Monopulse Radar Antenna Structure

Abstract

An antenna structure formed from slotted wave guide sections for producing monopulse arrays and incorporating means for reducing sidelobes. This is achieved by providing wave guide sections in the four quadrants of a monopulse antenna with adjacent ends of the wave guides in the quadrants being staggered such that certain wave guides in one quadrant extend into an adjacent quadrant and vice versa. The staggered configuration effectively reduces the slope of the transition in phase from one quadrant to the other and results in a reduction in amplitude of the side-lobes.

Description

The Invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Navy.

BACKGROUND OF THE INVENTION

In conventional radar systems, conical scanning is often used to obtain the angular position of a target with respect to the boresight axis of the radar antenna. Radiation from the antenna is in the form of a narrow pencil beam which is made to rotate circularly about the boresight axis so that the radiation pattern is in the form of a cone whose vertex is the center of the radiating antenna. By recording the position of the radiated energy pulses which are reflected by a target somewhere in the 360.degree. circular path traveled by the beam, the angular position of the target may be determined.

In a monopulse radar system, on the other hand complete angular information is obtained from a single radiated energy pulse. This is accomplished by using at least two antenna or antenna segments rather than one; and in the case where complete azimuth and elevation information is to be derived, the antenna must be divided into four quadrants, two above the other and separated along generally vertical and horizontal dividing planes. The antenna segments are spaced apart from a common center point and are aimed so that their cone-shaped lobes or radiation beams overlap. Any target in the field formed by the overlapping lobes will send reflected energy pulses back to the respective antenna segments. Unless the target lies at equal distances from two segments, the amplitudes and phases of the reflected energy waves arriving at the antenna segments will vary. By comparing the amplitude or phase differences, an angular error signal is derived whose magnitude and polarity indicates target distance and direction from the center point between the antenna segments. The monopulse system, therefore, determines target angularity by comparing the amplitudes of received signals at a plurality of antennas or antenna segments; whereas the conical scan system determines angularity by noting amplitude variations in reflected signals as the antenna and lobe travel through 360.degree..

It has been found desirable to employ in monopulse radar systems an antenna formed from slotted hollowpipe wave guides. In an antenna of this type, the wave guides are aligned in side-by-side relationship and provided with slots in the forward faces thereof. Four sets of such slotted wave guides must be provided to form the four quadrants of a monopulse antenna for determining azimuth and elevation information, the ends of the wave guides in two of the quadrants abutting or being closely adjacent the ends of the wave guides in the remaining two quadrants.

In an antenna of this type, it is necessary that the four quadrants be phased differently in order to obtain the angle error signal. This phasing results in a discontinuity in the illumination pattern of the antenna. Specifically, there is a discontinuity at the center of the antenna where the ends of the wave guides in the respective quadrants abut each other. At this point, the phase of the wave energy abruptly changes from a positive value of high amplitude to a negative value of high amplitude, with the result that excessive sidelobes occur.

SUMMARY OF THE INVENTION

In accordance with the present invention, the generation of sidelobes because of the abrupt change in phase at the abutting ends of wave guides in a slotted wave guide configuration for monopulse radars is reduced or eliminated. This is accomplished with the use of an antenna of the slotted wave guide type including at least two segments each formed from a plurality of parallel slotted wave guides. The ends of the wave guides in one segment are closely adjacent the ends of the wave guides in the other segment, and the wave guides in one segment are aligned with those in the other segment. The adjacent ends of the wave guides in the respective segments are staggered such that certain wave guides in one segment extend into an adjacent segment and vice versa to reduce the generation of sidelobes due to an abrupt discontinuity at the junction of the two segments.

In most cases, the antenna will include four segments in order that both azimuth and elevation information can be obtained. The outer peripheral configuration of the antenna is usually circular with the segments forming quadrants of the circular configuration; however, in any event, the outer periphery of the antenna will ordinarily define a geometrical configuration in which the four quadrants or sectors are all of the same size.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic diagram illustrating the general principle of a monopulse radar;

FIG. 2 is a schematic circuit diagram showing the manner in which the antenna of the invention may be connected to transmitting and receiving circuitry;

FIG. 3 is an elevational plan view of the antenna of the invention;

FIG. 4 is an enlarged perspective view of the slotted wave guide sections of the antenna of FIG. 3;

FIG. 5 is a plot of error signal voltage versus angle for the antenna of FIG. 3; and

FIG. 6 is a plot of amplitude versus angle showing the reduction in generation of sidelobes with the teachings of the invention.

With reference now to the drawings, and particularly to FIG. 1, the principle of a monopulse radar is shown for locating a target in one dimension, such as azimuth or elevation. Two antennas or feedhorns 10 and 12 are spaced at equal distances from a common center line 14. The radiated lobes or fields 16 and 18 overlap to form a combined field 20. If a target 22 is found within the combined field, simultaneous pulses emanating from the antennas 10 and 12 will be reflected back to their respective antennas. However, since in the present illustration the target is nearer to the center of the beam of antenna 12 than that of antenna 10, the reflected pulses arriving at antenna 12 will be greater in amplitude than those arriving at antenna 10. Likewise, the signal received at antenna 12 will lead in phase that received at antenna 10. By comparing the difference in amplitude or phase between the two received signals, an angular error signal can be derived whose magnitude and polarity will indicate the position of target 22 with respect to center line 14. The amplitudes of the received pulses will also indicate the range of the target 22.

A schematic circuit diagram of a monopulse radar system employing the antenna of the invention for locating a target in both azimuth and elevation is shown in FIG. 2. The antenna 24, hereinafter described in detail, includes four quadrants A, B, C and D, each of which acts as a separate antenna corresponding to one of the two antennas 10 or 12 in FIG. 1. The segments A and D are connected to the two arms of a hybrid junction 26 such as a "magic tee" or a short-slot coupler. The sum and difference signals appear at the other two arms 28 and 30 of the hybrid. Similarly, the segments B and C are connected to the two arms of a hybrid junction 32 with the sum and difference signals appearing at the other two arms 34 and 36. The sum signals 28 and 34 are applied to the two arms of a third hybrid junction 38 which produces, in channels 40 and 42 a sum signal and a difference signal, respectively. In addition, the difference signals at arms 30 and 36 are applied to a hybrid junction 44 which produces in channel 46 an elevation difference signal. The signals in channels 40, 42 and 46 are applied to mixers 48, 50 and 52, respectively, where they are mixed with the output of a local oscillator 54 and then applied through intermediate frequency amplifiers 56, 58 and 60 to an amplitude detector 62 and to two phase detectors 64 and 66, respectively. The output of the amplitude detector 62 is a signal proportional to the range of the target; the output of phase detector 64 is an azimuth angle error signal; and the output of phase detector 66 is an elevation angle error signal. The transmitter 68 is connected through duplexer devices 70 and 72 to the sum channel 40.

The antenna 24 is shown in detail in FIGS. 3 and 4. It is generally circular in configuration and includes the four quadrants A, B, C and D. Each quadrant, such as quadrant B, is formed from a plurality of parallel wave guide sections 74 provided with angled slots 76 spaced along their forward walls.

The wave guide sections 74 are shown in detail in FIG. 4. They comprise a piece of hollow-pipe wave guide which carries the energy from a transmitter. At the point where radiation is desired, the holes or slots 76 are cut and so shaped and spaced that radiation from them is of the desired form. Thus, the antenna feed and the antenna itself are really one and the same thing. Note that the wave guide sections 74 in quadrant B, for example, are aligned with those in quadrant C and that all wave guide sections in quadrants A, B, C and D are parallel to each other. Furthermore, ends of the wave guide sections 74 in quadrant B have terminating ends which are closely adjacent terminating ends of the wave guide sections in quadrant C. Similarly, the wave guide sections 74 in quadrant A have terminating ends which are closely adjacent the terminating ends in quadrant D.

It is common in slotted wave guide antennas of this type to align all of the terminating ends of the parallel wave guide sections 74 in quadrants B and C, for example, along a straight line. This results in the illumination pattern shown by the full-line curve 78 in FIG. 5. Suitable wave guide "plumbing," not shown, but within the skill of the art, is provided between the hybrid junctions 26 and 32 (FIG. 2) and the quadrants A, B, C and D to achieve this illumination pattern. It will be noted that the illumination pattern changes abruptly at the junction of the terminating ends of the wave guide sections in quadrants B and C. Specifically, radiation of positive phase increases in amplitude from the edge of the antenna to the center line thereof; whereupon the phase changes along the straight line edge 79 (FIG. 5) to a negative value of high amplitude and then decreases in amplitude as the other edge of the antenna is approached. Furthermore, with an arrangement of that type, sidelobes of considerable amplitude are generated as shown by full-line curve 80 in FIG. 6 which is a plot of amplitude versus angle off the boresight axis of the antenna.

In accordance with the present invention, reduction in sidelobes is achieved by staggering the terminating ends of the parallel wave guide sections 74 in quadrants B and C as well as in quadrants A and D. This creates a condition wherein the phases at the termination are more or less mixed and the phase reversal is not as steep as when the terminations are aligned, resulting in the illumination pattern indicated by the broken line 82 in FIG. 5. Furthermore, as can be seen from the broken-line curve 84 in FIG. 6, the amplitudes of the sidelobes are considerably reduced by staggering the ends of the wave guide sections.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

Claims

I claim as my invention:

1. In an antenna of the slotted wave guide type, at least two segments each formed from a plurality of parallel slotted wave guides, the wave guides in at least one segment being positioned end to end with the wave guides of at least one other segment, the longitudinal axis of all wave guides so positioned being parallel, with the adjacent ends of selected wave guides being staggered such that certain wave guides in one segment extend into an adjacent segment and vice versa to reduce the generation of sidelobes due to a discontinuity at the junction of the two segments, and means for feeding out-of-phase wave energy to the wave guides in the respective segments.

2. The antenna of claim 1 wherein the antenna is circular in configuration and there are four segments each defining a quadrant of a circle.

3. The antenna of claim 2 wherein the parallel wave guide sections are aligned along vertical axes such that two segments are above the other two.

4. The antenna of claim 1 including means for feeding wave energy to said wave guides to produce an illumination pattern where the phase of the wave energy reverses at the adjacent ends of the wave guides.