DESC:“A COMPOSITE MONOPULSE TRACKING FEED”
Field of the invention
The present invention generally relates to a tracking antenna system, more particularly it relates to a composite feed system used for tracking antenna, operating in widely separated dual frequencies.
Background of the invention
Generally, dual-band tracking function is achieved using single reflector antenna where a lower frequency (fL) feed is placed in prime focal configuration using dichroic sub-reflector and the higher frequency (fH) feed is employed in Cassegrain configuration. Generally, the dichroic sub-reflector is designed to pass lower frequency band with minimum insertion loss and reflect higher frequency with very high reflectivity. This technique has been successfully used in auto track antennas where the frequency separation is small, (fH / fL= 4). However, it is not very attractive when there exists a wider frequency separation than mentioned above. Relative large structural displacement due to physical separation of the two feeds leads to wandering of monopulse nulls. As a result it becomes extremely difficult to obtain null coincidence between the two bands for all the planes within the desired limits to satisfy tracking requirement. Consequently, tracking null at the lower frequency may lie beyond the pulling range of the tracking system at higher frequency. This will impair the accuracy needed for tracking switchover from lower to higher frequency bands. Ultimately, it imposes stringent constraints on the structural design of the Cassegrain geometry employing sub-reflector assembly to yield satisfactory performance.
Accordingly, there exists a need for the common aperture configuration to satisfy the compliance of stringent alignment accuracy requirement associated with Ka band operation, offering inherent structural integrity/rigidity due to composite structure of the two feeds.
Objects of the invention
The primary object of the present invention is to provide a monopulse composite tracking feed in Cassegrain geometry
Further object of the present invention is to facilitate the dual band operation where the frequency separation is more than 4 times between the two operating frequencies.
Summary of the invention
The state- of -the art- monopulse tracking composite feed has been realized to offer tracking slope better than 1.1 V/V/?bw , null depth better than 25 dB and null coincidence within 0.1 degree. In order to achieve circularly symmetric beams and suppressed cross polarized component, the diagonal horns are used at S-Band whereas, pyramidal horns surrounded by choke flanges at the aperture are used at Ka band. Over and above the circular symmetry of the beams, the required isolation between the two frequency bands has also been achieved by providing corrugations of suitable depth surrounding the array of horns at higher frequency. The tracking slope has been optimized for both the frequency bands by orienting the horn assemblies identically in both the bands along the principle axes by providing suitable truncation at the center of the diagonal horn arrays.
Brief description of the drawings
The objects and advantages of the present invention will become apparent when the disclosure is read in conjunction with the following figures, wherein
Figure 1 shows a composite monopulse tracking feed system in accordance with the present invention.
Figure 2 shows exploded view of the composite monopulse tracking feed system in accordance with the present invention.
Figure 3 shows higher frequency tracking feed array with 4 nos. of integrated septum polarizers in accordance with the present invention.
Figure 4 shows developed composite monopulse tracking feed, in accordance with the present invention.
Figure 5 shows developed composite monopulse tracking feed integrated in a Cassegrain reflector antenna, in accordance with the present invention.
Detailed description of the invention
The foregoing objects of the invention are accomplished and the problems and shortcomings associated with the prior techniques and approaches are overcome by the present invention as described below in the preferred embodiment.
A novel composite feed at S & Ka band is achieved by accommodating 2x2 array of choke flanged pyramidal horns at higher frequency inside 2x2 array of diagonal horns at the lower frequency by truncating its aperture suitably without affecting the overall performance in terms of coincidence of phase centers and monopulse nulls at both the frequencies. The design configuration provides simultaneous multi polarizations reception capability.
The present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description. The components of the invention and the reference numbers are also listed in the table below:
Ref. No. Component Name Ref. No. Component Name
10 Higher frequency tracking feed 4 Pyramidal horns
20 Lower frequency tracking feed 5 Second polarizer
22 First polarizer 6 Corrugations
24 Diagonal horns.
Figures 1 to 3 disclose the composite monopulse tracking feed system (100), hereinafter “the system (100)” in multi polarizations and widely separated multi-frequency operations. The system (100) comprises of a higher frequency tracking feed (10) accommodated in a lower frequency tracking feed (20) in a cassegrain geometry.
The lower frequency tracking feed (20) is an S band feed including 2x2 array of diagonal horns (24) as radiating elements, each diagonal horn (24) engaged with a first polarizer (22). The lower frequency monopulse feed (20) in Cassegrain geometry is centrally truncated to accommodate the higher frequency monopulse feed (10) without affecting the performance at lower frequency band. Diagonal horn configuration of lower frequency tracking feed (20) makes it further easier provision for accommodating monopulse comparator hardware at higher frequency band.
The higher frequency tracking feed (10) is a Ka band feed having a 2x2 array of pyramidal horns (4) surrounded by corrugations (6) at the aperture, wherein each pyramidal horns (4) is engaged with a second polarizer (5). The higher frequency feed (10) is an integrated structure peripherally configured with corrugations (6) thereon. Corrugation (6) is provided in order to achieve higher isolation from the harmonics at lower frequency band. The arrangement of high frequency feed (10) facilitates achieving higher accuracy in the assembly of horn arrays. A very compact plumbing has been designed for higher frequency tracking feed to optimize the insertion loss as well as axial ratio by matching the amplitudes and phases suitably at the input of monopulse comparator.
The higher frequency tracking feed (10) and the lower frequency tracking feed (20) thus share a common aperture and are aligned by virtue of this configuration.
Accommodation of higher frequency tracking feed (10) within the lower frequency tracking feed (20) in order to achieve a common aperture is achieved by separating radiating elements at lower frequency band to accommodate the radiating elements at higher frequency band. In this arrangement, there are chances of degradation of illumination efficiency of primary feed at lower frequency. This is overcome by designing a composite feed, wherein a central region of the lower frequency tracking feed (20) is truncated to accommodate the higher frequency tracking feed (10). The truncation is carried out in such a manner that it is restricted within 1% of the total aperture area at S-band, so that it will not impair the gain of lower frequency tracking feed (20).
In an embodiment, the first polarizers (22) and the second polarizers (5) are septum polarizers for generating multi-polarized signals. A monopulse tracking function in widely separated multi-frequency operations in multi polarization is achieved ensuring the accuracy of null coincidence.
The array of pyramidal horns, at higher frequency band, surrounded by choke flanges/ corrugations (6), has been optimized for maximum directivity in order to approach Gaussian feed criteria. The phase centers at both the frequency bands are made to coincide in order to minimize the defocusing of both the feeds in the same cassegrain geometry and thereby achieving maximum efficiencies at both the frequency bands. This could be achieved by optimizing the phase centers of higher frequency pyramidal horn array in multiple planes as well as over the operating bandwidth. This type of configuration employing composite feed will be extremely suitable for many applications where widely separated frequencies in multi polarizations are used for acquisition and tracking functions.
This invention is not limited to any particular dual band operation but extendable to a ratio of frequency separation of more than 4 and can be modified in multiple of ways within the framework of the invention as described in the patent claims attached herewith.
Advantages of the present invention
• The system (100) offers an attractive solution for the requirement of dual frequency monopulse tracking feed operating at largely separated (in a ratio of 1:4 or more) frequencies
• The system (100) also offers an attractive alternative to commonly employed concept of using prime focal feed at lower frequency and using dichroic hyperbolic surface designed for cassegrain assembly at higher band of frequency.
• The problem of null misalignment within the stringent specifications on the null coincidence due to Ka band operation is successfully resolved by developing a common aperture tracking feed.
• The composite monopulse tracking feed system (100) of the present invention offers tracking slope better than 1.1 V/V/?bw and null depth better than 25 dB in both the frequencies with a null coincidence within 0.1deg.
• The system (100) is extremely suitable for applications where widely separated frequencies in multi-polarizations are used for acquisition and tracking functions.
The foregoing objects of the invention are accomplished and the problems and shortcomings associated with prior techniques and approaches are overcome by the present invention described in the present embodiment. Detailed descriptions of the preferred embodiment are provided herein; however, it is to be understood that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure, or matter. The embodiments of the invention as described above and the methods disclosed herein will suggest further modification and alterations to those skilled in the art. Such further modifications and alterations may be made without departing from the spirit and scope of the invention.
,CLAIMS:We Claim:
1. A composite monopulse tracking Feed (100) for achieving optimum tracking performance simultaneously at higher frequency band and lower frequency band in multi polarizations and widely separated multi-frequency operations having frequency ratio more than 4, and for ensuring the accuracy of null coincidence, the composite monopulse tracking Feed (100) comprising of: a lower frequency monopulse feed (20) and a higher frequency monopulse feed (10);
the lower frequency monopulse feed (20) in Cassegrain geometry; centrally truncated to accommodate a higher frequency monopulse feed (10) without affecting the performance at lower frequency band and having a 2 x 2 array of diagonal horns (24) as radiating element and four first polarizers (22) wherein each of the diagonal horn (24) is engaged with one first polarizer (22); and
the higher frequency monopulse feed (10) having a 2 x 2 array of high gain pyramidal horns (4) as radiating element and four second polarizers (5) wherein each of the pyramidal horns (4) is engaged with one second polarizer (5); the array of high gain pyramidal horns (4) is configured with corrugations (6) at the periphery thereof in order to achieve isolation between the higher and lower frequency bands;
characterized in that, the composite monopulse tracking feed (100) receives the multiple polarizations and employs optimized routing of the outputs of lower and higher frequency bands reducing the overall insertion loss, compact envelope and minimal amplitude & phase imbalance amongst all the horn outputs.
2. The composite monopulse tracking feed (100) as claimed in claim 1, wherein the first polarizers (22) and the second polarizers (5) are septum polarizers.
Dated this 18th day of August, 2017

Ashwini Kelkar
(Agent for Applicant)
IN-PA/2461