US3444487A - Waveguide having corrugated exterior and smooth metal coated interior - Google Patents

Description

May 13, 1969 w. KRANK ET AL WAVEGUIDE HAVING CORRUGATED EXTERIOR AND SMOOTH METAL-COATED INTERIOR Sheet Filed Oct. 3, 1966 Fig.7

YA Fig. 2

May 13, 1969 w. KRANK ETAL 3,444,487

WA UIDE HAVING CORRUGATED EXTERIOR A SMOOTH METAL-COATED INTERIOR Sheet Filed Oct. 3. 1966 3 of 4 Inventors:

bOoL-fgoma Kvomk 3 ,8 Laws 5 P Flfizovvuaf s May 13, 1969 w. KRANK ET AL 3,444,487

WAVEGUIDE HAVING CORRUGATED EXTERIOR AND SMOOTH METAL-COATED INTERIOR Filed Oct. 5, 1966 May 13, 1969 w. KRANK ET AL WAVEGUIDE HAVING CORRUGATED EXTERIOR AND SMOOTH METAL-COATED INTERIOR Flled Oct 5 1966 Sheet lnvenfors;

hi M w awt m b KO 0 Ha m mm, "wh 0 G.. VJ 3 United States Patent C Int. 01. Hel 3/12, 3/14 US. Cl. 333-95 17 Claims ABSTRACT OF THE DISCLOSURE A flexible waveguide structure, which has a non-circular internal cross section, is provided with a hollow dielectric support tube whose outer surface is corrugated and defines the outer wall of such waveguide structure and a thin metal layer disposed on the inner surface of the support tube which defines a smooth inner wall for such structure.

The present invention relates to waveguides, and particularly to flexible waveguides for the transmission of linearly polarized electromagnetic waves.

It has recently been suggested to construct metal waveguides which are sufficiently flexible to be wound on a drum by constructing the waveguide in the form of a corrugated metal tube having an elliptical cross section, which tube is sufiiciently sturdy to prevent any deformations of the waveguide cross section. One such type of waveguide is described in German Patent No. 1,156,455. It has been found that waveguides of this type are inherently relatively heavy.

It has also been recently suggested, in German Patent No. 1,025,473, for example, to fabricate a flexible waveguide utilizing a dielectric foam as the waveguide support. The waveguide itself is constituted by longitudinally extending metallic foils disposed on the support. These waveguides, which have a rectangular cross section, have not proven to be satisfactory in practice because they barely satisfy existing radiation prevention requirements and because the nature of the dielectric foam is such as to cause these waveguides to have a relatively high attenuation.

It has also already been suggested, in German Gebrauchsmuster GM No. 1,744,052, for example, to provide a non-flexible waveguide whose inner wall is constituted by a metallic layer having a thickness of the order of the microwave current penetration depth, and whose outer wall is constituted by a layer of a synthetic resin.

It is a primary object of the present invention to provide a flexible waveguide having improved electrical and mechanical properties.

Another object of the present invention is to provide a flexible waveguide of relatively light weight.

Yet another object of the present invention is to provide a flexible waveguide having improved electrical characteristics.

These and other objects according to the present inven tion are achieved by the provision of a flexible waveguide structure having a non-circular internal cross section and including a hollow dielectric support tube whose outer surface defines the outer wall of the structure, and a thin metal layer disposed on the inner surface of the tube and defining the inner wall of the structure, at least one of the walls being corrugated.

Additional objects and advantages of the present invention Will become apparent upon consideration of the 3,444,458? Patented May 13, 1969 following description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a longitudinal, cross-sectional detail view of a first embodiment of the present invention.

FIGURE 2 is a view similar to that of FIGURE 1 showing another embodiment of the present invention.

FIGURE 3 is a view similar to that of FIGURE 1 of yet another embodiment of the present invention.

FIGURE 4 is a longitudinal, cross-sectional view of a further embodiment of the present invention.

FIGURE 5 is a view similar to that of FIGURE 4 of yet a further embodiment of the present invention.

FIGURE 6 is a view similar to that of FIGURE 4 of a still further embodiment of the present invention.

FIGURE 7 is an axial, cross-sectional view showing one form of construction of embodiments of the present invention.

FIGURE 8 is a view similar to that of FIGURE 7 of another form of construction thereof.

FIGURE 9 is a view similar to that of FIGURE 7 of yet another form of construction thereof.

FIGURE 10 is a view similar to that of FIGURE 7 of still another form of construction thereof.

FIGURE 11 is a view similar to that of FIGURE 7 of a further form of construction thereof.

It is known that in the transmission of energy at microwave frequencies, the depth of penetration of the high frequency current into the conductor is limited, due to the skin effect, to a relatively thin surface layer of the conductor. As a result, the current density in a solid conductor decreases toward its center. The radial distance below the surface of such conductor at which the current density has diminished to l/e of its value at the surface is known as the skin depth 0' for a particular frequency. The smaller the skin depth, the higher the equivalent resistance of the conductor. The present invention takes advantage of this effect by giving the waveguide conductor the form of a thin metal layer.

The present invention permits a relatively long waveguide, which is sufliciently flexible to be wound onto a drum, to be fabricated in one piece. This waveguide preferably has a non-circular internal cross section and has a corrugated inner and/or outer surface.

The hollow dielectric support can be made of any flexible dielectric, such as polyolefine, polyvinylchloride, or polytetrafluorethylene, for example, or foam bodies of these materials. The metal layer can advantageously be constituted, for example, by copper, copper alloys, silver, or aluminum.

According to a preferred embodiment of the present invention, the waveguide has a corrugated inner wall and a smooth outer Wall. The corrugations may have a zero pitch angle, i.e., they may be in the form of a succession of circular corrugations, or they may be in the form of a continuous helical corrugation having a pitch angle (,0.

The corruations may follow an approximately sinusoidal path in a direction parallel to the longitudinal axis of the wave-guide. They may also be formed so that each corrugation has a cosine-squared shape. The corrugations can also be in the form of a plurality of identical, spaced arcs of any suitable configuration.

In one particularly advantageous form of construction according to the present invention the waveguide has an elliptical internal cross section and is provided with a helical corrugation.

Turning now to FIGURE 1 of the drawings, there is shown a partial, longitudinal cross-sectional view of a waveguide constituting a first embodiment of the present invention, The x-axis of the drawing represents the longitudinal waveguide axis and the y-axis is taken along one radius of the waveguide cross section. The waveguide is constituted by a dielectric support 1 on the inner surface of which is dis-posed a thin metal layer 2 constituting the waveguide conductor.

The dimension W represents the minimum waveguide radius along the coordinate y. The dimension S represents the distance between a corresponding point of two successive corrugations. When the waveguide has a helical corrugation, the dimension S represents the corrugation pitch.

When the waveguide has a substantially elliptical cross section and a helical corrugation, the pitch S is preferably chosen so as to satisfy the following relationship:

where a represents the wavelength of the lowest frequency of the dominant waveguide mode.

In the embodiment of FIGURE 1, the outer wall of the waveguide is smooth and the inner wall thereof is corrugated. The corrugation has a depth t and is given an approximately sinusoidal shape which can be represented by the equation:

where n preferably has a value of +1.

This waveguide also preferably has an approximately elliptical cross section whose effective major axis is 20 and whose elfective minor axis is 212 When such a waveguide is provided with a helical corrugation having a pitch angle (,0, it has been found that the waveguide will be capable of transmitting a particularly satisfiactory bandwidth and will present a low reflection coefiicient if the corrugations are formed in accordance with the following equations:

where: y; is the shape of the corrugation along a longitudinal plane passing through the major axis of the waveguide elliptical cross section 3 represents the shape of the corrugations along a longitudinal plane passing through the minor axis of the waveguide elliptical cross section, t represents the corrugation depth in the direction of the major elliptical axis, and t represents the corrugation depth in the direction of the minor elliptical axis.

It has been found that the most satisfactory operation is achieved when n is made equal to +1, r is less than equal to 0.05M; and also less than or equal to 0.la and the pitch angle (p is equal to or less than 8. The corrugation depth I must have a value of between 0.7t and 1.4t, and is preferably made approximately equal to t.

The thickness of metal layer 2 is preferably made at least equal to the skin depth a. However, in order to reduce radiation of the microwave energy, and the accompanying attenuation of the transmitted wave, it is preferable to give the layer 2 a thickness which is double or triple the skin depth a. This is particularly desirable when two such waveguide are disposed adjacent one another because it is then extremely important that the signals being transmitted by the two waveguides be sufiiciently decoupled. This decoupling can be efiected either by giving the metal layer a sufficient thickness or by introducing into the dielectric support 1 a material, such as graphite for example, which will absorb any energy passing through the layer 2. This latter arrangement will thus produce a damping of the energy passing through the metal layer 2.

Turning now to FIGURE -2, there is shown another embodiment of the present invention which is similar to that of FIGURE 1 except for the form given the corrugation of the waveguide inner wall. The corrugation shown in FIGURE 2 is approximately parabolic and may be described by the following equation:

where Q represents the distance between the waveguide longitudinal axis and the longitudinally extending center line of the corrugations, and t is the corrugation depth.

When the waveguide of FIGURE 2 has a substantially elliptical cross section, the path defined by the corrugation along a longitudinal plane through the minor axis of the elliptical cross section may be represented by an equation which bears the same relation to the equation set forth above as that existing between the equations relating to y and y set forth earlier in connection with the embodiment of FIGURE 1. Similarly, the same conditions as those set forth in connection with the description of the embodiment of FIGURE 1 apply to the embodiment of FIGURE 2 insofar as concerns the pitch angle (p and the corrugation depths t and t for obtaining a minimum reflection coefficient.

FIGURE 3 shows another waveguide according to the present invention which is identical with that shown in FIGURES l and 2, with the exception that it is provided with a corrugation having the form of a train of cosine-squared corrugations, with each individual corrugation having a longitudinal extent of 2 5. The configuration of the corrugated inner wall in a direction parallel to the x-axis may be represented by the following equation:

sin 4) sin 2 where T is the minimum inner radius of the waveguiide along the y-axis. In this embodiment also the corrugation depth along the minor elliptical axis of the waveguide cross section may be different from that along the major axis thereof.

A special case exists when equals 1r.

When the waveguide has an approximately elliptical cross section and is provided with a helical corrugation, the values of the corrugation depths t and 1 along the major and minor elliptical axes, respectively, are determined in the same manner as that set forth above in connection with the embodiment of FIGURE 1.

Referring now to FIGURE 4, there is shown another embodiment of the present invention in which the waveguide is provided with a smooth inner wall and a corrugated outer wall. The corrugation is preferably arranged to yield the desired flexibility and can have any of the configurations described above in connection with FIG- URES 1 to 3.

FIGURE 5 shows another embodiment of the present invention in which both the inner and outer walls of the waveguide are corrugated. In this embodiment, the dielectric support 1 has a uniform thickness along the entire length of the waveguide. As a result, the same corrugation is formed on the outer waveguide wall as on the inner waveguide wall. This corrugation can be given any one of the configurations described above in connection with FIGURES 1 to 3.

Turning now to FIGURE 6 there is shown another embodiment of the present invention in which both the inner and outer waveguide walls are corrugated, with the inner waveguide wall corrugation being different from the outer wall corrugation. The outer wall corrugation is preferably formed in accordance with the mechanical requirements imposed on the waveguide with regard to its ability to be twisted and bent and its mechanical stability, while the inner corrugation is selected so as to impart the desired electrical properties to the waveguide,

such as a low reflection coefficient and a high degree of suppression of undesired wave types.

The outer wall of the support 1 is preferably covered with a second metal layers 5. The provision of two metal layers is highly advantageous because it results in the creation of a coaxial conductor system which can be used in the manner of an ordinary coaxial conductor to transmit an additional signal having a low frequency.

In those cases where the waveguide is to have a particularly high degree of flexibility, the outer conductive layer 5 can be arranged to have at least one interruption. This interruption may extend in a longitudinal direction or, when maximum twistability and flexibility are required, the interruption can extend along the trough of the corrugation in {the outer waveguide wall. When this outer corrugation is in the form of a plurality of spaced circular corrugations, the formation of a gap along the trough of each such corrugation eliminates the possibility of using the two metal layers as a coaxial transmission system.

Particularly in those cases where the outer waveguide wall is provided with a metal layer, it is desirable to enclose the entire waveguide with a protective sheath 6 preferably made of a wear-resistant and corrosion-resistant material.

In addition to being capable of being wound onto a drum and of being fabricated in one piece so as to have as low a weight as possible, waveguides according to the present invention are intended to effect a highly efficient transmission of linearly polarized waves. In order to achieve this result, waveguides according to the present invention are constructed to have a noncircular inner cross section. When such a cross section is provided, the plane of polarization of a transmitted wave is prevented from undergoing an undesired rotation, as occurs in waveguides having a circular inner cross section. The desired mode of operation according to the present invention can be achieved by modifying the inner cross section of a normally circular waveguide by attaching one or more metallic, dielectric, or metallized longitudinal bars to the inner waveguide wall. FIGURE 7 shows one such arrangement in which the generally cylindrical support 71 is provided on its inner wall with two longitudinally extending projections 3 and 4 which effectively destroy the circular configuration of the inner waveguide cross section and which give the device the form of a ridged waveguide. The metal layer 2 is applied to the inner wall of support 71 and upon the exposed surfaces of bars 3 and 4. It should be appreciated that this represents only one of the many possible configurations which can be employed in accordance with the present invention for the purpose of permitting an eflicient transmission of linearly polarized waves while preventing any rotation of their plane or polarization.

Depending on its intended use, the waveguide could also be given a configuration of the type shown in FIG- URE 8 wherein the dielectric support 81 has a generally rectangular cross section and is provided with rounded corners, or it may be given the form shown in FIGURE 9 wherein the dielectric support 91 has an approximately elliptical cross section.

Waveguides having the electrical characteristics contemplated by the present invention can also be constructed to have a cross section defining a symmetrically or asymmetrically flattened circle.

FIGURE 10 shows one embodiment of the present invention corresponding to an asymmetrically flattened circle, while FIGURE 11 shows an embodiment corresponding to a symmetrically flattened circle. The embodiment of FIGURE 10 is provided with a dielectric support 101, while the embodiment of FIGURE 11 is provided with a dielectric support 111. In each embodiment, the flattening corresponds to a respective one of the cross section axes 2:1 and 212 Waveguides of the types described above can also be provided with internally extending ridges for the purpose of increasing their conduction bandwidth.

When no separate junction is used for connection to other waveguides or for detuning the waveguides according to the present invention, it is possible to fabricate the ends of waveguides according to the present invention in such a Way as to permit them to perform the function of a junction. For example, the end of the waveguide can be originally fabricated in the form of a socket or can be subsequently connected to a suitable junction to form a complete structural unit.

In the construction of waveguides according to the present invention, the metal layers 2 and 5 are preferably applied by means of the known electroless metal plating processes. To prevent the metal layer from cracking or chipping off under high mechanical stress conditions, it is desirable to spray each metal layer with a low-loss, highly adherent lacquer, such as one having a polyester base for example.

It has been found that waveguides constructed according to the present invention have electrical characteristics which are comparable to those of prior art waveguides, while being substantially lighter in weight and more flexible and twistible, and while being capable of being produced in the form of relatively long, unitary pieces.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. A flexible waveguide structure having a non-circular internal cross section, said structure comprising, in combination:

(a) a hollow dielectric support tube whose outer surface defines the outer wall of said structure, said outer wall being corrugated; and

(b) a thin metal layer disposed on the inner surface of said tube and defining the inner wall of said structure, said inner wall being smooth.

2. An arrangement as defined in claim 1 wherein the thickness of said metal layer is at least equal to the com ductor skin depth 0' at the lowest frequency to be transmitted by said waveguide.

3. An arrangement as defined in claim 8 wherein the thickness of said :metal layer is equal to at least twice the skin depth 0' at the lowest frequency to be transmitted by said waveguide.

4. An arrangement as defined in claim 1 further comprising electromagnetic energy absorbing material distributed throughout said tube.

5. An arrangement as defined in claim 10 wherein said energy absorbing material is graphite.

6. An arrangement as defined in claim 1 wherein said outer wall is corrugated to have a series of circular corrugations, each extending completely around said waveguide outer wall.

7. An arrangement as defined in claim 1 wherein said outer wall is provided with a continuous helical corrugation.

8. An arrangement as defined in claim 1 wherein said outer wall is corrugated so as to have a corrugation which describes a plurality of parabolic arcs in a direction parallel to the longitudinal axis of said waveguide.

9. An arrangement as defined in claim 1 wherein said outer wall is corrugated to have a corrugation which describes a plurality of sinusoidal arcs in a direction parallel to the longitudinal axis of said waveguide.

10. An arrangement as defined in claim 1 further comprising an additional thin metal layer disposed on the outer surface of said tube.

11. An arrangement as defined in claim 10 wherein said additional metal layer forms a continuous current con duction path along the length of said waveguide for causing said metal layers to constitute a coaxial conductor system.

12. An arrangement as defined in claim 19 wherein said outer wall is helically corrugated and said additional metal layer is provided with a continuous interruption which extends along the trough of the resulting helical corrugation for imparting a high degree of flexibility and twistability to said waveguide.

13. An arrangement as defined in claim 1 wherein said waveguide has an approximately elliptical internal cross section.

14. An arrangement as defined in claim 1 wherein the internal cross section of said waveguide is in the form of a rectangle having rounded edges.

15. An arrangement as defined in claim 1 wherein the inner cross section of said waveguide has at least one circular arcuate portion and at least one substantially flat portion.

16. An arrangement as defined in claim 1 wherein said waveguide has the internal cross section of a ridged waveguide.

17. An arrangement as defined in claim 1 further comprising a protective sheath of wear-resistant and corrosion-resistant material enclosing said waveguide.

8 References Cited UNITED STATES PATENTS 2,908,746 10/1959 Fairhurst 3339S 2,758,612 8/ 1956 Zaleski. 2,950,454 8/1960 Unger 33395 3,016,502 1/1962 Unger 333-95 3,101,458 48/ 1963 Chandler 333-95 3,101,744 8/1963 Warnaka. 3,290,762 12/ 1966 Ayuzawa. 3,299,374 1/ 1967 Schickle. 3,364,446 1/ 196-8 Schickle 33395 FOREIGN PATENTS 739,488 11/1955 Great Britain.

557,003 5/1958 Canada. 1,193,125 5/1965 Germany.

OTHER REFERENCES Anderson, T.N., and Vega, 1., Double Ridge Wave Guide for Weather Radar, in Radio-Electronic Engineering April 1955, pps. 18, 19, 50, 51.

HERMAN KAZRL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner.

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