US3750183A

US3750183A - Multimode antenna system

Info

Publication number: US3750183A
Application number: US00208060A
Authority: US
Inventors: S Drabowitch
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1970-12-22
Filing date: 1971-12-15
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31
Also published as: FR2118848A1; SE375410B; NL160995B; GB1359830A; DE2163881A1; DE2163881B2; IT945571B; FR2118848B1; NL160995C; NL7117456A

Abstract

A waveguide structure, designed to operate as a wide-band multimode antenna, has an elongate main waveguide section centered on an axis and terminating at a radiating aperture, an intermediate waveguide section of lesser width joined to that main section, and a pair of still narrower input sections feeding the intermediate section, these input sections being cophasally or antiphasally excited with the same fundamental mode TE10. A first discontinuity, at a transverse plane representing the junction between the input and intermediate waveguide sections, and a second discontinuity, at a transverse plane representing the junction between the intermediate and main waveguide sections, give rise to mixed TEM modes which come into existence at the first or the second discontinuity, depending upon wavelength; at the originating discontinuity, the mixed mode is in phase with the generating fundamental mode so that, with proper dimensioning of the length of the main waveguide, they will recombine cophasally at the radiating aperture.

Description

0 n, r tt tates Patent 1 [111 3,75,t3 Drahewitch fly 31, R973 MULTHMODE ANTENNA SYSTEM [75] Inventor: Serge Drabowitch, Paris, France ABSTRACT [73] Assi e; Thom o -(13F, Pa i France A waveguide structure, designed to operate as a wideband multimode antenna, has an elongate main wave- [22] 1971 guide section centered on an axis and terminating at a [21] A pL No.1 208,060 radiating aperture, an intermediate waveguide section of lesser width joined to that main section, and a pair of still narrower input sections feeding the intermediate [30] Foreign Application Priority Data section, these input sections being cophasally or anti- Dec. 22, [970 France 7046249 phagaliy excited with the same fundamental mode TEW A first discontinuity, at a transverse plane representing ELS- (Hi. the junction between the input and intermediate wave. [5 flint. Cl. guide sections and a econd digeoni inuily at 3 "ans. [58] Field of Search /771, 333/6 verse plane representing the junction between the intermediate and main waveguide sections, give rise to References (med mixed TEM modes which come into existence at the UNITED STATES PATENTS first or the second discontinuity, depending upon wave- 3,573,838 4/1971 Ajioka 343/786 i f at the P s s dissfmtinuity, the mixed mode 3,373,431 3/1968 Webb IS in phase with the generating fundamental mode so 3,530,483 9/1970 Pierrot..... that, with proper dimensioning of the length of the 3,566,309 2/1971 Ajioka 343/786 main waveguide, they will recombine cophasally at the Primary Examiner-451i Lieberman Attorney-Karl F. Ross radiating aperture.

10 Claims, d Drawing Figures PATENIEU 1 3. 750. 1 83 SHEET 1 BF 2 Serge DRABOWITCH Inventor 5 By A g ()0 Attorney PATENTEB I975 3. 750, 1 83 sum 2 BF 2 WAVE- ENERGY INPUT Serge DRABOWITCH Inventor By WA g'iRoss Attorney 1 MULTIMODE ANTENNA SYSTEM The present invention relates to improvements in multimode antennas.

Multimode antennas are radiating devices which are based upon simultaneous radiation of different guided propagation modes which are generated by devices known as moders and whose amplitudes and phases are controlled. Under these conditions, the the resulting radiant-energy distribution or beam pattern is determined by the superimposition of the fundamental and harmonic guided-propagation modes. The creation of these modes and their superimposition have been achieved in structures, more specifically known as moders," which are constituted by waveguides and embody discontinuities designed to generate higher modes from the fundamental mode injected at the input ot the structure. My prior U.S. Pat. No. 3,308,469 discloses moders of this kind in which one or more waveguides known as excitation waveguides, each filtering its fundamental propagation mode, terminate in the input of a main waveguide which has a definite length and'whose open end in principle constitutes the radiating opening. This main waveguide is designed to propagate various higher modes and the harmonic modes are generated in principle at the planes of the discontinuities at which the excitation waveguides terminate. FIG. 1 of the accompanying drawing illustrates such a moder which can now be considered as conventional. The main waveguide l, of length L, communicates with two excitation waveguides 2 and 3, the latter terminating in the main waveguide at the plane 2 known as the discontinuity plane. The excitation waveguides 2 and 3 are supplied with the fundamental mode whereas harmonic modes are generated at this discontinuity plane; the harmonic modes, together with the fundamental mode, are propagated through the principal waveguide 1, whose dimensions are chosen accordingly, up to the opening or aperture plane Z L where they are radiated. Naturally, the moder of FIG. 1 has been illustrated purely schematically and may be supplemented with, for example, mode filters for eliminating unwanted modes, matching devices associated with the excitation waveguides, and also other discontinuities intentionally introduced in order to modify the ratio of the generated modes.

However, the fundamental and harmonic modes generated in accordance with the foregoing are propagated through the main waveguide at different phase velocities up to the radiating aperture at location 2 L. The result is that the precise phase combination required at the radiating aperture is strictly achieved only at a single frequency. However, if the modes utilized are not too close to cut-off it can be shown that very stable energy distribution can be obtained through frequency bands having a width on the order of 10 percent.

in certain applications, and in particular in space telecommunications, it is necessary to provide a passband whose width is on the order of 12 to 14 percent; a bandwidth on the order of 18 percent is also contemplated.

The object of the present invention, is to provide, in a moder of the class described in the aforementioned patent, a novel structure which operates over a substantially wider passband than has been obtained with selective moders of prior-art design.

An antenna system according to my present invention comprises a waveguide strucutre with at least two stepped junctions, or discontinuities, between an excitation stage, including a plurality of input waveguide sections, an intermediate waveguide section, and a main waveguide section terminating in a radiating aperture. The intermediate waveguide section has a crosssection which is smaller than that of the main waveguide section but larger than that of each input waveguide section and, in fact, of the several input waveguide sections combined. These input waveguide sections are simultaneously excited, with a common fundamental mode of propagation, from a source of wave energy at a frequency which is transmissible by the structure.

The main waveguide section may be centered on an axis and may have a circular cross-section, with the stepped junctions lying in planes transverse to that axis. The input and intermediate waveguide sections, in the embodiment described hereinafter, are of rectangular cross-section similar to corresponding waveguides in the antenna system of my prior U.S. Pat. No. 3,308,469.

As will be described more fully hereinafter, the length of the main waveguide section should be calculated to produce substantial in-phase relationship at the radiating aperture between the energy propagating at the fundamental mode from the input stage and energy propagating at a higher mode, generated at the last junction, for a minimum transmissible frequency giving rise to this higher mode.

The above and other features of my invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1, already referred to, is a perspective view of a prior-art moder according to my U.S. Pat. No. 3,308,469;

FIG. 2 is a similar schematic view of a moder in accordance with the invention;

FIG. 3 is a schematic axial sectional view of a simplified type-E moder embodying my invention; and

FIG. 4 is a similar view of another embodiment of the invention.

In accordance with the invention, the widening of the passband of a moder depends upon the phase velocity of the modes propagating within its structure, and upon its length which is determined in such a fashion that the modes are in phase in the plane of the radiating aperture.

FIG. 2 schematically illustrates my improved waveguide strucutre in wich the main waveguide 1 is pre ceded by a conventional moder of the kind disclosed in my above-identified patent. in this Figure, the composite unit 4 is a flat E-type moder whose excitation or

input sections

5 and 6 are both supplied in the fundamental mode (TE cophasally or in antiphase depending upon the parity of the modes which it is desired to generate in an intermediate section waveguide 8,'the dimensions of the latter section having been chosen accordingly. Another horn-type waveguide 9 is shown as well, with its discontinuity plane 10. These multimode excitation assemblies terminate in the main waveguide 1 whose input plane 11 constitutes a discontinuity plane or stepped junction. In this principal waveguide, the modes generated at the discontinuity planes such as 7 and 11 combine in phase and amplitude to produce the resultant in-phase modes at the radiating aperture.

The novel structure thus has two axially spaced discontinuities located in the junction planes between the several differently sized waveguide sections.

FIG. 3 illustrates another multimode structure embodying my invention which has the characteristics set out hereinbefore and is simplified for the sake of easier explanation of its operation. This schematic view illustrates a longitudinal section of an E-type moder but the explanation which follows applies equally to an H-type moder also coming within the scope of the invention.

This moder structure in accordance with the invention comprises

excitation stage

5, 6 between the input plane 12 and the plane of a first discontinuity 13. Between the plane 13 and the plane 14 of a second discontinuity, there is an intermediate waveguide section 15. Beyond the plane M, the main waveguide section 16 extends up to the plane 17 of the radiating aperture of the system.

The excitation of input sections and 6 are assumed to be supplied with the fundamental TE mode and the two successive discontinuities l3 and 14 create the TE and TM modes which have the same phase velocity. These two modes combine to produce the mixed mode TEM g.

Excited in phase at the discontinuity planes, these TE and TEM modes propagate at their own phase velocities and if they are to be in phase at the end of the main waveguide through which they propagate, then the length L of the latter must satisfy the condition where A and A are the wavelengths of the two modes in question.

Unter these circumstance, and in accordance with the dimensions of the intermediate and

main waveguides

15, 16 it is observed that at the low frequencies of the range the higher mixed mode TEM is cut off in the intermediate waveguide and does not start to propagate until beyond the second discontinuity 14. The system then operates like a conventional moder with only one discontinuity. At the high frequencies of the range, the higher mixed mode is generated at the first discontinuity 13, propagates through the intermediate waveguide and thus excites the main waveguide 16. At the second discontinuity 14 a function of the higher mode TEM is generated which combines in amplitude phase with the mode generated by the first discontinuity.

The two mode functions which combine at the second discontinuity are out of phase and, in accordance with equation (I), the length of the main waveguide is such that this equation is satisfied. Under these conditions, the wave components will attain a cophasal relationship at the plane of the radiating aperture where, for this length of waveguide, the desired distribution is achieved. In fact, in accordance with the frequency at which the system is operated, equation (I) is satisfied by the introduction ofan equivalent length L, as if the resultant TEM mode were generated in a plane located between the two discontinuity planes l3 and 14. Thus, when the frequency is relatively low, the plane of generation of the resultant mode is that of the second discontinuity, but as the operating frequency increases, this plane of generation of the higher mode tends to move further away from the plane of the second discontinuity l4 and to move towards the plane of the first discontinuity 13. The equivalent length L of the structure in accordance with the invention thus increases with the frequency, the physical length ot the main waveguide being chosen for the lowest frequency in the range.

Under these conditions, a bandwidth on the order of 20 percent has been observed, with phase errors at the radiating end of the multimode waveguide not in excess of 6.

FIG. 4 illustrates a further multimode antenna structure which constitutes a generalization of the embodiment hereinbefore described. In this flat E-typc moder structure the two

discontinuity planes

13 and 14 are still present but this time there terminate at the plane of the second discontinuity i. e., at the entrance end of main waveguide section 16, two

supplementary excitation waveguides

18, 19 designed to generate odd modes, e.g. TEM This kind of arrangement makes it possible to carry out range measurements in a monopulse system by the exploration of difference channels. The operation of this kind of system is similar to that already described. At the low frequencies in the band in question, the higher mode disappears at the first discontinuity 13 and is only propagated beyond the level of the second discontinuity 14. As the frequency rises, the system behaves as if the resultant mode produced in the principal waveguide had been generated at a plane moving from the second discontinuity 14 to the first in the tapering intermediate waveguide 20 of progressively increasing cross-section and a function of the higher mode is generated at the plane of the first discontinuity 13.

In this system, beyond the second discontinuity and in the principal waveguide 21, I provide shield means to produce theoretically perfect decoupling between the

supplementary waveguides

18, 19 and the composite waveguide 22 for the excitation of the even modes. This shield means consists of metal plates 23 24 disposed transversely to the electric field of the fundamental mode.

In FIGS. 3 and 4 the

intermediate waveguide section

15 or 20 is shown to be coaxial with main waveguide section 16 whereas the

input waveguide sections

5, 6 or .22a, 22b are symmetrically disposed with reference to their axis. Sections 22a, 22b of FIG. 4 are seen to open into a further waveguide section 220 ahead of section 20, with formation of an additional stepped junction or discontinuity at 22d.

Thus, a novel wideband multimode structure has been described which presents at least two successive, stepped discontinuities. It should be borne in mind in this context that the height of the steps is not a matter of arbitrary choice. It depends upon the proportion of the higher modes which have to be generated in order to produce the radiation pattern required at the plane of the radiating aperture of the system.

Similarly, it is evident that the number of discontinuities used is a function ofthe width of the desired passband. The number of cascaded discontinuities is in no way limitative and may be increased without departing from the principles of the invention.

It is likewise possible, without departing from the scope of the present invention, to provide discontinuities in a flat H-type moder and to combine a flat E-type moder and H-type moder.

Of course, the invention is not limited to the embodiments described and shown which are given solely by way of example.

What is claimed, is:

1. An antenna system comprising a waveguide structure with a main waveguide section terminating at a radiating aperture, an intermediate waveguide section coupled to said main waveguide section at an end thereof remote from said aperture, said intermediate waveguide section having a cross-section smaller than that of said main waveguide section, and a plurality of input waveguide sections of still smaller cross-section coupled to said intermediate waveguide section at an end thereof remote from said main waveguide section, said input and intermediate waveguide sections forming a stepped first junction, said intermediate and main waveguide sections forming a stepped second junction; and a source of wave energy of a frequency transmissible by said waveguide structure connected to said input waveguide sections for simultaneously exciting same with a common fundamental mode of propagation.

2. A system as defined in claim 2 wherein said main waveguide section is centered on an axis, said first and second junctions lying in planes transverse to said axis.

3. A system as defined in claim 2 wherein said input and intermediate waveguide sections extend parallel to said axis.

4. A system as defined in claim 2 wherein said input and intermediate waveguide sections are of rectangular cross-section, said main waveguide section being of circular cross-section. I

5. A system as defined in claim 2 wherein said input waveguide sections are a pair symmetrically positioned with reference to said axis.

6. A system as defined in claim 1 wherein the combined cross-section of said input waveguide sections is less than the cross-section of said intermediate waveguide section.

7. A system as defined in claim 1 wherein said intermediate waveguide section progressively increases in cross-section from said first to said second junction.

8. A system as defined in claim 1, further comprising supplemental input means entering said intermediate waveguide section at said first junction.

9. A system as defined in claim 8, further comprising conductive shield means in said intermediate waveguide section adjacent to said first junction for mutually decoupling said supplemental input means and said input waveguide sections.

10. A system as defined in claim 1 wherein said main waveguide section has a length calculated to produce substantial in-phase relationship at said radiating aperture between energy propagating at said fundamental mode and energy propagating at a higher mode, generated at said second junction, for minimum transmissible frequencies giving rise to said higher mode.

{k a: s m

Claims

4. A system as defined in claim 2 wherein said input and intermediate waveguide sections are of rectangular cross-section, said main waveguide section being of circular cross-section.