Title of Invention

A SINGLE APERTURE MULTIMODE TRACKING CUM COMMUNICATION FEED SYSTEM

Abstract

A multimode monopulse tracking scheme uses circular wave guide higher order modes, TE21 & Orthogonal-TE21 (TE21*), to generate pointing errors in azimuth and elevation planes respectively, while the dominant TEll mode is utilized to generate tracking sum signal as well as communication transmit and receive signals. The feed system requires a single radiating aperture (a conical or a corrugated horn); an innovative and compact cascaded turnstile-junction based multimode coupler for providing efficient coupling of orthogonal TE21 modes, A wave guide taper and a dual band OMT. (Fig 1)

Full Text

Field of invention: The invention relates to development of a single aperture (single horn) tracking cum communication antenna feed system. The invention further relates to use of circular wave-guide higher order modes to generate pointing errors and dominant mode for communication and sum signals. Background of invention: Since the advent of satellite communications, large ground-station antennas have been used to communicate with satellites. These antennas have high gain and thus have narrow beamwidth. With the arrival of multiple beam satellite antennas operating in high frequency bands (Ka-band and beyond), accurate beam pointing has become a major requirement, as the individual beams are extremely narrow. But due to gravitational effect the satellites nominally posses diurnal motion in their orbits which causes the apparent position of the spacecraft to wander in the space. Moreover, ground station antennas suffer perturbations due to mechanical performance and wind velocity. If these effects are uncompensated then they severely affect the performance of a satellite communication link. Hence, large ground station antennas and high efficiency on-board multiple-beam antennas need to have some form of tracking capability to realign them. Various tracking systems exist including manual, programmed and automatic. The manual and programmed tracking systems require operator intervention and are useable only where tracking accuracy requirements are not stringent. The automatic tracking (or auto-tracking) schemes represent closed-loop systems and once acquisition/alignment has been established, tracking continues with no requirement of operator's intervention. Various auto-tracking techniques include Monopulse or Simultaneous sensing, Sequential Amplitude sensing and Electronic Beam-squinting (EBS). All of these auto-tracking schemes rely on receiving a continuous beacon signal transmitted from ground for onboard tracking system or from satellite for ground station tracking system. The received beacon is used to derive the pointing error information, which is used to supply control signals to the antenna servomechanism. The antenna is driven so as to minimize the pointing error and hence null the tracking error. The monopulse technique is the most widely used accurate tracking method. The beacon energy received by the feed system of the tracking antenna is processed to generate a set of three patterns simultaneously, the £ (Sum) pattern, the AEL (Elevation Difference) pattern and the AAZ (Azimuth Difference) pattern. These three patterns together provide an accurate estimation of misalignment in two orthogonal angular coordinates. The conventional monopulse tracking feed system consisted of a 4-horn/5-horn feed and a magic-tee comparator network. They are bulky and not optimum for on-board communication along with tracking. Recently, a 4-probe-coupled single aperture monopulse tracking scheme and Monopulse micro strip antenna array have been reported. Although these schemes provide acceptable tracking performance, they do not provide transmit-receive communication feature. An attractive and lighter weight alterative is the multimode monopulse system that relies on detection of higher order modes excited in a single aperture/horn feed system to determine the pointing error. The sum signal and communication signal are obtained from the dominant mode. Circular wave-guide higher order modes TM01, TE21 and TE01 can be utilized for a circular horn along with dominant TEH mode. The null-on-boresight property of these higher order modes is utilized to generate the difference patterns. The choice of modes depends on the communication system requirement. For a circularly polarized tracking system any one of TM01, TE21 or TE01 modes is sufficient to generate error signals. While for a linearly polarized system two modes are necessary, possible combinations being TM01 & TE21 modes, TM01 & TE01 modes, TE21* & TE01 modes and TE21 & TE21* modes. Different modes require different sensing method owing to different propagation properties, cutoff frequency and field-distribution. Advantage with using TE21 and TE21* modes is an identical mode sensing technique. Various techniques to sense TE21 mode includes the multihole directional coupler and turnstile-junction. The former scheme is useful for wideband (10-20%) tracking requirements but has complex design, long size and high cost owing to stringent fabrication tolerances particularly for Ka-band frequencies and beyond. The later scheme offers efficient coupling for narrower bandwidth ( Summary of the Invention The invention relates to a multimode monopulse tracking scheme that utilizes circular wave-guide higher order modes, TE21 & TE21*, to generate pointing errors in azimuth and elevation planes, respectively. While the dominant TEH mode is utilized to generate the tracking sum signal as well as orthogonal linearly polarized communication transmit and receive signals. The null-on-axis property of the TE21 mode ensures that when the antenna is precisely pointed at the beacon location, no error signal is generated. Slight mispointing excites the TE21 & TE21* modes in the circular conical horn of the feed system, the level of these mode excitations being monotonically related to the angular misalignment from the boresight. These pointing error signals are sensed through an innovative cascaded-turnstile-junction type multimode coupler and supplied to a tracking receiver that in turn drives the antenna to boresight through an antenna servo system. A dual band OMT duplexes the orthogonal linearly polarized communication transmit and receive (along with sum) signals. The advantages of the invention are excellent tracking performance along with communication functions. It eliminates the requirement of separate feed system for tracking function in a communication antenna system. The compact, simple and lightweight design of the feed system makes it a very suitable for on-board satellite applications apart from ground use. Brief Description of Drawings In drawings that illustrate embodiments of the invention, Figure 1 - isometric view of the single aperture multimode tracking cum communication feed system for on-board and ground applications according to the invention. Figure 2 - expanded view of the feed system of Figure-1. Figure 3 - side view of the feed system of Figure-1. Figure 4 - bottom view of the feed system of Figure-1, showing small cross-section of the feed system. Figure 5 - illustrates two sets (8,9) of longitudinal rectangular coupling irises in the cascaded-turnstile-junction (2) of Figure-1. Figure 6 - split view of the E-Plane wave-guide power combiner (3,4) of Figure-1. Description of the Preferred Embodiment: The feed system illustrated in Figure-1 comprises a muitimode circular conical horn (1), a cascaded-turnstile-junction (2), two E-plane wave-guide power combiners (3,4), a circular-to-circular wave guide taper (5), a circular-to-rectangular wave-guide transition (6) and an orthomode transducer (OMT) (7). The cascaded-turnstile-junction (2), the two E-plane wave-guide power combiners (3,4) and the circular-to-circular wave guide taper (5) together forms the multimode coupler (MMC). The multimode coupler is the heart of the invention and is a simple, compact yet efficient alternative to the multihole directional coupler type of mode couplers. The input section of the multimode circular conical horn (1) is an overmoded circular wave guide that allows propagation of TE21 mode at tracking frequency along with the dominant TE11 mode at tracking frequency as well as communication transmit-receive frequency bands. Other higher order modes are not allowed to propagate. In Figure 5 Orthogonal TE21 modes are degenerate modes and their field distributions are spatially rotated by 45° with respect to each other. The cascaded-turnstile-junction (2) has a couple of set of two longitudinal rectangular resonant coupling irises (8,9) located diametrically opposite on a circular wave-guide periphery. The two sets are longitudinally separated from each other by approximately one guide-wavelength (A,g) of TE21 mode and rotated by 45° on circular wave-guide axis with respect to each other. The longitudinal separation aids in fabrication and further improves the isolation between the orthogonal TE21 modes. According to TE21 mode's field property, outputs of coupling irises of a set are combined in opposite phase through an E-plane wave-guide power combiner whose geometry is optimized for impedance matching. This structure rules out possibility of sensing TEll and TM01 modes. The set of coupling irises (9) that lie in the plane of polarization of the sum signal together with the E-Plane wave guide power combiner (4) forms a TE21 mode sensor, while the 45° rotated set (8) forms the TE21* mode sensor with another E-Plane wave guide power combiner (3). Simple Magic-Tees can also be used instead of the E-plane wave guide power combiners, these are connected to the irises in the cascaded-turnstile-junction via round or mitered wave guide bends The input arms of the magic tee are connected to the irises, H-Plane port is terminated and output is taken from the E-plane port. The circular-to-circular wave-guide taper (5) acts as a virtual short for TE21 and TE21* modes and allows propagation of TEll mode at tracking frequency and receive frequency band along with orthogonal - TEll mode at transmit frequency band of communication signals. The position and size of the irises and slope of the circular-to-circular wave-guide taper are optimized to achieve OdB (approx.) coupling of TE21 and TE21* modes in their respective mode sensors and high isolation (>40dB) with TEll mode. Instead of the circular-to-circular wave guide taper a TE21 mode cutoff circular wave guide straight section can also acts as virtual short for TE21 mode and TE21* mode. Any misalignment of the tracking antenna excites the TE21 & TE21* modes in the circular conical horn of the feed system along with dominant TEll mode. All the modes travel down through the horn into the multimode coupler region where signal corresponding to Azimuth pointing error is sensed through the TE21 mode sensor while signal corresponding to Elevation pointing error is sensed through the TE21* mode sensor. The dominant TE11 mode keeps traveling through the multimode coupler without getting affected. A circular-to-rectangular wave guide transition (6) and transforms TEH and orthogonal TEH modes in the tapered circular wave guide section to TE01 and TE10 modes in the common rectangular wave guide section of an orthomode transducer. The OMT (Figure-1, Part-7) consists of a longitudinal step taper section and a wave-guide branching perpendicular to the taper axis. The longitudinal step taper provides an asymmetrical transition of the common rectangular wave-guide cross-section to standard WR51, this forms the transmit port. The wave-guide branching perpendicular to the taper axis leads to a standard WR28 interface that forms the receiving and tracking sum port. Industrial applicability. The feed system is suitable for on-board satellite, ship and ground application. Owing to its compact size and less volume it can be used as one of the feeds in satellite Multiple Beam Antenna for communication. Due to high mode purity resulting in deep nulls in difference patterns, it is used as feed for Inter-Satellite Link (ISL) antennas. It can be used on ships (where the antenna platform is not stable) to track and communicate with satellites. It can be used for applications requiring precision tracking like Radars, Rockets and Aircrafts etc. Finally as it easy to design and cheap, it can be used in developing low cost ground based antennas with low insertion loss. Constructional and Test data. A hardware based on the scheme described above has been developed in Ka-band. The details of the developed feed system are as follows: Tracking Frequency : 29.51 GHz Receive Frequency : 29.6 - 30.2GHz Transmit Frequency : 20.6 - 21.2GF£z Conical Horn Aperture Diameter : 33 mm Conical Horn Input wg Diameter : 10.8mm Iris Dimensions : 4.94mm x 0.4mm Spacing between iris sets : 25.3mm Circular-circular taper : O10.8 to 09.38mm and 22.4mm long Circular-rectangular transition : 09.38mm to 9mm x 5.34mm and 40mm long Tracking Error ports : WR28 interface (7.12mm x 3.56mm) Receive and sum port : WR28 interface (7.12mm x 3.56mm) Transmit port : WR51 interface (12.95mm x 6.48mm) Measured feed performance : Null-depth > 40dB Sum peak-to-Error peak difference ~ 6dB Isolation between all ports > 40dB Return-Loss WE CLAIM: 1. A Multimode Coupler comprising: a cascaded-turnstile-junction (2) having two sets of longitudinal rectangular coupling irises(8,9) located diametrically opposite on a circular wave guide periphery, wherein the said irises (8,9) are longitudinally separated from each other by one guide-wavelength of TE21 mode and rotated by an angle of 45° with respect to each other; two combiner means (3,4) for combining the outputs of both sets of rectangular coupling irises (8,9) providing output ports for calculating the Azimuth Pointing error in TE21 mode and Elevation Pointing error in TE21* mode; and a circular-to-circular wave guide taper (5) or a cutoff circular wave guide straight section that acts as virtual short for TE21 mode and TE21* mode 2. The Multimode Coupler as claimed in claim 1, wherein the said irises (8,9) are rounded-corner rectangular or elliptical. 3. The Multimode Coupler as claimed in claim 1, wherein the said combiner means is a E-plane wave guide power combiners. 4. The multimode Coupler as claimed in claim 1 wherein, the said combiner means is a simple wave-guide Magic-Tees connected to the irises such that H-Plane port is terminated and output is taken from the E-plane port. 5. The Multimode Coupler as claimed in any one of the preceding claims wherein, the input arms of the combiner means (3,4) are connected to the irises in the cascaded- turnstile-junction via round or mitered wave-guide bends. 6. A Single aperture multimode tracking cum communication feed system comprising: multimode conical horn (1) as a single radiating element; multimode coupler as claimed in any of the preceding claims 1-4; circular-to-rectangular wave guide transition (6) which transforms TE11 mode and orthogonal-TEll mode of the circular-to-circular wave guide taper to TE01 mode and TE10 mode respectively in the rectangular cross-section; and a dual band Ortho Mode Transducer (OMT) (7) that duplexes the orthogonal linearly polarized communication transmit and receive (along with sum) signals. 7. The Single aperture multimode tracking cum communication feed as claimed in claim 6, the radiating element is a multimode corrugated conical horn wherein the input circular wave guide allows propagation of TE21 and TE21* modes at tracking frequency along with the dominant TEll mode at tracking frequency and communication receive and transmit frequency bands 8. The Single aperture multimode tacking cum communication feed as claimed in Claim 6 wherein, the said Ortho Mode Transducer (OMT) (7) comprises a longitudinal step taper section and a wave-guide perpendicular to the taper axis 9. The Single aperture multimode tacking cum communication feed as claimed in Claim 6 wherein, in the said Ortho Mode Transducer OMT (7) the taper section is connected to the Standard WR51 interface and the perpendicular branching wave-guide connected to the Standard WR28 interface. 10. A method of operating the tracking cum communication feed comprising the steps of receiving the TE21 and TE11 modes at tracking frequency at the conical horn (1); sensing the TE21 mode by means of coupling irises(9) and E plane wave guide combiner(4) to check for azimuth pointing error; sensing the TE21* mode by means of coupling irises(8) and E plane wave guide combiner(3) to check for elevation pointing error; shorting the TE21 and TE21* by means of the circular- to-circular waveguide taper(5); transforming the TEH and TEH* mode to TE01 and TE01* modes by means of circular to rectangular wave guide section(6); receiving and tracking by means of standard W28 interface connected to the wave guide perpendicular to the taper axis; and transmitting by means of longitudinal step taper section standard WR51 and perpendicular wave guide leading to a standard WR51 interface.

Documents:

2295-che-2006 abstract duplicate.pdf

2295-CHE-2006 ABSTRACT.pdf

2295-che-2006 claims duplicate.pdf

2295-CHE-2006 CLAIMS.pdf

2295-CHE-2006 CORRESPONDENCE OTHERS.pdf

2295-CHE-2006 CORRESPONDENCE PO.pdf

2295-CHE-2006 DESCRIPTION (COMPLETE).pdf

2295-che-2006 descrption (complete) duplicate.pdf

2295-che-2006 drawings duplicate.pdf

2295-CHE-2006 DRAWINGS.pdf

2295-CHE-2006 FORM 1.pdf

2295-CHE-2006 FORM 18.pdf

2295-che-2006 form-18.pdf

2295-che-2006 form-26.pdf

2295-che-2006 form-8.pdf

2295-che-2006-abstract.pdf

2295-che-2006-abstractimage.jpg

2295-che-2006-claims.pdf

2295-che-2006-correspondnece-others.pdf

2295-che-2006-description(complete).pdf

2295-che-2006-drawings.pdf

2295-che-2006-form 1.pdf

2295-che-2006-form 26.pdf

2295-che-2006-form 3.pdf