US3703690A

US3703690A - Dielectric waveguides

Info

Publication number: US3703690A
Application number: US98242A
Authority: US
Inventors: Ivor Albert Ravenscroft; Lynden Ashrooke Jackson
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1969-12-17
Filing date: 1970-12-15
Publication date: 1972-11-21
Anticipated expiration: 1989-11-21
Also published as: NL7018357A; FR2070897A1; DE2061052B2; FR2070897B1; DE2061052A1; GB1338384A; DE2061052C3

Abstract

A dielectric waveguide for millimetric wavelengths comprising a core of polymer material supported by a body of foamed polymeric material and enclosed in a protective jacket.

Description

KR '3a703s690 United States Patent Ravenscroft et al.

rag/a1 5] Nov. 21, 1972 DIELECTRIC WAVEGUIDES Inventors: lvor Albert Ravenscrolt, Woodbridge; Lynden Ashrooke Jackson, Ipswich, both of England Assignee: The Post Office, London, England Filed: Dec. 15, 1970 Appl. No.: 98,242

Foreign Application Priority Data Dec. 17, 1969 Great Britain ..6l,589/69 US. Cl. ..333/95 5, 333/98 M, 350/96 Int. Cl. ..H0lp 3/16, HOlp 3/18 Field of Search ..333/95, 95 S, 98, 98 M References Cited UNITED STATES PATENTS 11/1964 Hicks, Jr. et al ..333/95 X 3,040,278 6/1962 Griemsmann ..333/95 2,769,148 10/1956 Clogston ..333/95 X 2,794,959 6/1957 Fox ..333/95 X 3,386,787 6/1968 Kaplan ..333/95 X 3,434,774 3/1969 Miller ..333/95 X 3,542,536 11/1970 Primary Examiner-l-lerman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Hall & Houghton ABSTRACT A dielectric waveguide for millimetric wavelengths comprising a core of polymer material supported by a body of foamed polymeric material and enclosed in a protective jacket.

4 Claims, 7 Drawing Figures OR IN; 333795 1 p Flam et al. ..333/95 X P'A'TENTEDnuvz: m2

sum 1 nr 6 FIG. 2.

.74 01? 4 1% vsmrno F77 'INVENTOR? BY 7am 7% ATTORNEY imam/21 I972 WENT SHEET 2 [IF 6 9 B S 3253mm mwwzqzmoz k 3 2 E 3 g g g Q 3 I 29%: 38m 5 we? ma 3m Us EBQQ v -mwm -wR 85m HE E m: as mzfim is 335- 3 4 INVENTORS 'BY ATTORNEY P'A'TE'N'TEDnum I972 SHEET 5 UF 6 FREQUENCY (GHZ) FIG. 6.

I 1 01? A PA VE/I/SCROFT INVENTOR ATTORNEY P'A'TE'NTEDInm I972 SHEET 6 0F 6 Ecimg z kqbzmtq INVENTORS DIELECTRIC WAVEGUIDES BACKGROUND OF THE INVENTION It has been proposed to employ a rod of dielectric material as a waveguide but in the past low loss transmission has been achieved only with rods of diameter small in relation to wavelength. Dielectric rods of such small diameter do not possess adequate guiding properties and therefore have to be employed in straight lengths which limits their usefulness.

It is an object of the present invention to provide an improved dielectric rod waveguide.

SUMMARY OF THE INVENTION According to the present invention a dielectric rod waveguide comprises a core of a polymeric material of a loss angle about 50 microradians or less and of a diameter from about 0.5 A a to about 0.85 A where A, is the free space wavelength of the electromagnetic energy to be transmitted along the waveguide, the core being supported by a body of polymeric material in foamed form, having an effective dielectric constant substantially equal to unity and of a size such that the overall diameter of the core and support body is of a value from about 1.6 A, to about 5.0 A,,, the whole lying within a jacket providing protection against mechanical stresses, the ingress of moisture and providing electrical screening.

The core may be composed of polypropylene, a particularly suitable form of which is that known as PXC 3391 manufactured by Imperial Chemical Industries Limited.

The polymeric material in foamed form supporting the core may be the same material as that of which the core is composed.

Preferably, the loss angle should be below microradians.

The core may be in the form of a solid rod or it may consist of a number of strands of the dielectric material or it may be a rod of foamed dielectric material. Both these alternative forms have a lower density than the solid rod and both have a lower effective dielectric constant and lower effective loss angle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged cross-section of the waveguide,

FIG. 2 shows on a difierent scale from FIG. 1 and in diagrammatic form the field configuration of the HE mode in the waveguide of FIG. 1, and,

FIGS. 3, 4, 5, 6 and 7 are graphs illustrating various relationships.

DESCRIPTION OF PREFERRED EMBODIMENTS The waveguide shown in FIG. 1 consists of a core 1 comprising a solid rod of polypropylene of electrical grade having a dielectric constant e, 2.26 of a loss angles equal to 50 microradians. The diameter of the core 1 is 5.3 mm and it is covered by a sheath 2 of low-loss foamed polypropylene preferably of the same grade as that from which the core is made. The overall diameter of the core 1 and sheath 2 is 28 mm. The outer surface of the sheath is covered with a layer 3 of a loss'y" foam having the same effective dielectric constant as the low loss foam. The final covering of the waveguide is a protective jacket 4 of a suitable polymer. The jacket 4 pro- 7 vides protection against mechanical stresses and electrical screening. Embedded in or otherwise forming part of the jacket is a water barrier providing adequate protection against ingress of moisture.

The waveguide supports the HE mode and operates over the frequency range 29-39 Gl-Iz. The field configuration of the HE. mode is indicated in FIG. 2.

The core 1 may be formed by extrusion, care being taken to the diameter of the core 1 to within very close tolerances about i 1 percent and to ensure that the core is totally homogeneous and of a high quality surface finish. The core 1 may be extruded by a method in which molten material is extruded through a nozzle along whose length the rate of cooling of material passing through it is varied in such manner as to perrnit the application to the molten material of an extra or back-up pressure which substantially prevents the appearance of voids. The nozzle has a bore which has an extremely smooth surface thereby ensuring the requisite high quality surface finish for the core.

The sheath 2 may be made by conventional foaming techniques and may be applied to the core during extrusion of the latter or after. The core 1 may be passed into a chamber inside which the core is held at a temperature below the softening of the material from which the core is made, sheathing material being fed to the chamber and therein bonded to the core. The sheathing material may be allowed to foam or be foamed after entry into the chamber or the sheathing material may be fed in foamed form into the chamber.

Application of the lossy material comprising the layer 3 may be effected by the introduction of a substance having a high loss angle, for example carbon, into the outer regions of the foamed sheath 2, or alternatively, the lossy material may be introduced as part of a separately extruded layer.

The group velocity of the HE mode depends to a large extent on the diameter in wavelengths of the core 1, and varies between limits obtained by propagation in free space, when the core diameter tends to zero, and by propagation entirely through the material of the core. FIG. 3 shows, for material of dielectric constant e, 2.26, the relationship between group velocity and normalized frequency (core diameter D/free space wavelength) obtained by numerical methods. If dispersion is to be kept at acceptably low values, rod diameters below about 0.3 A, and from about 0.5 A and above must be used, say from about 0.5 A, to 0.85 A the upper limit being reasonably close to the cut-off point for higher modes of operation of the waveguide.

The relationship between group velocity and frequency for the core 1 of the waveguide of FIG. 1 is shown in FIG. 4.

Since the core 1 must be supported in some suitable manner, practical values for the extent of the radial field of the I-IE, mode must be ascertained. To indicate the extent of the radial field, the graph of FIG. 5 has been prepared showing the diameter of D1 1A,, of the concentric cylinder through which 99 percent and 99.99 percent of the total power in the propagated wave flows. This indicates that the variation of the diameter of percentage power flow with frequency is fairly uniform over the range D X 0.5 A, and 0.72 A

FIG. 5 also shows that 99 percent of the total power is transmitted through a cylinder of a diameter D1 of about 1.6 A over the range of D just mentioned whilst 99.9 percent of the total power is transmitted through a cylinder of a diameter D1 of about 1.4 A However, if the latter value is increased to about 5.0 A a downward extension of transmitted bandwidth is possible.

FIG. 6 shows the relationship between extent of radial field and frequency of operation for the core 1 of the waveguide of FIG. 1. FIG. 6 shows that the overall diameter of the cylinder containing 99.99 percent of the energy is within 28 mm over the frequency range 29-39 GHz.

The attenuation of the HE mode is given approximately by the equation a g 1 nepers/metre where A, free space wavelength in meters 6 dielectric constant of core 1 to sheath 2 -y loss angle of core 1 in radians N /N millimetric of power inside rod to the total power transmitted.

FIG. 7 shows the variation of the attenuation of the waveguide of FIG. 1 with frequency over the range 29-39 GHz.

Thus, by minimizing losses in the material, attenuation, for a given set of operating parameters, is reduced. The dielectric loss may also be reduced by adopting a core of foamed or standard form.

We claim:

1. A dielectric rod waveguide for the transmission of electromagnetic energy at frequencies in the millimetric waveband, the waveguide comprising in combination a core through which a proportion but not all of said electromagnetic energy is transmitted, said core having a diameter lying within the range from about 0.5 A to about 0.85 A where A is the free space wavelength of the electromagnetic energy to be transmitted and being of a polymeric material with a loss angle not greater than about 50 microradians, a core support body round the core through which substantially the balance of said electromagnetic energy is transmitted, the support body comprising a tubular body of polymeric material in foamed form and having a dielectric constant of substantially equal'to unity and of a size such that the overall diameter of core and support body lies within the range from about 1.6 A, to about 5.0 A lossy material associated with said support body to absorb electromagnetic energy incident upon it, and, surrounding the core support, a jacket providing protection against mechanical stresses, the ingress of moisture and electrical screening.

2. A dielectric rod waveguide as claimed in claim 1 in which the core is of stranded form.

3. A dielectric rod waveguide as claimed in claim 1 in which the core is of foamed form.

4. A dielectric rod waveguide as claimed in claim 1 in which the polymeric material of which the core is formed is the same as that of which the core support is formed.

Claims

1. A dielectric rod waveguide for the transmission of electromagnetic energy at frequencies in the millimetric waveband, the waveguide comprising in combination a core through which a proportion but not all of said electromagnetic energy is transmitted, said core having a diameter lying within the range from about 0.5 lambda o to about 0.85 lambda o where lambda o is the free space wavelength of the electromagnetic energy to be transmitted and being of a polymeric material with a loss angle not greater than about 50 microradians, a core support body round the core through which substantially the balance of said electromagnetic energy is transmitted, the support body comprising a tubular body of polymeric material in foamed form and having a dielectric constant of substantially equal to unity and of a size such that the overall diameter of core and support body lies within the range from about 1.6 lambda o to about 5.0 lambda o, lossy material associated with said support body to absorb electromagnetic energy incident upon it, and, surrounding the core support, a jacket providing protection against mechanical stresses, the ingress of moisture and electrical screening.