Optimized monopulse antenna feed

Abstract

The invention relates to a monopulse feed, contained in an antenna, which supplies at three outputs sum and difference signals which are a function of the angular difference between the line along which an object in space lies and the axis of the antenna. The feed is optimized, in one plane, by adding to the central horn, two lateral horns which transmit only the difference signals, the central horn being used solely for the sum signal, and in the other plane, the feed is optimized by using high order modes.

Description

The present invention relates to an optimised monopulse antenna feed of the amplitude-comparison type.

Monopulse systems are either active systems, that is to say radars which emit and receive emitted energy after it has been reflected from an object in space, or passive systems which receive the energy radiated directly by the object. Whether it is active or passive systems with which one is concerned, an indication of the direction of the object as represented by two parameters, generally elevation and azimuth, is obtained from a single received echo pulse. A monopulse feed associated with a focussing device forms the antenna of the system. Each echo received from the space is focused to a "spot" or "image" having finite dimensions. This spot generally is circular and has a cross section shape of the forme sinX/X. The spot is centered in the focal plane when the target is on the antenna axis and moves off center when the target moves off axis. The antenna feed is located at the focal point to receive maximum energy from an object on axis. An amplitude-comparison monopulse feed is designed to sense any lateral displacement of the spot from the center of the focal plane. The radar senses the object displacement by comparing the amplitude of the echo signal excited in each part of feed. The feed is so designed that each received echo produces, at two separate outputs, two signals termed "difference signals and referred as .DELTA. E and .DELTA. H. The angular differences in elevation and azimuth, between the line along which the object lies and the axis of the antenna, are obtained by comparing each of the "difference" signals with a reference signal termed the "sum" signal .SIGMA.. This signal is generally obtained from another output.

A monopulse feed thus contains a sum output channel, an elevation difference output channel and an azimuth difference output channel. In the case of a radar system the feed also contains an input channel which in general is identical with the sum output channel, these various channels being connected to the radar by more or less complex circuitry.

Externally of the monopulse feed, the three outputs are connected to circuits for comparing the sum and difference signals. In the case of a radar system the input channel is connected to the transmitter; if the input channel is identical with the sum output channel, a duplexer is inserted between this output on the one hand and the transmitter and the comparison circuits on the other hand.

There is a wide variety of possible configurations for monopulse feeds. FIGS. 17, 18, 19, 20, 21 in chapter 21-4 of the Radar Handbook by Merril Skolnick published by MacGraw-Hill show monopulse feeds containing four or more radiating apertures, FIGS. 23, 25 and 26 show other feeds having a single aperture which use high-order propagation modes for their waves in order to extract the difference signals.

In all cases, optimum sum and difference signals, low side lobe levels, minimum spill-over, omnipolarization capability and simplicity cannot all be fully satisfied simultaneously. The more important problem is to optimize simultaneously the sum and difference signals. For this purpose the spill-over must be reduced, since it causes high sidelobes which increase radar susceptibility to interference from offaxis sources of radiation in addition to loss of efficiency. For obtaining an optimized feed, with optimum coupling of the feed to the antenna for each of the three signals, the electrical field intensity in the aperture of the feed should smoothly taper to the edges.

The usual aim is for the optimisation to exist in the two planes of symmetry of the monopulse feed. In the following description, these planes will be termed "E-plane" and "H-plane." The E plane is the plane of symmetry which contains the electrical field vector E at the centre of the sum source. The H plane is the plane of symmetry at right angles to the E plane.

Monopulse feeds are basically formed from one or more sections of wave guide, usually rectangular wave guide. The guides are open at one end, and may or may not be flared into the shape of a horn, in order to form radiating apertures. At the other end more or less complex junction circuitry connects the guides to the sum and difference outputs.

Four or five horn feeds are optimized by adding extra horns to enlarge the apertures of the difference feeds. The consequence is that the feed may comprise up to 12 horns but the microwave circuitry is very complicated and hardly compatible with the size and weight requirements in the case of airborne equipment.

A subsequent development in monopulse feed techniques provides the monopulse sum and difference signals with a single feed aperture. It makes use of hybrid modes of propagation in addition to the conventional TE.sub.10 mode to separate the sum and difference signals. Such a feed may be optimized by using high-order waveguide modes. These modes are, however, difficult to use and require complicated microwave circuitry.

The monopulse feed according to the invention does not have these drawbacks. It enables the use of high-order propagation modes to be avoided and employs very simple microwave circuitry.

In accordance with a feature of the invention, an optimized monopulse feed for supplying at three outputs a sum signal .SIGMA. and two difference signals .DELTA.E and .DELTA.H which are a function of the angular deviation of an object in space with respect to the axis of the antenna, the said feed comprising on the one hand, a rectangular main wave-guide for propagating simultaneously at least the fundamental mode for the sum signal and at least one odd mode for a difference signal, the main guide terminating at one end in a radiating aperture and at the other and in a first coupling microwave circuit for separating said fundamental mode and said odd mode and delivering respectively the sum signal at a first output and the difference signal at a second output, and, on the other hand, two lateral rectangular wave guides each of which terminates at one end in a radiating aperture and has, at the aperture, a large side adjoining one large side of the main guide, the two lateral guides being connected at the other end to a second coupling microwave circuit for extracting and delivering the waves which are in phase opposition in the lateral guides and correspond to the other difference signal, and for reflecting the waves which are in phase in said lateral guides.

The coupling circuitry, which are completely separate, are thus very much simplified. It is therefore easy to optimize the feed independently in the E plane and the H plane.

Other features will appear in the following description which is illustrated by the Figures, which show:

FIG. 1, a perspective view of a monopulse feed according to the invention from which the output signal for the difference signal .DELTA.H has been omitted;

FIGS. 2 and 3, views from above of the feed which shows the direction of the electrical field in various sections of it;

FIGS. 4 and 5, schematic diagrams showing the distribution of the electrical field in the aperture for the sum and difference signals;

FIg. 6, a particular type of coupling circuitry between the lateral horns and the output for the difference signal .DELTA.E;

FIG. 7, an embodiment of a complete feed;

FIGS. 8, 9 and 10, views showing the direction of the electrical field in the three parts of the radiating aperture for the three types of received signals.

FIG. 1 shows an example of a monopulse feed according to the invention. It comprises a main rectangular wave guide 1 which flares to form a horn and is contained between two lateral rectangular guides 2 and 3 which are also flared. In the following description reference will simply be made to the main horn 1 and lateral horns 2 and 3. At the end at which their major opening is situated the horns open into free space. These openings form the radiating apertures of the monopulse feed. At the end at which their minor openings are situated the horns are connected to a sum output 4 and a difference output 5, by suitable microwave circuits.

To simplify the description, the feed shown contains only the output for the difference signal .DELTA.E and for which the optimization is effected solely in the E plane. The second difference output and the associated microwave coupling circuits are not shown in FIG. 1. These circuits are independent of those shown and they are described in the explanation below.

The invention is, of course, not restricted to monopulse feed capable of determining an angular separation in only one plane.

The main horn 1 is directly connected to the sum output 4 by a rectangular wave guide 6. A suitable coupling microwave circuit is coupled to the output 4 for separating the sum signal and the .DELTA.H difference signal which are carried respectively by the fundamental mode and an odd mode. This microwave circuit is only shown in FIG. 7 and will be described below. The lateral horns 2 and 3 are connected to the difference output 5 by means of a coupling microwave circuit 7 such as a magic-T for example. Two channels of the magic-T are coupled to the two horns 2 and 3 and the other two channels are shown by sections of wave guide 5 and 8. Guide 8 is closed by a short-circuit 9. Guide 5 forms the difference output of the monopulse feed.

The operation of the feed will be better understood from FIGS. 2 and 3, which are views of the feed from above, and from FIGS. 4 and 5 which are schematic diagrams showing the distribution of the electrical field in the aperture for the sum .SIGMA. signals and difference .DELTA.E signals respectively.

Upon transmission, the energy to be transmitted is applied to the sum input 4. Electrical field E, shown in the Figures by arrows, is similarly oriented at any cross-sectional plane of the wave guides 6 and is propagated with the fundamental mode known internationally as TE.sub.10 in technical literature.

Arriving at the mouth of horn 1, the energy is radiated into space (after reflection on a reflector which is not shown). Because the large side of the lateral horns are joined onto the large side of main horn in the region of the respective aperture, part of the energy radiated by the main horn is propagated into the lateral horns as a result of coupling. The waves introduced into horn 2 and those introduced into horn 3 are in phase. They combine in the magic-T in a known way and are propagated solely in the section of guide 8. Since these waves are in phase and are of equal amplitude, no energy is propagated in guide 5 due to the characteristics of such a junction. Guide 8 is closed off by a short-circuit 9 which causes the waves to be reflected. The energy is then transmitted back to the mouth of horns 2 and 3. The electrical length between the respective apertures and the short circuit 9 should be such that the waves are re-emitted in phase with those of the main horn.

The overall illumination of the opening of the feed, i.e. the amplitude distribution of the electrical field E in this aperture is shown in FIG. 4. This distribution is a discontinuous equi-phase function made up of a central step and two lower level lateral steps. The electric field intensity is taperred to the edges of the aperture resulting in a good optimization of the feed in the E plane.

Distribution of the electrical field is thus related to the size of the aperture. The shape of the radiation pattern of the feed is a result of this distribution. Starting from the fact that the pattern must correctly enclose the focusing device for the optimization, the dimensions of the apertures and in particular those of the main horn are calculated therefrom. It is also possible for the waves re-emitted by the lateral horns to be in phase opposition with respect to those from the main horn. In this way the monopulse feed is better matched to the "spot" focused by the associated antenna. This can be achieved, for example, by altering the length of the section of wave guide 8.

Upon reception, if object in space is not situated on the axis of the antenna, the focussing device forms spot in the plane of the apertures which is not centred on the axis. This spot, which contains almost all the received energy, may be broken down into two components, one in which the electrical field distribution is of even symmetry, whilst in the other it is of odd symmetry.

The distribution of energy in the field of even symmetry will excite the three horns in phase. The energy collected by the main horn passes to output 4. That collected by the lateral horns 2 and 3 is reflected by short circuit 9 and passes to output 4, partly as a result of coupling to the main horn. The type of operation is the reverse of that on transmission.

The component whose symmetry is odd excites horns 2 and 3 in phase opposition. It is combined in the magic-T 7 and transmitted to output 5. The direction of the electrical vector in this case in the laternal horns and in the magic-T as far as output 5, is shown in FIG. 3.

The main guide is excited by a hybrid mode TE.sub.11 + TM.sub.11. This mode of propagation should not be allowed to exist. The dimension of the horn are calculated to cut-off this mode. This being so, the mode is reflected and causes only transversely polarised waves to appear which can be suppressed by adding a polarisation filter positioned in the aperture of the main horn. The filter may be formed by a grid of wires parallel to H plane i.e. parallel to the direction of polarisation to be suppressed.

The field distribution as shown in FIg. 4 is that which gives rise solely to the sum signal .SIGMA.. Curve A in FIG. 5 is that which gives rise solely to the difference signal .DELTA.E. In particular, in the case of the difference signal, the field is a nul at the main horn and is a maximum at the lateral horns.

A radiation diagram equivalent to this field distribution is one which should encompass the focussing device in order to optimise the feed in E plane. As a first approximation, the directional characteristic given by field distribution A is the same as the first term of a Fourrier series development which is a sine-wave arc, as shown by curve B of FIG. 5. This directional characteristic is related to the overall dimensions of the feed, whereas the directional characteristic of the sum signal may be adjusted by altering the dimension of the main horn. The directional characteristics may thus be adjusted independently.

Another parameter of adjustment consists in lengthening or shortening the dividing walls between the main horn and the lateral horns. In the second case, a cavity is formed at the front of the main horn in which it is possible for upper-level hybrid modes to exist. This process provides a further parameter which enables the directional characteristic of the feed to be altered with a view to achieving the desired optimization. This process is used, for example, to make it easier to match the radiating aperture, i.e. to reduce the effects of the discontinuity in the propagation of the waves where they pass from the feed into free space and vice versa.

FIG. 6 shows another type of junction circuit between the lateral horns and the difference output.

The lateral horns 2 and 3, which are shown in broken lines, extend into lateral rectangular sections of waveguide 20 and 30. The difference output, which bears the same reference numeral 5, is situated at an opening of a third rectangular section of waveguide 10 which is applied to the small side of the lateral guides 20 and 30 along one large side. The lateral guides are closed off by short circuits 21 and 31. Guide 10 is also closed off by a short circuit 11. All the axes of symmetry of the guides are parallel to one another.

The lateral guides and guide 10 are coupled to one another via two slots 12 and 13 parallel to the axis of symmetry of the guides which are pieced in the common walls of the guides in a symmetrical fashion on either side of the axis of guide 10.

Guide 10 only operates in the fundamental mode TE.sub.10, which is equivalent to slots 12 and 13 being excited in phase opposition. For this, the waves propagated in lateral guides 20 and 30 must be in phase opposition. This condition is satisfied when the line along which a detected object lies does not coincide with the axis of the antenna. A difference signal then appears at output 5.

The positions of short circuits 21 and 31 and 11 are calculated to produce the best coupling between the lateral guides and guide 10.

When in-phase waves are propagated in the lateral guides, the coupling slots excite mode TE.sub.20 in guide 10. Since this mode is cut off, energy cannot be propagated in the guide and is therefore reflected to the lateral guides, the length of which is adjusted so that this energy is once again in phase with the incident waves at the aperture. It is also possible to adjust the length so that the reflected waves are in phase opposition at the aperture.

For reasons of clarity, the feed which has just been described contains only one difference channel. It is only capable of detecting an angular divergence in a single plane, i.e. E plane. It is however quite possible to use this feed to establish angular divergences both in E plane and also in H plane at right angles to it.

In this case, the TE.sub.10 mode is used, in a known way, in the main horn for the sum signal and the TE.sub.20 mode for the difference signal in plane H. An appropriate microwave circuit is utilized for separating the TE.sub.10 and TE.sub.20 modes. This circuit is shown on FIG. 7.

FIG. 7 shows an embodiment of a complete monopulse feed according to the invention.

The horns 1, 2 and 3 and the central guide 6 are the same as in FIG. 1. In contrast, the "magic-T" 7 used in FIG. 1 has been replaced by the type of connection in FIG. 6 although all that actually appears are the lateral guides 20 and 30 and the output guide 10 from which the difference output for E plane (i.e. 5) emerges.

The difference output for plane H is designated 18. It emerges from a section of waveguide 17 the narrow sides of which are perpendicular to the axis of guide 6. The plane of symmetry of guide 17 is coincident with the horizontal plane of symmetry of guide 6.

Guide 6 extends into a guide 16 after a constriction 15. At the end of guide 16 is situated the sum output 4. It is assumed that all the outputs 4, 5 and 18 are connected to external circuitry via guides in which it is only possible for the TE.sub.10 mode to exist.

The way in which the difference channel for E plane operates has already been described. The displacement of the spot in the E plane provide a .DELTA.E signal which appear at output 5, in addition with a sum signal .SIGMA. at output 4. In H plane the displacement of the "spot" excites the TE.sub.20 mode which is the unsymmetrical waveguide mode used as the difference signal in the H plane. TE.sub.20 and TE.sub.10 modes coexistent in the waveguide 6. Thus the TE.sub.20 mode coupled from the feed has an amplitude increasing with object displacement from the antenna axis and a phase that differs by 180.degree., depending on the direction of displacement, providing the direction sense used by the monopulse receiver. TE.sub.10 mode is extracted by the microwave circuit 6-17 and appears at output 4. TE.sub.20 mode is extracted and transmitted to guide 17 to output 18. The feed is optimized in H plane in a very simple way by widening guide 6 at a suitable distance from its mouth.

After passing through this widened portion a TE.sub.10 mode will give rise to a combination of modes TE.sub.10 and TE.sub.30.

Optimation is thus achieved in H plane by using at least an upper-level mode which alters the aperture field distribution independently of E plane.

All these modes are excited in the lateral horns with the same coefficient of coupling as the fundamental mode and are then reflected by appropriate reflecting means, such as short circuits judiciously positioned in the lateral guides 20 and 30 and are again in phase at the aperture. The short circuits are individual to each mode since propagation is different between one mode and another. These reflecting means could also be positioned for reflecting the higher-order mode waves in phase opposition at the apertures.

The short circuits may be done away with if the TE modes utilised are cut off in the lateral horns. Reflection then takes place without phase-inversion at the aperture.

The dimensions of the central horn are calculated as a function of the directional characteristic of the difference pattern in H plane. The dimensions of the lateral horns may either be the same or smaller, which prevents high-level modes from being excited.

When calculating the dimensions of the horn it is also necessary to take into account the nature of the dielectric media which fill the horns and the guides and also any ridges which might be placed there in order to facilitate the propagation of upper-level modes without increasing the dimensions of the guides to an excessive degree.

FIGS. 8, 9 and 10 are front views of the three horns 1, 2 and 3 in which are shown the directions of the electrical field vector E for the three different types of operation, these being "sum," "difference in E plane," "difference in H plane," respectively.

In "sum" operation, the fields in the three horns are in phase.

In "difference in E plane" operation, there is no field in the central horn 1 and the fields are in phaseopposition in the lateral horns.

In "difference in H plane" operation, the fields are in phase opposition between one end of a horn and the other in the case of each horn.

Claims

What is claimed is:

1. In an antenna system, an optimized monopulse feed for supplying at three outputs a sum signal .SIGMA. and two difference signals .DELTA.E and .DELTA.H respectively which are a function of the angular deviation of an object in space with respect to the axis of the antenna, comprising:

a rectangular main waveguide for propagating simultaneously at least the fundamental mode and one odd mode, said main guide terminating at one end in a radiating aperture;

a first coupling microwave circuit coupled to the other end of said main guide for extracting said fundamental mode corresponding to the sum signal, at a first output, and said odd mode corresponding to a first difference signal, at a second output;

two lateral rectangular wave guides placed at each side of the main guide respectively, for propagating the fundamental mode, each lateral guide being terminated at one end in a radiating aperture, and,

a second coupling microwave circuit having two inputs connected at the other end of the two lateral guides for extracting the waves which are in phase opposition in the lateral guides, corresponding to the second difference signal, at an output thereof, and for reflecting the waves which are in phase in said lateral guides.

2. A monopulse feed according to claim 1, wherein each lateral guide has, at its aperture, a large side adjoining one large side of the main guide.

3. A monopulse feed according to claim 1, wherein said second coupling microwave circuit is a magic-T having two inputs, a first output for extracting the waves which are in phase-opposition in its two inputs, and a second output for extracting the waves which are in phase in its two inputs, said second output being closed by a short-circuit for reflecting said in-phase waves, wherein said short-circuit is arranged at a distance from the respective apertures of the lateral guides such that the reflected waves are in phase at each aperture with the incident waves.

4. A monopulse feed according to claim 1, wherein said second coupling microwave circuit is a magic-T having two inputs, a first output for extracting the waves which are in phase-opposition in its two inputs, and a second output for extracting the waves which are in phase in its two inputs, said second output being closed by a short-circuit for reflecting said in phase waves and wherein said short-circuit is arranged at a distance from the respective apertures of the lateral guides such that the reflected waves are in phase-opposition with the incident waves.

5. A monopulse feed according to claim 1, wherein the second coupling microwave circuit comprises a section of rectangular waveguide, having one large side which is applied to a small side of the two lateral guides respectively, means for coupling the lateral guides and said section of waveguide, said means being constituted by two slots which are provided in the common walls of said guides and are oriented in a parallel direction with the axes of symmetry of the lateral guides, in a symmetrical fashion with respect to the plane of symmetry of said microwave circuit, and three short-circuit means respectively located at the end of the two lateral guides and one end of said section of waveguide, the other end of said waveguide constituting the output of said second microwave circuit.

6. A monopulse feed according to claim 5, wherein said slots and said short-circuit means are arranged at respective distances from the apertures of the lateral guides such that the reflected waves in phase with the incident waves at each aperture.

7. A monopulse feed according to claim 5, wherein said slots and said short-circuit means are arranged at respective distances from the apertures of the lateral guides such that the reflected waves are in phase opposition with the incident waves at each aperture.

8. A monopulse feed according to claim 1, wherein the main guide 6 is widened to allow even TE modes of a higher order to exist at its aperture in addition to the fundamental mode of propagation, for optimizing said feed in H plane.

9. A monopulse feed according to claim 8, wherein individual reflecting means for reflecting the said higher-order even modes are positioned in the lateral guides and are arranged at a distance from the apertures such that the reflected waves are then phased with the incident waves at each apertures.

10. A monopulse feed according to claim 8, wherein individual reflecting devices for reflecting the said even higher-order modes are positioned in the lateral guides and are arranged at a distance from the apertures such that the reflected waves are in phase opposition with the incident waves at each aperture.

11. A monopulse feed according to claim 1, wherein the plane of the aperture of the main guide is set back from the plane of the apertures of the lateral guides.

12. A monopulse feed according to claim 1, wherein the plane of the aperture of the main guide projects in front of the plane of the apertures of the lateral guides.