US2736866A

US2736866A - Filter for transmission line

Info

Publication number: US2736866A
Application number: US152132A
Authority: US
Inventors: Andre G Clavier; David L Thomas
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1950-03-27
Filing date: 1950-03-27
Publication date: 1956-02-28
Anticipated expiration: 1973-02-28
Also published as: DE898339C; NL98681C; FR1034212A; GB683352A; NL160075B

Description

Feb 28 1956 A. G. CLAVIER ET AL I FILTER FOR TRANSMISSION LINE Filed March 27, 1950 lNvENToRs ANDRE G. cLAv/ER OAV/o L. THOMAS A ORNE Unit@ gStates Patent Office 2,736,866 Patented Feb. 28, 1956 FILTER FOR TRANSMISSION LINE Andr G. Clavier, Nutley, N. J., and David L. Thomas,

Lee, London, England, assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 27, 1950, Serial No. 152,132

13 Claims. (Cl. 3253-73) This invention relates to microwave transmission systems and more particularly to a filter and means for coupling the filter in a microwave transmission system.

In addition to the transmission of microwave energy over radio links, coaxial cables and dielectric waveguides, it has recently been discovered that high frequency energy covering a very wide band may be transmitted by means of a single wire properly insulated, the transmission of energy being confined closely adjacent the surface of the wire in the electric and magnetic fields formed thereabout. It has been recognized that when a single wire had electric energy applied thereto that electric and magnetic fields formed about the wire but it was believed that these fields extended outwardly without any definite limits or boundary. lt has been found, however, that if the conductor is coated with a given thickness of insulation the electromagnetic fields are conp centrated and substantially confined within a cylindrical volume of a given radius about the conductor, the given radius being determined by the size of the conductor and the quality and thickness of the insulating coating. By way of example, an ordinary No. 12 enameled copper wire was found to have an electromagnetic field.

concentrated within a three to four inch radius about the Wire and that the high frequency energy fiowed in this field. This manner of propagation now referred to as the surface wave transmission presents very small loss and is substantially free from electrical and 'other disturbances where this confined cylindrical field is substantially unobstructed.

One of the objects of this invention is to provide a filter for passage of certain microwave frequencies.

Another object is to provide a combined filter and coupling arrangement for coupling the filter to certain types of microwave transmission conductors.

One of the features of the invention is to utilize a conductor corresponding to the so-called surface wave conductor as a filter. This is done by disposing enlarged conductive sections such as discs or plates at spaced in-` tervals along the conductor, the spacing depending upon'v the frequency energy to be passed. To avoid or minimize perturbation in the propagation of microwave energy, the disc sections at the ends of the filter are made gradually smaller in diameter or cross-sectional dimension until the last one at each end is nearly the same cross-sectional size as the Wire conductor. The filter conductor and disc sections may be bare of insulation but in order to confine the electromagnetic field throughout the length ofthe filter, it is preferable to coat with insulation the conductor and disc sections similarly as the single wire surface wave conductor associated therewith. The disc sections may be circular, elliptical, rectangular, triangular, or any other shape that does not produce undue perturbation in the wave propagation.

Another feature of tne invention is the manner in which the filter is coupled between a coaxial line or waveguide and a surface wave conductor. The end of the waveguide structure or the outer conductor of a coaxial line is ared outwardly and the tapered end sections of the filter are disposed at least partly within the said fiared portion. Thus when the electromagnetic field is permitted to extend radially in the fiared portion of the waveguide or coaxial outer conductor, the tapered filter section or sections are disposed axially of this expanding zone so that a minimum amount of perturbation occurs. 'This relationship of the tapered section or sections of the filter and the fiared portion of the associated dielectric waveguide or coaxial line bears out for both directions of wave propagation, that is, whether or not the microwave propagation is being launched from the fiared portion or being received thereby.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. l is a view in side elevation of a filter according to the principles of this invention;

Fig-2 is a longitudinal sectional view of the same type of filter shown with tapered end sections coupled to a surface wave conductor;

Fig. 3 is a longitudinal sectional view of the filter coupled to a dielectric waveguide; and

Fig. 4 is a longitudinal sectional view showing a modified filter structure coupled to a coaxial line.

Referring to Fig. 1, the filter basically comprises a conductor 1 which may correspond in size to the surface wave conductor with which the filter is associated, although it may be of another size depending upon the overall filter construction and how it is coupled in the transmission system. The conductor 1 is provided with enlarged sections in the form of discs 2 spaced at intervals therealong. A basic filter section comprises a pair of such discs. The discs may be circular, elliptical, rectangular, triangular or any other shape that will not produce undue perturbation in the wave propagation. The conductor line may also be of shapes other than the usual round form and where desired the conductor may be of a shape similar to the shapes of the discs carried thereby. The spacing intervals d have a direct bearing upon the frequency band that may be passed by the filter. The upper cut-off wave length of the filter may be represented by the formula )\c|r=g while the lower cut-off may be represented by the formula cL=d A narrow band pass filter may be constructed by using two such filters in series, one of the filters being proportioned as a high pass filter and the other as a low pass filter.

The above equations are effective providing the manner of propagation is non-radiating. The thickness W being too great compared with the spacing d would result in undesired energy radiations. The thickness W should therefore be kept small. The ratio of the cross-sectional dimension or diameter D to the diameter of the conductor 1 will affect the sharpness of cutoff and attenuation of the filter. The difference between the radius of the discs and the radius of the conductor should not be greater than one quarter of a wavelength of the upper cut-off frequency.

The filter shown in Fig. 1 is bare of insulation but may be coated with insulation, such as enamel, glass, poly styrene, polyethylene, etc., where it is desired to have the y electromagnetic field along the filter confined closely 'grasses adjacent the filter sections. in Fig. 2 the filter therein shown is provided with such an insulating coating as indicated at 3. The filter structure of Fig. 2 is provided with end portions which minimize the perturbation of the microwave propagation in either direction with respect to the filter. The end sections are shown to comprise a plurality of discs 4, the diameters of which are progressively diminished from adjacent one of the discs of maximum diameter to the endmost disc 5 of the filter, the end disc 5 being slightly larger than the diameter of the conductor 1. The conductor 1 beyond the endmost section 5 may be coupled to or made integral with a surface wave line conductor 6 or may itself comprise a part of the line conductor 6. The conductor 1 and the disc line sections of the filter are all shown to be coated with a layer of insulation 3 of a thickness corresponding to the insulation on the line conductor 6. It will be clear, however, that the coating of insulation on the filter conductor and sections may be either thicker or thinner than the coating on the conductor depending on the permissible diameter of electromagnetic field about the filter as well as the filtering action thereof.

In Fig. 3, a filter with terminatingl tapered sections as shown in Fig. 2 is shown coupled to a hollow conductor 7 which may comprise a circular wall structure of a waveguide or the outer conductor of a coaxial line. The end portion 8 of the hollow conductor may be plane but preferably is fiared in the form of a horn and certain of the end sections of the lter are disposed within this flared portion. The end lter section 5 is disposed closely adjacent the inner end of the iiared portion 8 so that the taper of the end sections associated therewith corresponds proportionately to the angular degree of the ared portion 8 which is concentrically disposed thereabout. Where the hollow conductor 7 is provided with a center conductor 9, as in the case of a coaxial line, the insulation 3 is also tapered from the periphery of the disc section 5 to the surface of the conductor 9 as indicated by the tapered body 10. The disc sections are preferably the same shape as the cross-section of the waveguide or coax.

In Fig. 4, a similar microwave transmission line of the coaxial type is shown as comprising an outer conductor 11 and an inner conductor 12. The outer conductor is also flared as shown by the end portion 13. The hollow conductor 11 contains a solid dielectric 14 which extends at the same diameter outwardly through the flared portion 13. The filter in this embodiment is provided with a tapered end section 15 which has a maximum diameter D corresponding to the diameter of the other sections 16 of the filter. The end section 15 is tapered from its end of maximum diameter to the surface of the conductor 12 substantially in radial alignment with the inner end 17 of the ared portion 13. The section 15 is proportioned to extend for a length equal to or slightly larger than the length of the fiared portion 13 of the outer conductor.

In both forms shown in Figs. 3 and 4, the tapered end portion of the filter is disposed within the flared horn end of the outer conductor. This relationship provides for smooth transfer of energy from manner of propagation to another with a minimum of perturbation. In Fig. 4 the dielectric body contained in the conductor 11 is gradually diminished in thickness along the section 15 until it reaches the uniform thickness of insulation on the filter. This gradual change in the dielectric also aids in minimizing the occurrence of perturbation of the wave propagation.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that many variations are contemplated, for example, the discs may be of various shapes so long as the longitudinal cross-sections thereof are symmetrical. Also, the discs may be made separately and secured to the conductor or they may be made integral therewith. This description is therefore to be regarded as being made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

l. A high frequency band pass filter for use in conjunction with an insulated line conductor adapted singly to propagate high frequency energy through van electromagnetic field confined closely adjacent the surface of such conductor, comprising a single conductor and a plurality of groups of axially separated enlarged conductor sections extending outwardly about said conductor, each group consisting of a plurality of serially connected conduct-or sections, adjacent conductor sections of one group being spaced one wavelength apart, and adjacent conductor sections of a second lgroup being spaced two wavelengths apart, the two groups being serially connected and the wavelength 'spacings representing the upper and lower cut-off wavelengths of said band pass filter.

2. A high frequency filter according to claim l, wherein said enlarged sections `are coated with insulating material.

3. A high frequency filter according to claim l, wherein the conductor and the enlarged sections thereof are coated with insulating material.

4. A high frequency filter according to claim l, wherein the end sections of the filter are graduated in size with the endmost section slightly larger than the diameter of the conductor.

5. A high frequency filter according to claim 4, wherein the conductor and the enlarged sections are coated with insulating material with the insulation Iadjacent the end section being tapered from the periphery of said end section to closely adjacent the surface of said conductor.

6. A high frequency filter according to claim l, wherein the diameters of the conductor sections are approximately equal and the end section is tapered from la diameter Isubstantially equal to the diameter of the other sections down to the diameter of said conductor.

7. A high frequency filter according to claim 6, wherein the sections of said filter are coated with insulation.

8. A high frequency filter according to claim l, further including a hollow waveguide conductor coupled to one end of the filter, and the end portion of said filter being disposed at least partly within the end portion of said end section being disposed substantially within the ared portion of said hollow conductor.

l1. A high frequency filter according to claim 9, wherein the end lsection is tapered from a diameter corresponding to the diameter of the other sections to the diameter of the line conductor.

12. Ahigh frequency filter according to claim 9, Wherein the end sections are of progressively smaller diameter in the direction of the endmost section.

13. A high frequency filter according to claim 9, wherein the hollow conductor -contains a body of dielectric material extending outwardly of said flared portion with the end section of said filter imbedded therein.

References Cited inthe file of this patent UNITED STATES PATENTS 2,207,845 Wold July 16, 1940 2,342,736 Herzog Feb. 29, 1944 2,387,783 Tawney Oct. 30, 1945 2,419,855 Roosenstein Apr. 29, 1947 2,438,795 Wheeler Mar. 30, 1948 (Gther references on following page) UNITED STATES PATENTS FOREIGN PATENTS 2,438,913 Hansen Apr. 6, 1948 287,503 Great Britain Feb. 14, 1929 2,479,687 Linder Aug. 23, 1949 2,497,707 Wetherill Feb. 14, 1950 OTHER REFERENCES 2,567,718 Larson Sept' 11 1951 5 A Survey of the Principles and Practice of Wave 2,588,610 Boothroyd etal Mar. 11, 1952 Guides, L. G. H. Huxley, Macmillan Co., New York, 2,643,353 Dewey june 23, 1953 1947, pages 198-203- (Copy in SCintC Library.) 2,659,817 Cutler NOV 17, 1953 Science Abstracts, vol. II (April) 1899, pages 233- 2,688732 Kock Sep/[ 7, 1954 234, Gresham Press, London. (Copy in Scientific Li- 10 brary.)