WO2012172565A1

WO2012172565A1 - Wideband waveguide turnstile junction based microwave coupler and monopulse tracking feed system

Info

Publication number: WO2012172565A1
Application number: PCT/IN2012/000413
Authority: WO
Inventors: Rajeev Jyoti; Khagindra Kumar SOOD; Shashank SAXENA; Jigar PANDYA; Yogesh Harshadray TRIVEDI
Original assignee: Indian Space Research Organisation
Priority date: 2011-06-14
Filing date: 2012-06-12
Publication date: 2012-12-20

Abstract

The present invention relates to a waveguide turnstile junction based coupler (1) for selectively coupling the higher order modes of propagating microwave energy while allowing the dominant mode of said microwave energy to propagate unperturbed comprising of: - an over-moded circular waveguide (2), - a single-mode circular waveguide (5), - a taper region (3) of circular cross-section having two ends, a broad end (6) and a narrow end (7), the broad end (6) of the taper region (3) is connected to said over-moded circular waveguide (2), while the narrow end (7) of the taper region (3) is connected to said single-mode circular waveguide (5) of smaller cross-section after tapering down in the axial direction; - a branch-coupling section (4) comprised of a plurality of equispaced coupling arms (10) symmetrically disposed around the circumference of the junction of said over-moded circular waveguide (2) and said taper region (3); each said coupling arm (10) is in signal communication with the over-moded circular waveguide section (2). The present invention also provides a monopulse tracking feed system for generating tracking sum signal and tracking difference signal to be used for directing an antenna.

Description

"WIDEBAND WAVEGUIDE TURNSTILE JUNCTION BASED MICROWAVE COUPLER AND MONOPULSE TRACKING FEED SYSTEM"

Field of invention:

The present invention relates to a wideband waveguide turnstile junction based microwave coupler for selectively coupling the higher order modes of propagating microwave energy while allowing the dominant mode of said microwave energy to propagate unperturbed. The present invention also relates to a monopulse tracking feed system for generating tracking sum signal and tracking difference signal to be used for directing an antenna.

Background of invention: Communication satellites generally employ autotracking systems to achieve accurate antenna pointing for point-to-point communication. One of the most preferred and accurate autotracking systems is Monopulse system. In conventional Monopulse systems 4/5-horn feed system is utilized. However, in advanced systems a Single Aperture Multimode is preferred where higher-order modes are used to generate Difference patterns, independent of Sum pattern, from a single horn feed system. This results in an antenna system having high efficiency, compact structure, low loss, less weight, low aperture blockage and excellent boresight stability. Various combinations of higher-order modes of the circular waveguide, like TMoi/TE₂i_; TE₂i/TE2i*, TE₂i*/TE₀i etc. may be used for extracting errors in orthogonal planes. The major advantage of the combination of TE21/TE21* modes is that being degenerate modes one is able to maintain the phase equality over the entire frequency band and use similar coupling mechanism and structures for both the modes. Most of the Monopulse systems utilize a carrier signal within the entire communication receive frequency band as the beacon signal that may be linearly or circularly polarized. Hence, a TE21 mode coupler that works for bandwidth of 10-15% is required to generate linear/ circularly polarized tracking signals.

Basically, two types of TE21 mode couplers exist:

a) Travelling Wave Coupler (TWC), and

b) Turnstile Junction (TJ).

Turnstile junction (TJ), reported till-date, has narrow (<3%) bandwidth, while TWCs reportedly work upto 40% bandwidth. Moreover, for moderate bandwidth requirements (upto 15%) a TWC has been the only available option, till present. US patent document 4566012 discloses a TWC, exclusively for orthogonal TE21 modes, based on S.E. Miller's Coupled Wave Theory [1, 2, 3]. It employs four auxiliary rectangular waveguides symmetrically oriented around the periphery of a circular waveguide with a series of coupling slots along the length of the common wall. The coupling between ΎΕ10 mode of the rectangular waveguide with the desired TE21 mode of the circular waveguide and isolation with undesired modes like TEn, TM₁₁ modes depend on the geometry (size, shape & spacing) of the coupling slots and energy distribution function along the coupling length. The outputs of these four auxiliary lines are connected to a 4: 1 Magic-Tee Combining Network in proper phase and orientation to construct a TE21 mode. Orthogonal-TE2i mode (TE21) is generated with a similar set of four auxiliary lines and a Magic-Tee network rotated 45° around the periphery of the circular waveguide. Hence, the complete coupler consists of a circular waveguide and eight peripheral rectangular waveguides each with 10 to 40 coupling slots, depending upon the bandwidth required. The disadvantages associated with such coupling mechanism are as follows:

(i) Amplitude and phase imbalance between individual coupling arms, due to deviation in any of the large number of coupling slots in each arm, leads to Null-shift with respect to Sum-peak even with a balanced Magic-Tee network resulting in the antenna getting locked off-boresight &/or poor aperture efficiency.

(ii) Higher TE21 mode Insertion-Loss due to long coupling region leads to deterioration of SNR for the Difference-pattern signal.

(iii) Heavy, bulky and excessively long construction makes it unsuitable for On-board spacecraft use.

(iv) Complicated design and fabrication tolerances on large number of coupling slots leads to high cost.

In US patent documents US5402089, US5410318 and US5736907 simplified TWCs with four or two peripheral rectangular waveguides instead of eight have been disclosed. But such arrangements have following disadvantages-

(i) Asymmetric coupling mechanism, leading to depolarization of TEii mode, causes increase in cross-polarization /axial ratio for linear/ circular polarizations, respectively.

(ii) Increased number of coupling slots per peripheral waveguide arm for the same coupling requirements leads to further increase in the length of the coupler.

The turnstile junction couplers disclosed in prior publications US6657516 and US2002/0187760A1, couples TEn modes i.e. TEn/TEu* and is utilized for communication uplinking and downlinking. Turnstile junction based tracking couplers that utilize the hybrid combination of TE21 + TM01 modes for generating elevation-plane tracking signal and TE12 mode for azimuth-plane tracking signal are also known from US patent documents US3906508, US3936838 and US4030048.

These devices have following disadvantages:

- they are inherently narrowband because of the requirement of phasing between TE21 & TM01 modes, phasing between TE12 & hybrid TE21+TM01 modes and mode compensation for higher-order modes like TE13 etc.

- they are not suitable for tracking , cum transmit /receive communication as the larger-end of the turnstile junction is immensely over-moded (to allow TEi2 mode) leading to conversion of TEn mode into undesirable modes like TMn etc. which can severely deteriorate the crosspolarization performance of the horn.

The turnstile junction disclosed in US patent document US4742317 generates TE21 mode by developing standing waves in a cavity that is tuned to the resonant frequency of the TE21 mode; while in US publication US2005/0007287A1, and in EP patent document EPOO 14692 , and in Indian patent publication 2295/CHE/2006A describe turnstile junctions for TE21 mode generation through resonant coupling slots located entirely in an over-moded circular waveguide with a short-circuit plane for TE21 mode created by a taper. Hence, prior art for TJ type TE21 mode coupler is strictly resonant in nature that limits their operational bandwidth to 2-3% only.

With a view, therefore, to overcome the disadvantages associated with conventional turnstile based junctions the inventors felt the need of developing a turnstile junction coupler t at would fill up the gap between the narrowband turnstile junction based couplers and wideband traveling wave coupler and may be termed as wideband turnstile junction based coupled that would work over moderate (15-20%) bandwidth. Such coupler would selectively couple the higher order orthogonal modes, such as TE21 modes, while maintaining isolation from the dominant modes carrying communication signals, such as TEn modes. Such coupler can be used as tracking cum communication uplink and downlink coupler.

Summary Of The Invention:

Accordingly, the present invention provides a wideband waveguide turnstile junction based microwave coupler for selectively coupling the higher order modes of propagating microwave energy while allowing the dominant mode of the said microwave energy to propagate unperturbed unperturbed comprising of:

an over-moded circular waveguide to allow multiple modes to propagate at a given frequency band,

a single-mode circular waveguide to allow only dominant mode to propagate at said frequency band ;

- a circular cross-section taper region in which the higher order modes of the propagating microwave energy become evanescent and get reflected during receive mode of operation of the coupler; said taper region having two end, a broad end and a narrow end , the broad end of the taper region is connected to said over-moded circular waveguide while the narrow end of the taper region is connected to said single- mode circular waveguide after tapering down in the axial direction;

a branch-coupling section for coupling the higher order modes of propagating microwave energy that are reflected from said circular cross-section taper, said branch-coupling section is comprised of a plurality of equispaced coupling arms symmetrically disposed around the circumference of the junction of said over-moded circular waveguide and said taper region; each said coupling arm is in the form of a rectangular waveguide, and is in signal communication with the over- moded circular waveguide and is provided with:

- a longitudinal coupling slot, for signal communication, opening into the junction region of said over-moded circular waveguide and said taper region,

- a rectangular waveguide interface for providing the output of respective coupling branch; and

- and an impedence matching section extending between said longitudinal coupling slot and said rectangular waveguide interface.

Preferably the higher order modes propagating through the coupler are TE21 modes, i.e. TE21 and TE21* modes, while dominant mode is TEn mode, and the impedence matching and coupling takes place for TE21 modes only. According to a preferred embodiment of the invented microwave coupler said taper region is conical in shape with linear profile, and said coupling arms are disposed perpendicular to the axes of said over-moded circular waveguide section and said taper region. Preferably, the longitudinal coupling slot provided with each coupling arm extends from said over- moded circular waveguide section into said taper region and opens into said over-moded circular waveguide at the junction of the over-moded circular waveguide and said taper region.

According to a particular embodiment of the invention said branch- coupling section is comprised of a set of four equispaced coupling arms oriented in the directions of polarization of orthogonal TEn modes. According to another embodiment of the invented waveguide turnstile junction based coupler, the branch- coupling section is comprised of two sets of four equispaced coupling arms; a first set of coupling arms and a second set of coupling arms; said first set of coupling arms is oriented in the direction of polarization of orthogonal TEu modes while said second set of coupling arms is oriented at 45° with respect to the first set of coupling arm. Thus, each coupling arms of said first set of coupling arm couples ../4^th of the propagating TE21 mode while each coupling arms of said second set of coupling arm couples 1/ 4^th - of the propagating TE21* mode signals and thus said taper region becomes a TE21 cut-off taper.

By placing the coupling slots in the TE21 cut-off taper region their effective length is varied over the entire frequency band of operation. Advantageously, since the impedance of the TE21 mode signal varies along the length of the tapered region and depends on the cross- sectional dimension of the taper at a particular position, impedance matching is possible over a much wider frequency band as compared to prior art TJs. Preferably, the coupling arms are integrally formed with said over-moded circular waveguide section and with said taper region.

It is advantageous to have a rectangular profile for the coupling slots and a stepped profile for the impedence matching section of the coupling arms. The coupling and impedance matching in the invented coupler depends on the optimization of various design parameters like the width, length and location of the coupling slots, diameter of the over-moded and smaller circular waveguide sections along with the profile and length of the taper region. According to a preferred embodiment of the invention, a linear taper with flare angle 13.2° was selected to achieve TE21 mode coupling over frequency range of 10.95GHz-12.75GHz. One skilled in the art would know how to optimize these parameters for a particular frequency band.

The invention also provides a monopulse tracking feed system for generating tracking sum signal and tracking difference signal to be used for directing an antenna; said monopulse tracking feed system comprising of:

- an antenna feed horn for conveying the propagating microwave energy,

- a wideband waveguide turnstile junction based microwave coupler for separating higher order modes of microwave energy, being propagated, from multiple modes of microwave energy while allowing the dominant mode to propagate unperturbed;

- atleast a magic-tee network communicatively connected to said microwave coupler,

- a pair of tracking error ports, a first error port and second error port, and

- an orthomode transducer and a diplexer network for receiving and transmitting the dominant mode;

said microwave coupler comprises of:

a circular cross- section taper region in which the higher order modes of the propagating microwave energy become evanescent and get reflected during receive mode of operation of the coupler; said taper region having two end, a broad end and a narrow end , the broad end of the taper region is connected to said over-moded circular waveguide while the narrow end of the taper region is connected to said single-mode circular waveguide after tapering down in the axial direction;

- a branch-coupling section for coupling the higher order modes of propagating microwave energy that are reflected from said circular cross-section taper, said branch-coupling section is comprised of a plurality of equispaced coupling arms symmetrically disposed around the circumference of the junction of said over-moded circular waveguide and said taper region; each said coupling arm is in the form of a rectangular waveguide, and is in signal communication with the over-moded circular waveguide and is provided with:

According to a particular embodiment of the invented monopulse tracking feed system the higher order modes propagating through the coupler are TE21 modes, i.e. TE21 and TE21* modes, while dominant mode is TEn mode, and the impedence matching and coupling in said taper region takes place for TE21 modes only.

According to a preferred embodiment of the invented monopulse tracking feed system said taper region is conical in shape with linear profile and said coupling arms are disposed perpendicular to the axes of said over- moded circular waveguide section and said! taper region. Preferably, the

i

longitudinal coupling slot provided with each coupling arm extends from said over-moded circular waveguide into saicl taper region and opens into said over-moded circular waveguide at thej junction of the over-moded circular waveguide and said taper region.

According to a particular embodiment of the monopulse tracking feed system said branch-coupling section is comprised of a set of four

i

equispaced coupling arms oriented in thej direction of polarization of orthogonal TEn mode.

According to a particular embodiment, the invented monopulse tracking feed system is provided with a single magic tjee network and wherein said branch-coupling section is comprised of a set of four equispaced coupling arms oriented in the direction of polarizatiojn of orthogonal TEn modes; said magic-tee network combines the outputs of the coupling arms in suitable phase relationship to generate the orthogonal TE21 mode signal.

According to another embodiment, the invented monopulse tracking feed system is provided with a pair of magic tee networks, a first magic-tee network and a second magic-tee network, ! and wherein said branch- coupling section is comprised of two sets of four equispaced coupling arms; a first set of coupling arms and a second set of coupling arms; said first set of coupling arms is oriented in the direction j of polarization of orthogonal TEn modes while said second set of coupling arms is oriented at 45° with respect to the first set of coupling arm; said first magic-tee network combines the outputs of the first set of coupling arms in suitable phase relationship to generate the orthogonal TE21 niode signal, while the second

1

magic-tee network combines the outputs of the second set of coupling arms in suitable phase relationship to generate !the orthogonal TE21* mode

i

signal. Thus, each arm of said first set of coupling arm couples 1 /4^th of the propagating TE21 mode while each arm of said second set of coupling arm couples .. /4^th of the propagating TE21* mode signals and said taper region is a TE21 cut-off taper. This symmetry in the structure of the invented coupler, alignment of the coupling slots and phasing in the magic-tee combining network allows the dominant TEn mode to pass through the taper/ coupling region to the single-mode circular waveguide without getting perturbed thereby resulting in high isolation between Sum and Error ports.

According to a preferred embodiment of the invented monopulse tracking feed system, each said magic tee network comprises of four H-Plane S- shapes bends, four E-Plane 135° bends, a pair of H-Plane 90° bends, a pair of E-Plane 90° bends and three magic-tees; all with rectangular waveguide ports.

In a particular embodiment of the invented monopulse tracking feed system, the generated TE21 mode signal is combined separately with TM01 mode coupler to generate tracking- error signal in respective azimuth and elevation planes. In another embodiment of the invented system, generated TE21* mode signal is combined separately with TE01 mode couplers to generate tracking-error signals in respective elevation and azimuth planes.

According to a preferred embodiment of the invented system, said coupling arms are integrally formed with said over-moded circular waveguide and with said taper region. Preferably, said longitudinal coupling slot has a rectangular profile. The impedence matching section of the waveguide coupler used in the invented system may have a stepped profile. According to a preferred embodiment of the invention, a linear taper with flare angle 13.2° was selected to achieve TE21 mode coupling over frequency range of 10.95GHz-12.75GHz. One skilled in the art would know how to optimize these parameters for a particular frequency band.

Brief Description of the Drawings:

For better understanding an illustrative embodiment of the invention will now be described with reference to the _. accompanying drawings. It will however be appreciated that the embodiment exemplified in the drawings is , merely illustrative and not limitative to the scope of the invention.

Figure 1 is a schematic perspective view of the invented waveguide turnstile junction based coupler, according to a particular embodiment.

Figure 2 is a schematic side view of the invented coupler together with a feed horn, according to a particular embodiment.

Figure 3 is a schematic cross-sectional view of the invented coupler in transverse direction, according to a particular embodiment. Figure 4 is a schematic cross-sectional view of the invented coupler in longitudinal direction, according to a particular embodiment.

Figure 5 is a schematic cross-sectional view of a coupling arm of the branch coupling section of the invented coupler, according to a particular embodiment.

Figure 6 shows the electric field distribution of dominant mode ΤΕπ propagating unperturbed through invented coupler and that of the higher order modes TE21 and TE21*, produced as output of the invented coupler, according to an embodiment of the invention. Figure 7 schematically shows a perspective view of an assembly of the invented coupler with magic tee networks, according to an embodiment of the invention.

Figure 8 is a schematic top view of the assembly depicted in figure 7.

Figure 9 is a block diagram of the as sembly of the invented coupler with magic tee network, according to a particular embodiment of the invention.

Figure 10 is a block diagram of the assembly of the invented coupler with magic tee network, according to another embodiment of the invention.

Figure 11 shows the measured return i-loss performance of a particular embodiment of the invented coupler with a wideband-corrugated horn

Figure 12 shows the measured isolation between the between sum and difference channels of a particular embodiment of the invented monopulse tracking feed system.

Figure 13 shows the measured trackihg pattern of a particular embodiment of the invented monopulse tracking feed system.

Brief description of the drawings;

For better understanding, an illustrative embodiment of the invention will be described with reference to the accompanying drawings. It will however be appreciated that the embodiments) exemplified in the drawings is merely illustrative and not limitative to the scope of the invention, because it is quite possible, indeed often desirable! , to introduce a number of variations in the particular embodiment that has been shown in the drawings Detailed description of the drawings:

Referring to figures 1 to 4 of the accompanying drawings, the invented wideband waveguide turnstile junction based coupler (1) comprises of an over-moded circular waveguide (2), a taper region (3) having a circular cross- section (shown in figure-4 only), a branch coupling section (4) and a single-mode circular waveguide (5) of smaller diameter than that of the over-moded circular waveguide (2). Referring to figure 4, the taper region (3) has two ends, a broad end (6) connected to the over-moded circular waveguide (2) and a narrow end (7) connected to the single-mode circular waveguide (5) after tapering down in the axial direction. An antenna feed horn (8) is connected to the outer end of the over-moded circular waveguide(2) as seen in figure 2 and an Orthomode Transducer may be connected to the of the second waveguide (5) that forms the sum port (9).

The branch coupling section (4) is formed of two sets of four equispaced coupling arms (10a to lOh) disposed symmetrically around the junction of said first circular waveguide (2) and said taper region (3). The first set of coupling arms (10a, 10c, lOe and lOg) is oriented at 45° with respect to the second set of coupling arms (10b, lOd, lOf and lOh). In the description, which follows hereinafter, the coupling arms have sometimes been commonly referred, for the sake of convenience, by the reference numeral 10. The coupling arms (10a to 10h) are perpendicular to the axes of said over-moded circular waveguide section (2) and said taper region (3).

As shown in figure 3 of the accompanying drawings, the coupling arms (10a to lOh) are in signal communication with the over-moded circular waveguide section (2). Each of the coupling arms (10) is in the form of a rectangular waveguide (11) and is provided with a rectangular longitudinal coupling slot (12) and an impedence matching step-transition section (13). The impedence matching step-transition section (13) extends between the coupling slot (12) to the interface of the rectangular waveguide (11).

According to a preferred embodiment of the invention the turnstile junction based coupler (1) is a waveguide device that separates microwave energy in the TE21 mode from the over-moded circular waveguide (2) in such a manner as to avoid perturbation to the TEu mode. The over-moded circular waveguide (2) allows propagation of TEn as well as TE₂1 modes. The dominant TEn mode from the horn (8) passes through the over-moded circular waveguide (2) and taper (3) and is supplied to a sum-port (9). The orthogonal-TE2i modes simultaneously become evanescent within the taper region (3) and are reflected and coupled to the respective coupling arms (10a to lOh). The rectangular coupling slots (12) inductively couple the axial component (H_z) of the magnetic fields of the TE21 modes to the transverse component (H_x) of the TE10 mode in the rectangular waveguides (11). Thereby each arm of the two sets of four spatially orthogonal coupling arms (10) couples ¼^Λ of the respective TE21 mode signal. The orthogonal TE21 modes are degenerate modes and their field distributions are spatially rotated by 45° with respect to each other as shown in figure 6. The output of the set of four spatially orthogonal coupling branches (10a, 10c, lOe and lOg) that are oriented in the direction of polarization of orthogonal TE1 1 modes are combined in suitable phase through the Magic-Tee network (14) to generate the complete TE21 mode signal. Similarly the other set of four spatially orthogonal coupling branches (10b, lOd, 10f, lOh) that are oriented 45° with respect to the first set are combined in suitable phase through another Magic-Tee network (15) to generate the complete TE21^* mode signal. These two outputs form the two Difference Ports (16 and 17) where Tracking Error signals in orthogonal planes, Azimuth 8s Elevation respectively, are obtained. The present invention also provides a monopulse tracking feed system. When the invented coupler is used in a tracking feed system, each set of coupling arms (10a, 10c, lOe and lOg) or (10b, lOd, lOf, and lOh) is communicatively connected to magic tee network (14 and 15) as shown in figures 7 to 10 of the accompanying drawings. Thus in the tracking feed system a pair of magic-tee networks (14 and 15) is used to combine the outputs of the coupling arms (10) in suitable phase relationships to construct the complete signal of the higher-order TE21 modes. Each magic-tee network (14 or 15) is basically a 4: 1 power combiner network that combines the components/ signals from the four coupling (10a, 10c, lOe and lOg) or (10b, lOd, lOf, and lOh) to form the complete TE₂₁/TE₂i* mode. Each magic-tee network (14 or 15) consists of four H-Plane. S-shapes bends (18), four E-Plane 135° bend (19), a pair of H-Plane 90° bends (20), a pair of E-Plane 90° bends (21) and three magic-tees (22), all with rectangular waveguide ports as shown in figures 7 and 8.

The operation of a magic-tee network (15) in combining the outputs of a set of four coupling arms (10b, lOd, 1 Of, and lOh) of the invented coupler (1) has been schematically shown in figure 9 for TE21* mode. The center region shows the electric field distribution of the TE21* mode. This arrangement of the components of the magic tee network (14 and 15) i.e. the bends (18, 19, 20 & 21) and the magic-tees (22) (not shown in figure 9), ensures that the TE21/TE21* modes are generated with full integrity and high purity.

Another possible configuration of the coupler (1) and magic-tee networks (14 and 15) assembly for generating orthogonal TE21 modes has been shown in figure 10.

The orientation of equi-phased electric field in the combining network has been shown by arrows in figure 8, 9 and 10. Measured return-loss performance of a particular embodiment of the invented coupler with a wideband-corrugated horn has been graphically shown in figure 11.

The magic-tees (22) have been specially designed to achieve return-loss better than 25dB, amplitude and phase imbalance better than O. ldB and 5° respectively, and port-to-port isolation better than 35dB over the entire tracking frequency band (> 15% bandwidth; example 10.95-12.75GHz). The measured isolation between the between sum and difference channels of a particular embodiment of the invented monopulse tracking feed system has been graphically shown in figure 12 and the measured tracking pattern has been illustrated in figure 13. As already mentioned, the foregoing description is illustrative of the invention and not limitative to its scope, because it will be apparent to persons skilled in the art to device other alternative embodiments without departing from the broad ambit of the disclosures made herein.

Claims

We claim:

1. A wideband waveguide turnstile junction based microwave coupler for selectively coupling the higher order modes of propagating microwave energy while allowing the dominant mode of said microwave energy to propagate unperturbed comprising of:

a circular cross-section taper region in which the higher order modes of the propagating microwave energy become evanescent and get reflected during receive mode of operation of the coupler; said taper region having two end, a broad end and a narrow end , the broad end of the taper region is connected to said over-moded circular waveguide while the narrow end of the taper region is connected to said single- mode circular waveguide after tapering down in the axial direction;

- a longitudinal coupling slot, for signal communication, opening into the junction region of said over-moded circular waveguide and said taper region, - a rectangular waveguide interface for providing the output of respective coupling branch; and

- and an impedence matching section extending between said longitudinal coupling slot and said rectangular waveguide interface .

2. The microwave coupler as claimed in claim 1 wherein said higher order modes are TE21 modes, TE21 and TE21*, and said dominant mode is TEn mode; and wherein coupling and impedence matching takes place for the TE21 modes.

3. The microwave coupler as claimed in any of claims 1 and 2, wherein said circular cross-section taper region is conical in shape with linear profile.

4. The microwave coupler as claimed in any of claims 1 to 3, wherein the longitudinal coupling slot provided with each coupling arm extends from said over-moded circular waveguide into said taper region and each coupling arm opens into said over-moded circular waveguide at the junction of the over-moded circular waveguide and said taper region.

5. The microwave coupler as claimed in any of claims 1 to 4, wherein said coupling arms are disposed perpendicular to the axes of said over-moded circular waveguide section and said taper region.

6. The microwave coupler as claimed in any of claims 1 to 5, wherein said branch-coupling section is comprised of a set of four equispaced coupling arms oriented in the direction of polarization of orthogonal TE11 modes.

7. The microwave coupler as claimed in any of claim 1 to 6, wherein said branch-coupling section is comprised of two sets of four equispaced coupling arms; a first set of coupling arms and a second set of coupling arms; said first set of coupling arms is oriented in the direction of polarization of orthogonal TEu modes while said second set of coupling arms is oriented at 45° with respect to the first set of coupling arm.

8. The microwave coupler as claimed in any of claims 1 to 7, wherein each coupling arms of said first set of coupling arm couples 1/ 4^th of the propagating TE21 mode while each coupling arms of said second set of coupling arm couples 1/4^Λ of the propagating TE21* mode signals and said taper region is a TE21 cut-off taper.

9. The microwave coupler as claimed in any of claims 1 to 8, wherein said coupling arms are integrally formed with said over-moded circular waveguide section and with said taper region.

10. The microwave coupler as claimed in any of claims 1 to 9, wherein said longitudinal coupling slot has a rectangular profile.

11. The microwave coupler as claimed in any of claims 1 to 10, wherein said impedence matching section has a stepped profile.

12. The microwave coupler as claimed in any of claims 1 to 11, wherein said taper region has a flare angle of 13.2°.

13. The microwave coupler as claimed in any of claims 1 to 12, wherein said microwave coupler couples TE21 mode over frequency range of 10.95GHz- 12.75GHz.

14. A monopulse tracking feed system for generating tracking sum signal and tracking difference signal to be used for directing an antenna; the dominant mode is used for generating tracking sum signal and higher order modes for tracking difference signals in azimuth 86 elevation planes; said monopulse tracking feed system comprising of:

- an antenna feed horn for conveying the propagating microwave energy,

- a pair of tracking error ports, a first error port and second error port, and

said microwave coupler comprises of:

a circular cross-section taper region in which the higher order modes of the propagating microwave energy become evanescent and get reflected during receive mode of operation of the coupler; said taper region having two end, a broad end and a narrow end , the broad end of the taper region is connected to said over-moded circular waveguide while the narrow end of the taper region is connected to said single-mode circular waveguide after tapering down in the axial direction;

a branch-coupling section for coupling the higher order modes of propagating microwave energy that are reflected from said circular cross-section taper, said branch-coupling section is comprised of a plurality of equispaced coupling arms symmetrically disposed around the circumference of the junction of said over-moded circular waveguide and said taper region; each said coupling arm is in the form of a rectangular waveguide, and is in signal communication with the over-moded circular waveguide and is provided with:

The monopulse tracking feed system as claimed in claim 14 wherein said higher order modes are orthogonal TE21 modes , TE21 and TE2i*and said dominant mode is TE11 mode; and wherein coupling in said taper region and impedence matching takes place for orthogonal TE21 modes.

The monopulse tracking feed system as claimed in any of claims 14 and 15, wherein said circular cross-section taper region is conical in shape with linear profile.

17. The monopulse tracking feed system as claimed in any of claims 14 to 16, wherein the longitudinal coupling slot provided with each coupling arm extends from said over-moded circular waveguide into said taper region, and each coupling arm opens into said over-moded circular waveguide at the junction of the over-moded circular waveguide and said taper region.

18. The monopulse tracking feed system as claimed in any of claims 14 to 17, wherein coupling arms are disposed perpendicular to the axes of said over-moded circular waveguide section and said taper region.

19. The monopulse tracking feed system as claimed in any of claims 14 to 18, wherein said monopulse tracking feed system is provided with a single magic tee network and wherein said branch-coupling section is comprised of a set of four equispaced coupling arms oriented in the direction of polarization of orthogonal TEn mode; said magic-tee network combines the outputs of the coupling arms in suitable phase relationship to generate the TE21 mode signal.

20. The monopulse tracking feed system as claimed in any of claims 14 to 18, wherein said monopulse tracking feed system is provided with a pair of magic tee networks, a first magi-tee network and a second magic-tee network, and wherein said branch-coupling section is comprised of two sets of four equispaced coupling arms; a first set of coupling arms and a second set of coupling arms; said first set of coupling arms is oriented in the direction of polarization of orthogonal TEn mode while said second set of coupling arms is oriented at 45° with respect to the first set of coupling arm; said first magic-tee network combines the outputs of the first set of coupling arms in suitable phase relationship to generate the orthogonal TE21 mode signal, while the second magic-tee network combines the outputs of the second set of coupling arms in suitable phase relationship to generate the orthogonal TE21* mode signal.

21. The monopulse tracking feed system as claimed in any of claim 19 and 20 , wherein generated TE21 mode signal is combined separately with TM01 mode coupler to generate tracking-error signals in respective azimuth and elevation planes.

22. The monopulse tracking feed system as claimed in any of claims 19 and 20, wherein generated TE21* mode signal is combined separately with TEoi mode couplers to generate tracking-error signals in respective elevation and azimuth planes.

23. The monopulse tracking feed system as claimed in any of claims 19 to 22, wherein each said magic tee network comprises of consists of four H-Plane S-shapes bends, four E-Plane 135° bends, a pair of H- Plane 90° bends, a pair of E-Plane 90° bends and three magic-tees; all with rectangular waveguide ports.

24. The monopulse tracking feed system as claimed in any of claims 14 to 21 wherein each arm of said first set of coupling arm couples 1 /4^th of the propagating TE21 mode while each arm of said second set of coupling arm couples l/4^th of the propagating TE21* mode signals and said taper region is a TE21 cut-off taper.

25. The monopulse tracking feed system as claimed in any of claims 14 to 22, wherein said coupling arms are integrally formed with said over-moded circular waveguide and with said taper region.

26. The monopulse tracking feed system as claimed in any of claims 14 to 25, wherein said longitudinal coupling slot has a rectangular profile.

27. The monopulse tracking feed system as claimed in any of claims 14 to 26, wherein said impedence matching section has a stepped profile.

28. The monopulse tracking feed system as claimed in claims any of claims 14 to 27, wherein said taper region has a flare angle of 13.2°.

29. The monopulse tracking feed system as claimed in any of claims 15 to 27, wherein said microwave coupler couples TE21 mode over frequency range of 10.95GHz- 12.75GHz.