US2951222A

US2951222A - Bends for circular wave guides

Info

Publication number: US2951222A
Application number: US777874A
Authority: US
Inventors: Marie Georges Robert Pierre
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-12-19
Filing date: 1958-12-03
Publication date: 1960-08-30
Anticipated expiration: 1977-08-30
Also published as: FR1190730A; DE1068321B; GB857889A

Description

Aug. 30, 1960 a. R. P. MARIE BENDS FOR CIRCULAR WAVE GUIDES Filed Dec. 5, 1958 2 Sheets-Sheet 1 iiI/IIIIII/IIIA a,

EXM'HNER Aug. 30, 1960 G. R. P. MARIE szmas FOR CIRCULAR wm: cums 2 Sheets-Sheet 2 Filed Dec. 3, 1958 United States Patent BENDS FOR CIRCULAR WAVE GUIDES Georges Robert Pierre Mari, 16 Rue de Varize, Paris, France Filed Dec. 3, 1958, Ser. No. 777,874 Claims priority, application France Dec. 19, 1957 Claims. (Cl. 333-98) This invention relates to bends for circular wave guides, and more particularly to bends for circular wave guides for the propagation of TE, mode electromagnetic waves.

The bends according to the invention preferably consist of metal elements in the form of rectangular toroidal segments. The elements are in the shape of a wedge in cross-section substantially square and will hereinafter be referred to as segmental metal elements. The bends comprise: segmental metal elements having two converging plane surfaces forming a first angle between them and pierced with a circular aperture forming a section of the guide; dielectric prisms introduced between two segmental metal elements and having plane surfaces forming between them a second angle; and means for securing a number of such elements and prisms relatively to one another.

However, while the just described arrangement is a preferred form of embodiment of the invention, it must be understood that said prisms might as well be mounted inside ordinary bent wave guide tubing, as the main advantage of said arrangement is greater facility for securing said prisms inside said guide. The main original feature of the invention is a new method of construction of said prisms.

In the device of the invention, the angle of said prisms is without effect on the direction of wave propagation and is, for instance, equal to the angle between the surfaces of a segmental metal element, while the apparent permittivity and therefore the apparent refractive index of each prism vary from place to place therein in such manner that the phase wave length in the bend at any part thereof is proportional to the radius of the bend at such part. This is effected by providing the plane surfaces of said prisms with grooves of suitable size, shape and arrangement.

The invention will hereinafter be described in detail with reference to the accompanying drawings wherein:

Figure 1 is a perspective view of a segmental metal element;

Figures 2 and 3 are a view in longitudinal section and a view in cross-section, respectively, of a preferred embodiment of the invention, and

Figures 4 and 5 are explanatory diagrams of the construction of the bend illustrated in Figures 2 and 3.

Figure 1 illustrates a segmental metal element 1 of a bend of a circular wave guide. The element 1 is in the shape of a prism or wedge without any edge. The plane surfaces 2, 3 converge along an imaginary edge 25 at a reduced angle 3, for instance, of the order of 5 or less. Each element 1 is pierced with a circular aperture 4, the axis 5 of which is perpendicular to the plane 6 bisecting the dihedron formed by the surfaces 2, 3. When a number of said elements are stacked to form the bend, the internal wall of the guide is formed by the inner walls of the apertures 4 of consecutive elements.

The four corners of the element 1 are pierced with transverse apertures 7, 8, 7', 8' which extend parallel with the axis 5 and which are adapted to receive arcuate metal rods such as 10, 11. The elements 1 are clamped on the rods 10, 11 by clamping screws 9, and against one another by nuts 12, which bear against flanges 13 of straight sections 14 of the waveguide at each end of the bend. The method of securing the prisms between the metal elements will be explained later on, in connection with Fig. 2.

It has been found that if a stack of metal and dielectric elements according to the invention is used to produce a deflection of electromagnetic waves, the TE mode is maintained satisfactorily and with a very reduced power loss, even with a bend having a much shorter curvature radius than in known systems.

Such loss is mainly caused by parasitic waves produced by reflections on the surfaces of the prisms 20 within the bend. The power associated with these higher modes is radiated outside the waveguide, for the higher modes are associated with longitudinal currents which can travel out of the bend in the dielectric-filled spaces between the segmental metal elements.

In the embodiment of a bend illustrated in Figures 1 and 2, to obviate reflections at transitions from a dielectric surface to the air, the apparent refractive index of the dielectric prisms varies with the distance from a point on the prism to the axis of the annulus.

To explain the construction of the novel dielectric prisms, it will be noted with reference to Figure 4 that, in a medium consisting of dielectric plates 44 which are of thickness a and which have parallel surfaces and which are disposed parallel with one another and spaced apart by a distance b between the central planes of two consecutive plates, the apparent permittivity e' of such medium is related to the permittivity e of the dielectric substance forming the plates by the formula:

for a plane wave which travels in a direction 40 parallel with the plane of the plates 44, has its electric field parallel therewith and is of a wavelength considerably greater than 2b.

Referring to Figures 2 and 3, three

segmental metal elements

27, 28 and 29 similar to those illustrated in Figure 1 are assembled by means of rods 30 which extend through apertures 31 and are provided with clamping nuts 32. Two variable-refractive-index dielectric prisms 33 and 34 have an apex angle equal to the apex angle of the segmental metal elements. In Figure 2, the prisms 33 and 34 are shown as being slightly spaced apart but when the nuts 32 are tightened they engage by way of their surfaces 48 and 48'. The prisms 33 and 34 are formed with

grooves

35, 36 and 37 of a shape to be described hereinafter, annular cylindrical projections 41 to 43 being left therebetween. The grooves are of a depth such that a solid wall 49 is left in the plane bisecting each prism. For the sake of simplicity, only three such grooves and projections are shown in Figures 2 and 3 but in practice more can be provided. The periphery of each prism is formed by a thin plate 38 engageable between two consecutive segmental metal elements. The rods 30 extend through apertures 39 in the fixing plates 38.

The prisms 33 and 34 are made, for instance, of moulded polyethylene.

The grooves 35 to 37 and the cylindrical projections 41 to 43 have a general substantially circular shape but in actual fact their shape is complex, since their thicknesses vary and the bounding lateral walls are not coaxial.

The construction of the annular grooves and projections will be explained with reference to Figure 5.

Let be the center of the thin wall 49 and coincident with the centers of the circular apertures pierced in the

elements

27 and 28. The wall 49 is situated in a plane passing through the axis of revolution of the annulus, a part of which forms the bend according to the invention.

Let Or and 0y be two reference axes taken in the plane of the wall 49. The axis Ox is perpendicular to the axis of the annulus. The axis Oy perpendicular to Ox is parallel with the axis of the annulus. A third axis Oz is perpendicular to the cross-section of the bent circular guide--i.e. tangential to the center line 45 of the bend (Figure 2)--and forms a trirectangular trihedron with the axes Ox and Oy.

The axis Ox is the common axis of symmetry of all the grooves 35 to 37 and projections 41 to 43.

Let:

N=the number of annular projections or rings in the D=the diameter of the circular guide;

R=the radius of curvature of the bend-Le. the radius of the circle formed by the center line 45.

The annular projection of order k (the ring of order unity is the smallest diameter ring) is included between two cylinders of radius (R -p/2) and (R +p/2), the axes of revolution of which meet the axis Ox at abscissae points .7c=e, and x: -e respectively.

R: will be called the mean radius" of the ring of order k.

p will be called the mean thickness" of the rings and is independent of k.

The mean radius of the ring is found from the formula:

In Figure 5, a circle 46 of center 0 and radius R =D/4 N is the mean circle of the ring 41, while circles 47 and 47' have centers 0 and 0, respectively and radii R -p/2 and R,+p/2 respectively. The points 0 and O: are on the x axis and have as abscissae e and e respectively.

If the quantity e is small as compared with the guide diameter D, the thickness of the ring of order k in de pendence upon the azimuth (calculated in the plane xOy and taking Ox as the axis of origin) is found from the formula:

pt()= t cos Since cos =x/R Formula 3 can be written:

ek i

It will therefore be apparent that the various rings form a medium similar to that of Figure 4, except that the plates 26 are replaced by annular cylindrical plates 41 to 43, a is replaced by p and b is replaced by (R -1K =D/2N Formula 1 applied to rings of the prism 33 gives:

4 revolution of the annulus of which the bend forms a part. Hence:

If the bend radius R is fairly large relative to the wave guide diameter D, only the first x/R term of Formula 7 is required.

By identifying the second members of Formulae 5 and 7, the latter being reduced to the first x/R order, there As a rule, the diameter D is predetermined; the radius R is made fairly large to exclude non-linear x/R terms.

Advantagcously, to reduce losses in the dielectric of the rings 41 to 43 the same are extremely thin. The least thickness is that of the Nth ring on the x axis. Let this thickness be denoted by p It is equal, according to Formula 3, to:

Of course, this minimum value must be compatible with moulding requirements, and Formula 8 combined with Formula 9 appears as a first-order equation in p, giving:

Once p is known, e and more generally a quantities can be calculated from the formula:

so that all the parameters of the moulded dielectric prism according to the invention are provided.

To prevent parasitic reflections of the kind already mentioned, the prismatic shape of the dielectric system illustrated in Figures 2 and 3 is such that the boundary planes 48 and 48 of two consecutive prisms 33 and 34 are contiguous; in other words, the rings of the same mean diameter of two consecutive systems engage with one another.

In conclusion, a calculation example will be given for the simple case of a bend for a circular waveguide having a diameter D of 7 cm. and serving for the propagation of electromagnetic waves of a frequency of 10,000 mc./ s. A =3 cm.). The bend radius R is 70 cm.

The moulded polyethylene prismatic member of permittivity e=2.6 comprises three rings; the distance between the rings is much less than one half-wavelength and so there is no likelihood of parasitic modes being produced.

In order that the prismatic member may be readily moulded, the thickness p of the third ring on the x axis is 0.5 mm.

The mean thickness of each of the three rings is p=l mm. The eccentricities of the ring-bounding cylinders are found from Formula 11 to be:

The mean radii of the rings corresponding to these three eccentricities are:

What is claimed is:

1. A low loss bend for a circular wave guide propagating a 'I'E wave, comprising a length of circular crosssection metallic wave guide the axis of which is bent with a given radius and a plurality of wedge-shaped prisms of dielectric material mounted inside said guide length and having two converging plane faces substantially perpendicular to said bent axis, wherein at least part of said prisms have their converging plane faces provided with grooves of progressively varying width and depth from the inside towards the outside of said bend, said grooves provided on said faces being so shaped as to permit said TE wave to propagate around said bend with no substantial distortion.

2. A bend as claimed in claim 1, wherein the widths and depths of said grooves progressively vary in such a manner that the phase wave length in said bend at any part thereof be proportional to the radius of said bend in latter said part.

3. A bend as claimed in claim 1, wherein said grooves are limited by non-concentric circular cylindrical surfaces substantially parallel to said axis and the radii of which progressively increase from said axis towards the inside surface of said guide.

4. A bend as claimed in claim 1, wherein said guide length is made of spaced metal elements having the shape of rectangular toroidal segments and having two converging plane faces which form an angle, each one of said elements being provided with a circular aperture.

5. A bend as claimed in claim 4, wherein spacings are provided between said metal elements and wherein said prisms are secured to said guide length by means of projections provided in said prisms and tightly held in said spacings between said elements.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,712 Southworth Sept. 13, 1938 2,596,251 Kock May 13, 1952 2,779,006 Albersheim Jan. 22, 1957 2,785,397 Rust et a1. Mar. 12, 1957 OTHER REFERENCES Steutzer: Proceedings of the IRE, Sept. 1950, vol. 38, No. 9, pages 1053-1056.