide section  110  is approximately 0.686ID×1.5 inches. 
     Referring again to FIG. 1, there is shown in the wall of circular waveguide section  110  signal cable connectors ( 100  and  100 ′), highly linear radio frequency (RF) electric (E)-field probes (E-field Probe- 1   130  and E-field Probe- 2   130 ′) and RF impedance posts ( 105  and  105 ′). The signal cable connectors ( 100  and  100 ′) provide a signal transmission means for the electromagnetic energy that is injected or removed from circular waveguide section  110  from the E-field probes ( 130  and  130 ′). However, signal transition means accomplished by the signal cable connectors ( 100  and  100 ′), may take a number of forms, such as by direct connection to low noise amplifiers (LNA) transmitter printed circuit boards, which are readily apparent to one of ordinary skill in the art. E-field probes ( 130  and  130 ′) are used to inject or remove energy from circular waveguide section  110  and are arranged in an orthogonal and nonplanar relationship for signal detection means and for high probe-to-probe isolation when used in conjunction with the RF Impedance posts ( 105  and  105 ′). 
     It should now be noted that the orthogonal and nonplanar relationship of the E-field probes ( 130  and  130 ′) and positioning of RF Impedance posts ( 105  and  105 ′) within circular waveguide section  110  is a novel aspect of this invention that not only permits the electromagnetic signal extraction, but more importantly results in the polarization diverse characteristics of this high-isolation waveguide feed assembly  140 . In order that this aspect of the invention may be properly understood and appreciated, it is essential to first examine the structure that defines the sense of electromagnetic wave polarization. 
     There is shown in FIG. 3 diagrams of the means by which electromagnetic signal energy is extracted by the E-field probes ( 100  and  100 ′) from circular waveguide section  110 . It is a well known relationship that an arbitrary electric field, that oscillates on a straight line within a X-Y reference plane perpendicular to the transmission direction, can be resolved into two orthogonal components, E x , electric field strength in the X-direction, and E y , electric field strength in the Y-direction, that are aligned with a reference coordinate system. FIG. 3A depicts an example illustration  300  of how an arbitrary dual linear polarized wave can be described by two linear orthogonal E-field components E x  and E y , which may have amplitude difference, but no phase variation. Additionally, FIG. 3B shows another example illustration  305  of how a perfectly circular polarized wave can be described by two linear orthogonal field components, E x  and E y , which exhibit identical magnitude and a phase difference of 90°. When the phase difference is +90° the electromagnetic wave is right-hand circular polarized (RHCP), while a phase difference of −90° indicates a left-hand circular polarized (LHCP) electromagnetic wave. There is also shown in FIG. 3B example illustration  305  a calibrated waveguide dispersion phase shift Δ φ  (f) that results from the nonplanar arrangement of E-field probes ( 300  and  300 ′) and whose magnitude is a function of the operating frequency, which is removed in the signal recovery circuitry that interfaces with the waveguide feed assembly. 
     Referring again to FIG.  1  and to the cutaway and section views of FIG. 2, it is readily seen the orthogonal arrangement of the E-field probes ( 130  and  130 ′) permits linear decomposition of any elliptically polarized electromagnetic wave into a vertical component, detected by E-field Probe- 1   130 , and a horizontal component, detected by E-field Probe- 2   130 ′, both having amplitude and phase, which together determine the polarization angle of the electromagnetic wave in circular waveguide section  110 . The nonplanar arrangement of the E-field probes ( 130  and  130 ′) allows for positioning RF Impedance posts ( 105  and  105 ′) in a manner to provide high isolation between the linear decomposed electromagnetic waves detected by the probes. RF impedance posts ( 105  and  105 ′) are constructed with material of sufficient conductivity for the frequency of operation, positioned in-line with each other and parallel to E-field Probe- 1   130 , disposed between E-field Probe- 1   130  and E-field Probe- 2   130 ′, extending through circular waveguide section  110 , and electrically and physically joined to circular waveguide section  110  by fusible metals or methods, interference fit, or other machining method. The configuration, size, spacing, and characteristics of the RF impedance posts ( 105  and  105 ′) are chosen to present a low impedance (short) to vertical polarized signal component energy at E-field Probe- 1   130 , such that vertical polarized signal component energy does not pass through to E-Field Probe- 2   130 ′, and to present high impedance (open) to horizontal polarized signal component energy, which propagates in circular waveguide section  110  to E-field Probe- 2   130 ′. 
     Referring now to the section and cutaway views of FIG. 2, there is shown insulating sleeves ( 205  and  205 ′) comprising a suitable dielectric material known in the art surrounding the E-field probes ( 130  and  130 ′) shafts. The thickness, length, and type of dielectric material chosen for the dielectric encasements ( 205  and  205 ′) and the center pin length and diameter for the E-field probes ( 130  and  130 ′) are chosen to provide optimal impedance matching over the useful bandwidth of electromagnetic energy of the desired frequency range. Affixed concentrically to the tip of E-field probes ( 130  and  130 ′) are electrically and physically coupled isotropic E-field probe enhancements ( 200  and  200 ′), which are fabricated from metal of sufficient conductivity for the frequency of operation, and having size and shape chosen to provide a means to increase the bandwidth of the electromagnetic energy propagating in circular waveguide section  110 . 
     For operating ranges between 10.95 GHz to 12.7 GHz, X and Ku band, E-field probes ( 130  and  130 ′) are approximately 50 mils in diameter and protrude about midway into circular waveguide section  110 , RF impedance posts ( 105  and  105 ′) approximately 50 mils in diameter, located nearly two-thirds the distance from E-field probe  130  to E-field  130 ′, and positioned laterally in circular waveguide section  110  proportionally dividing its diameter into three roughly equal segments, insulating sleeves ( 205  and  205 ′) constructed of 56 mil thick Teflon material having length that is approximately flush with the interior surface of circular waveguide section  110 , and E-field probe enhancements ( 200  and  200 ′) resembling circular disks with approximate diameter of 90 mils and thickness about 20 mils. 
     It is understood that, while the detailed drawings, specific examples, and particular values given describe preferred exemplary embodiments of the present invention, they are for the purpose of illustration only. The apparatus and method of the present invention is not limited to the precise details of the conditions disclosed. Accordingly, changes may be made to the details disclosed without departing from the spirit of the invention the scope of which should be determined by the following claims.