GB2438261A

GB2438261A - Microwave antenna with a cross-polarization feed built in a multimode monopulse feed

Info

Publication number: GB2438261A
Application number: GB9102063A
Authority: GB
Inventors: Jean Bouko; Daniel Casseau
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1990-02-02
Filing date: 1991-01-31
Publication date: 2007-11-21
Anticipated expiration: 2011-01-31
Also published as: ES2291148A6; GB2438261B; ITRM910028A0; FR2902936A1; GB9102063D0; ITRM910028A1

Abstract

A multimode monopulse feed has an E-plane multimode structure comprised of a section (1) of rectangular waveguide fed at its input by an array of excitation waveguides (2,3), and extended at its output by a rectangular horn (6). The cross-polarization feed comprises at least one dipole (9) disposed in the aperture of the horn (6) of the multimode monopulse feed with an orientation such that its electric field is orthogonal to that of the multimode monopulse feed, and a grid made up of metal bars (13) parallel to each other and to the electric field of the dipole (9), disposed at the rear of the dipole (9) in the aperture of the horn (6) of the multimode monopulse feed.

Description

Microwave antenna with a cross-polarization feed built in a multimode

monopulse source The present invention relates to microwave antennas and more particularly to a tracking antenna for a trajectory plotting radar allowing to know the polarization signature of the targets.

It is known to add to the feed with nominal polarization of a tracking antenna an external feed with a cross-polari- zation, and to separate the polarizations by means of a sys-tern of polarization filters which has the disadvantage of causing a loss and of being costly.

It is also known, in particular from the US patents 4 241 353 and 4 357 612, to implement multimode rnonopulse feeds.

A purpose of the present invention is to implement a cross-polarized channel within a nominal-polarization multi- mode monopulse feed without disturbing the latter polariza- tion while having a cross-polarized channel with good radia-tion characteristics.

It consists in integrating a cross-polarized channel into a multimode monopulse feed in such a way that the nomi-nal-polarization and cross-polarization radiation patterns have their phase centers coincident and, rather similar widths at -3dB and -10dB.

The present invention comprises a microwave antenna with a cross-polarization feed built in a multimode mono-pulse feed. The multimode monopulse feed includes a E-plane mul.timode structure formed by a section of rectangular wave-guide fed through an input wall by an array of waveguides excited to the fundamental mode, and extended by a rectan-gular horn. The cross-polarization feed includes at least one dipole disposed in the aperture of the horn of the mul-timode monopulse feed, in a plane parallel therewith, and with an orientation such that its electric field is ortho-gonal to the electric field of the multimode monopulse feed.

The cross-polarization feed may also include a grid disposed in the aperture of the horn of the multirnode monopulse feed, at the rear of said dipole or dipoles and made up of metal bars parallel to each other and to the direction of the elec-

tric field of said cross-polarization feed.

Advantageously, the grid of the cross-polarization feed has a V-shaped cross section perpendicular to its bars, the base of the V being directed toward the inside of the aper-ture of the horn of the multimode monopulse feed.

The cross-polarization feed can also be of the monopulse type. In this case, it includes four dipoles disposed in a diamond-shaped pattern in the rectangular aperture of the horn of the multimode monopulse feed.

Other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments given as a non-limitative example with reference to the accompanying drawings, in which -Figures 1 and 2 are longitudinal sectional views, respec-tively in the E plane, containing the electric field vector, and in the H plane, containing the magnetic field vector, of a multimode monopulse feed; -Figure 3 is a partially cut out perspective view of a mi-owave antenna in which the invention is,odied, with a cross-polarization feed built in a multimode monopulse feed; -Figure 4 shows the radiation pattern of the cross-polari- zation feed seen in Figure 3, the grid being removed, plot-ted in the E and H planes of the multimode monopulse feed for a frequency of 5,650 MHz; -Figure 5 shows the radiation pattern of the cross-polari-zation Feed seen in Figure 3, equipped with the grid, as well as the phase law of this radiation pattern plotted in the E and H planes of the multimode monopulse feed for a frequency of 5,650 MHz; and -Figures 6, 7 and 8 are longitudinal sectional views in the E-plane and the H-plane, as well as a front view of a multimode monopulse feed equipped with a cross-polarization monopulse feed.

A monopulse feed allows to take advantage of several radiation patterns. These radiation patterns are often three in number: a sum radiation pattern used as a reference, with an even symmetry in the vertical and horizontal planes, a difference radiation pattern, with an odd symmetry in the vertical plane, giving signals of angular deviation in ele-vation, and a difference radiation pattern with odd symmetry in the horizontal plane, giving signals of angular deviation in azimuth.

A multimode monopulse feed is a monopulse feed in which the various radiation patterns are obtained from various laws of illumination of its aperture resulting from various superpositions, within its structure, of various modes, Fun-damental and harmonic, of guided propagation.

Figure 1 shows, as seen in a sectional view along a lon- gitudinal plane containing the electric field vector (E pla- ne) assumed here to be the vertical plane, a multimode mono-pulse feed with an E plane multimode structure, while Figure 2 shows the same multimode monopulse feed as seen in a sec- tional view along a longitudinal plane containing the mag-netic field vector (H plane) which happens to be, with the above assumption, the horizontal plane.

The feed comprises essentially an E-plane mode former comprised of a section of rectangular waveguide 1 closed at its input by a wall in which four rectangular waveguides 2, 3, 4 and 5 open, and extended at its output by a rectan- gular horn 6 through which the feed radiates into free spa-ce.

All four excitation rectangular waveguides 2, 3, 4 and 5 open in the input wall of the E-plane mode former at the apexes of a rectangle, with their wide sides horizontal and parallel to the H-plane. The openings of both upper excita-tion waveguides 2, 3 are separated from those of the two lower excitation waveguides 4, 5 by a median horizontal rib 7 protuding toward the inner of the E-plane mode former.

The openings of the two upper excitation waveguides 2, 3, as well as those of the t.wo lower excitation waveguides 4, 5, are adjacent by their small side and occupy the entire width of the cross section of the rectangular waveguide 1 delimiting the E-plane mode former.

The excitation waveguides 2, 3, 4 and 5 propagate the fundamental mode TE10, whose electric field vector is ver-tical, and cause in the cavity of the feed the creation of harmonic modes retaining a vertical electric field vector, which combines between themselves and with the fundamental mode to give various illumination laws with vertical pola-rizat ion.

In short, to explain the formation of the illumination law in the E plane, we shall consider a single pair of su-perposed excitation waveguides 2, 4 or 3, 5 and the half of the cavity of the feed cut by a vertical median longitu-dinal plane, in which they open. When they are excited in phase, the excitation waveguides of the pair of interest generate in the discontinuity plane of their openings even upper modes TE12 and TM12 which combine between themselves to form a mixed mode EM12 and which are superimposed in pha-se at the aperture of the rectangular horn 6 terminating the mode former E, with the fundamental mode TE10 to form an even illumination law in the sum channel in the E plane.

When they are excited in phase opposition, these same exci-tation waveguides produce in the discontinuity plane of their openings odd upper modes TE11 and TM11 which combine between themselves to form a mixed mode EM11 and give an odd illu-mination law in the difference channel in the E plane.

To explain the formation of the illumination law in the H plane, we shall consider a single pair of adjacent excita-tion waveguides 2, 3 or 4, 5 and the half of the cavity of the reed cut by an horizontal median longitudinal plane, in which they open. When they are excited in phase, only the even fundamental mode TE10 propagates and generates an even illumination law in the I-I plane. When they are excited in phase opposition, these same excitation waveguides gene-rate at the input of the rectangular horn 6 an odd tipper mode TE20 which forms an odd illumination law in the diffe-rence channel in the H plane.

For further explanations on the operation of a multimode monopulse feed, reference is made to the descriptions in the above-mentioned patents.

Figure 3 shows, as seen in partially cut out perspective view, a microwave antenna in which the present invention is embodied with a cross-polarization feed built in a multimode monopulse feed.

In this Figure, there can be seen the structure of a multimode monopulse feed with the rectangular waveguide sec-tion 1 of the E-plane mode former interposed between a base 8 within which the four excitation waveguides open, of which two 2, 3 are visible, and the rectangular horn 6.

A dipole 9 comprising the cross-polarization feed is disposed in the aperture of the rectangular horn 6. It is carried by the rod 10 which is formed by a rigid coaxial line which is fixed in the median horizontal rib 7 in the bottom of the cavity of the multimode monopulse Feed. Its two radiating elements 11 and 12 are disposed in the rectan- gular horn 6, in a plane parallel to its aperture and orien-ted horizontally so as to have an electric field orthogonal to that of the multimode monopulse feed.

Due to the size of the cavity of the multimode monopulse feed, the dipole 9 produces upper modes therein. These upper modes are eliminated by means of a metal grid which makes monomode for the cross-polarized channel the cavity of the multimode monopulse feed. This grid closes the aperture of the cavity of the multimode monopulse feed at the rear of the dipole. It is made up of metal bars which are disposed horizontally, parallel with the radiating elements 11 and 12 of the dipole, and which do not interfere with the ver-tical electric field of the multimode monopulse feed. It has a V-shaped cross section in a vertical cutting plane perpendicul3r Li its ears 13, the base of the V being direc- ted toward the inside of the cavity of the multimode mono-pulse feed. The apex angle of the V and the position of the dipole relative to the apex of this V have an effect on the radiation pattern of the dipole and the position of the pha- se center. Through a suitable adjustment, they allow to ob-tain for the dipole 9 a radiation pattern whose aperture at -3dB and phase center position coincide with those of the radiation patterns of the multimode monopulse feed.

Figure 4 shows, plotted in the E plane and in the H pla-ne of the multimode monopulse feed, the radiation pattern obtained with the dipole 9 disposed in the aperture of the rnultimode monopulse feed without the grid, at an operating center frequency of the multimode monopulse feed and of tu-ning of the dipole 9 of 5,650 MHz. This radiation pattern exhibits a small central lobe recessed between two signifi- cant side lobes. This configuration is due to the upper mo-des generated by the dipole 9 in the cavity of the multimode rnonopulse feed.

Figure 5 shows, plotted in the E plane and in the H pla- ne of the multimode monopulse feed, the new radiation pat-tern and its phase law obtained with the grid interposed at the rear of the dip.ole, in the aperture of the multimode monopulse feed. The center lobe is enlarged to the point where it forms a single lobe with the side lobes, whi.ch il-lustrates the suppression of the upper lobes in the cavity of the multimode monopulse feed.

It is possible to implement a cross-polarization feed which is of the monopulse type by replacing the single dipo-le by four dipoles disposed in a diamond-shaped pattern in the aperture of the rectangular horn of the multimode mono-pulse feed.

Figures 6, 7 and 8 illustrate such a disposition, res- pectively seen in a longitudinal sectional view of the mul-timode monopulse feed in a vertical plane (E plane), then in an horizontal plane (H plane), and in front view.

The four dipoles A, B, C, D are disposed at the apexes of a diamond in the aperture of the rectangular horn 6. The upper dipole A is attached by a rod comprised of a rigid coaxial line 20 to the wall 21 separating the openings of the upper excitation waveguides 2 and 3. The lower dipole C is attached by a rod comprised of a rigid coaxial line 22 to the wall 23 separating the openings of the lower exci-tation waveguides 4 and 5. The median dipoles B and D are placed at the end of rods 24, 25 comprised of a rigid coa-xial line attached side by side to the horizontal median rib 7.

The sum radiation pattern of the cross-polarization feed is obtained by summing the signals picked up by the four dipoles A, B, C and D, the difference radiation pattern in elevation by computing the difference between the signals picked up by the dipoles A and C, and the difference radia-tion pattern in azimuth by computing the difference between the signals picked up by the dipoles B and D. Building a cross-polarization feed in a multimode mono-pulse feed has the advantage of being achievable without increasing the size of the multimode monopulse feed while allowing to obtain radiation patterns with nominal and cross polarizations of comparable width and with coincident phase centers.

It also has the advantage of being achievable in multi-mode monopulse feeds of already existing radar antennas.

It is possible, without departing from the basic princi- ples of the present invention, to modify certain disposi-tions or to replace certain means by equivalent means. It is in particular possible to improve the directivity of the dipole or dipoles by adding directors attached either to their rod by means of an insulating support, or to the wall of a radome covering the aperture of the cavity of the mul-timode monopulse feed.

Claims

Claims 1. A microwave antenna with a cross-polarization feed built in a

multimode monopulse feed, comprising -a multimode monopulse feed with an E-plane multimode struc-ture comprised of a section of rectangular waveguide fed at its input by an array of excitation waveguides, and ex-tended at its output by a rectangular horn; and -a cross-polarization feed comprising at least one dipole disposed in the aperture of said horn of said multimode mo-nopulse feed in a plane parallel to said aperture and with an orientation such that its electric field is orthogonal to that of said multimode monopulse feed.

2. An antenna according to claim 1, wherein said cross-polarization feed comprises in addition a grid made up of metal bars parallel to each other and to its electric field, and disposed at the rear of said dipole, in said aperture of said horn of said multimode monopulse feed.

3. An antenna according to claim 2, wherein said grid has, in a plane perpendicular to its bars, a V-shaped cross sec-tion, the base of the V being directed toward the inside of said aperture of said horn of said multimode monopulse feed.

4. An antenna according to claim 1, wherein said cross- polarization feed comprises four dipoles disposed in a dia-mond-shaped pattern in said aperture of said rectangular horn of said multimode monopulse feed.

-10 - 5. An antenna according to claim 1, wherein said dipole or dipoles of' said cross-polarization feed each includes two radiating elements disposed in alignment with each other in a plane parallel to said aperture of said horn of said multimode monopulse feed, in a direction orthogonal to that of said electric field of said multimode monopulse feed, and carried by the end of a rod attached to the input wall of said rectangular waveguide of said E-plane multimode struc-ture of said multimode monopulse feed on a wall separating the openings of said excitation waveguides.

6. An antenna according to claim 1, wherein said dipole or dipoles are provided with directors improving their di-rectivity.

7. A microwave antenna substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figures 1 to 3, or modified as hereinbefore described with reference to and as shown in Figures 6 to 8.