US3144624A - Coaxial wave filter - Google Patents

Description

Aug. 11, 1964* c. A. RYPINSKI, JR

COAXIAL WAVE FILTER 2 Sheets-Sheet 1 Filed Aug. 1, 1960 OUTPUT $ECTION FILTER EQECTION lNPUT SECTION -OuTPUT INPUT CHA N005 A. RYP/N5K4JQ Aug. 11, 1964 c. A. RYPINSKI, JR 3,144,624

coAxIAL WAVE FILTER Filed Aug. 1, 1960 2 Sheets-Sheet 2 DB DOWN 5 IO ATTEN'UATION I5 l I 500 I000 2.000 3000 4000 5000 FREQU ENCY "III DOWN 2O 25 ATrEN UAT\ON 5O 4o NO SPURlOUSTRANSMlSSDN 4 [GREATER THAN 5on5 DowN o 4000 MC A 7TORNE Y6 United States Patent 3,144,624 COAXIAL WAVE FILTER Chandos A. Rypinski, Jr., Sherman Oaks, Calif., assignor to C. A. Rypinski Company, a corporation of California Filed Aug. 1, 1966, Ser. No. 46,539 3 Claims. (Cl. 333-43) This invention relates to electromagnetic wave trans mission systems, and more particularly to filters for coaxial waveguide systems which are required to transmit electromagnetic energy at high voltage levels.

In systems in which electromagnetic energy is transmitted along a hollow, coaxial or other transmission line structure, the physical size required for the transmission line structure generally becomes smaller with increased frequencies. However, in systems which operate with relatively high power transmission levels, there is a danger of voltage breakdown within the transmission line structure when small sizes are used. Therefore, when transmitting signals involving high power and voltage levels, transmission lines of larger sizes are required, up to six inches and more in cross-section.

In wave transmission systems utilizing waveguide structures, filters are used as basic circuit elements to pass only waves of selected frequencies. For the purpose of blocking the passage of harmonics of a wave of fundamental frequency, a low pass filter is particularly useful in Wave transmission systems utilizing waveguide structures, inasmuch as the appearance of harmonic frequency waves is apt to deleteriously affect the operation of a system.

The desirable features of a low pass filter include minimum attenuation in the pass band, sharp cutoff at the upper limit of the pass band, and a marked degree of attenuation over a broad band of frequencies above the cutoff frequency. In accordance with a prior arrangement of low pass filters, it was thought that certain geometrical relationships precluded achieving the above mentioned desirable features.

Accordingly it is a primary object of the present invention to provide a new and improved coaxial waveguide structure which functions as a filter.

It is another object of the present invention to provide a new and improved filter for use in electromagnetic wave transmission systems at high power levels.

It is still another object of the present invention to provide a new and improved filter for use in a transmission line utilizing shunt elements.

Electromagnetic wave filters in accordance with the present invention employ conductive elements on one conductor in opposed relation to shunt capacitance elements of another conductor of a wave transmission section. The conductive elements are disposed and configured relative to the shunt capacitance elements such that currents conducted along the two conductors traverse substantially equal path lengths so as to nullify the eifect of wavelength radial dimensions.

In a specific example of a filter in accordance with the invention, the inner conductor of a coaxial waveguide section may be defined by alternate series inductance elements of relatively small diameter and shunt capacitance discs of relatively large diameter. A number of annuli may be mounted on the inner surface of the outer hollow conductor, with each of the annuli encompassing a different one of the conductive discs of the inner conductor. When employed in large waveguide sizes, this configuration permits the shunt capacitance elements to exhibit low impedance so as to provide improved filtering characteristics over a broader frequency band than has heretofore been possible.

A better understanding of the invention may be had 3,144,624 Patented Aug. 11, 1964 by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view, partly broken away, of a single section coaxial filter in accordance with the present invention;

FIG. 2 is a side sectional view of the arrangement of FIG. 1;

FIG. 3 is a simplified schematic circuit diagram, showing equivalent circuit elements for the arrangement of FIGS. 1 and 2;

FIG. 4 is a graph of attenuation versus frequency on which is displayed performance characteristics for a filter arrangement as shown in FIGS. 1 and 2;

FIG. 5 is a side sectional view of another arrangement in accordance with the invention which utilizes a number of sections to provide a low pass coaxial filter; and

FIG. 6 is a graph of attenuation versus frequency on displayed performance characteristics for a filter as shown in FIG. 5.

A coaxial filter section in accordance with the present invention may be coupled between two sections of coaxial wave transmission line. For convenience, the two sections which are coupled together by the filter may be referred to as an inputcoaxial line 10 and an outputcoaxial line 12, although it will be recognized that either or both may serve at different times to provide inputs and outputs to the filter section. Thefilter section 20 consists of a coaxial wave transmission section which is joined at its opposite terminal ends byflanges 14 and 15 to the input and outputcoaxial lines 10 and 12 respectively.

Thefilter section 20, referring now to both FIGS. 1 and 2, is defined principally by anouter conductor 21 and a center orinner conductor 22, with theouter conductor 21 being hollow and disposed concentrically about a central axis with which theinner conductor 22 is aligned. The principal body of theouter conductor 21 is defined by a cylindrical shell of the type usually employed for the outer conductor of coaxial waveguide sections. Theinner conductor 22, however, is defined by sections disposed along the central axis which have selected dimensions in both length and diameter relative to the central axis. Each terminal end of theinner conductor 22 is defined by a coupling ortransition element 24 or 25 having a diameter corresponding to that of the center conductor in the input andoutput lines 10 and 12 respectively. Thecoupling elements 24, 25 may have conventional fittings (not shown) at the points at which they engage mating fittings of the associated coaxial line sections.

The central section of theinner conductor 22 comprises aseries inductance element 27 of relatively small diameter to provide a high impedance in thefilter section 20. Each of the opposite ends of theseries inductance element 27 is coupled to a different one of thecoupling elements 24 or 25 by ashunt capacitance element 28 or 29 respectively. Theshunt capacitance elements 28, 29 are in the form of discs of relatively large diameter and relatively shorter length than theseries inductance element 27. All of theelements 24, 25, 27, 28 and 29 are of circular cross-section and are concentric with the central axis of thefilter section 29. Theinner conductor 22 is maintained in desired position relative to theouter conductor 21 byinsulating rings 31 and 32 set into matching recesses in thecoupling elements 24 and 25 respectively. Theinsulating rings 31 and 32 may be of foam polystyrene or some other material having a dielectric constant of substantially unity, or electrically compensated for in the circuit.

Theouter conductor 21 also includes a pair of annuli ordiscs 33, 34, each of which is coupled to the inner surface of theouter conductor 21 and protrudes inwardly therefrom toward and in opposing facing relation to a are ges- 3 different one of theshunt capacitance elements 28 or 29, respectively. A concentric radial spacing exists between each of the low impedanceshunt capacitance elements 28 and 29 and its immediatelyadjacent annulus 33 or 34.

Thefilter section 20 and the associated input and output lines and 12 may be of relatively large dimension, with theouter conductor 21 being, for example, of the order of six inches in diameter where it is desired to transmit, i.e. propagate in a confined mode, at frequencies up to approximately 800 megacycles.

As mentioned above, the use of fundamental frequencies of 500 megacycles may also involve the unwanted transmission of harmonics of the fundamental frequency, thus necessitating the use of low pass filter sections to attenuate the harmonics. The equivalent circuit elements of theseries inductance element 27 and theshunt capacitance elements 28 and 29 are theinductance element 36 and theshunt capacitance elements 37 and 38 shown in FIG. 3. Theinductance element 36 corresponds to the high characteristic impedance section and thecapacitance elements 37 and 38 correspond to the low characteristic impedance sections. Theshunt capacitance elements 37 and 33 also include the discontinuity capacitances which are introduced by the abrupt changes in line diameter due to termination of the electric, E, field on the transverse surfaces of the capacitance discs.

It has been shown in the published literature (Very High-Frequency Techniques, v01. 2, pp. 688691, Radio Research Laboratory, Harvard University, McGraw-Hill Book C0,, Inc., 1947) that the relationships I ;2(r r and l ;2(r r should be observed, where is the axial length of a capacitive element, is the spacing between opposing faces of adjacent capacitive elements, r is the internal radius of the outer conductor, r is the radius of the capacitive discs, and r is the radius of the series inductance. While the first condition is generally satisfied without difficulty, the second has heretofore been considered to place a definite restriction on the largest allowable radius r Previously also, it had been thought necessary to reduce r as much as possible so as to minimize discontinuity susceptances at the changes in inner conductor diameters. Where energy of high voltage and power is to be transmitted, so that large waveguide sizes are required, or where the frequency to be transmitted is in the higher reaches of the microwave region, so that the waveguide must be very small, the desired proportionality between the difference r r and the axial spacing I is difficult to obtain. Stated in another way, with previously known devices it is difficult to obtain good pass band characteristics and concurrently have high attenuation across a frequency band which extends to many times the cutoff frequency.

In the present arrangement, the capacitive element of the filter illustrated in FIGS. 1 and 2 is defined by the pairedinner discs 28 or 29 andouter annuli 33 or 34 respectively. The relative sizes of the various elements in theinner conductor 22 and the outer conductor 21 'are arranged such that currents passing along each of theconductors 21 and 22 traverse substantially equal path lengths. The paired elements, such as 28 and 33 and 29 and 34 which lie in the same transverse planes, provide the desired capacity, but exhibit low impedance for a wider range of frequencies than has been heretofore feasible.

By virtue of the fact that the currents traverse relatively equal path lengths, the capacitive shunt elements continue to exhibit low impedance at the frequencies of the higher harmonics, at which the spacing between the inner and outer conductors becomes greater than a quarter wavelength. By equal path lengths is meant that currents are in phase as they appear to each other at opposite points on the paired discs andannuli 28, 33 and 29, 34. By relatively equal is meant that there is less than a quarter wavelength displacement at the highest frequency under consideration. With the currents in phase at the shunt elements, the filter is free from the effects of spurious lines which appear to be in series with the shunt elements. These spurious lines arise from out-of-phase currents and constitute, effectively, transmission line sections with impedance characteristics which are dependent upon frequency. In consequence, the operation of the filter remains substantially the same to many multiples of the cutoff frequency. This is illustrated by the curve of attenuation versus frequency shown in FIG. 4 for the arrangement of FIGS. 1 and 2. With a nominal transmission frequency of approximately 500 megacycles and a cutoff frequency selected of 580 megacycles, there is a sharp increase in attenuation beyond the cutoff frequency, and attenuation of 25 decibels or greater for the majority of the stop band for the single filter section. Spurious responses occur, as is well known, at frequencies where the length of the series inductor is one-half wavelength, and at a higher harmonic, at which the capacitor dimension is one-half wavelength.

The superior filter action provided by the single filter section may be augmented by the use of a number of elements, with the elements being staggered as to frequency characteristics so as to provide attenuation at all microwave frequencies over a wide band above the cutoff frequency. Thus, as shown in FIG. 5, a low pass coaxial filter may be constructed of a number of filter sections 4-0. Each of thefilter sections 40 may include aseries inductance element 41 and shunt capacitances, with the capacitance elements being defined bydiscs 42 on the center conductor and annuli 44 on the outer conductor. The dimensions of thediscs 42 and theannuli 44 are selected relative to the associated inner and outer conductors such that the path lengths traversed by currents passing along the two conductors are equal.

The effective response of each filter section may be designed in accordance with well known techniques to achieve the desired overall response. Network synthesis may be used, for example, to establish the inductance and capacitance values which are needed for a desired response, and these values may then be realized in coaxial transmission line form, with suitable corrections being made for discontinuity reactances. The values of the inductance and capacitance parameters may correspond to a Butterworth, Chebishev or fiat time delay network. The inductance and capacitance values may be obtained with a variety of mechanical proportions to satisfy, at the same time, voltage, current and spurious response requirements. In addition, as is shown in FIG. 5, the series offilter sections 40 may be terminated at opposite ends at the middle of either an inductance element, e.g., 46, or a capacitance element, e.g., 48.

The use of the arrangement of FIG. 5 permits a sharp cutoff, but also a high degree of attenuation over a wider range of harmonics than has heretofore been possible. As may be seen in FIG. 6, in a practical example, attenuation of more than db is achieved with sharp cutoff to approximately ten times cutoff frequency. Further, an insertion loss of only 0.05 db with variations of 0.05 db peak-to-peak, is maintained in the pass band. With techniques heretofore available, at 2.5 times the cutoff frequency the diameter of the line would be such as to introduce a spurious half wavelength response and etfectively place a limit upon the upper frequency which would be attenuated, with the use of multiple sections.

With a specific device corresponding to the arrange ment of FIG. 1, the following dimensions were used:

r =2.86 inches r 1.5 inches r =.375 inch l =2.25 inches l =.87 inch Annulus inner radius=1.87 inches The transmission line employed had a nominal frequency of 500 megacycles and a design cutoff frequency of 3 decibels down at 580 megacycles.

A specific device constructed in accordance with the arrangement of FIG. 5 utilized a series of filter sections made up of 18 elements, with amid-capacitance termina 5 tion at one end and a mid-inductance termination at the other. A Chebishev distribution was employed for the various filter sections. In this instance, the pass band encompassed 225 to 400 megacycles and the stop band extended to greater than times the cutoff frequency.

In a practical example of the arrangement of FIG. 5, operating with the performance characteristics of FIG. 6, the following dimensions were employed. Here the successive inductance and capacitance elements are successively designated L C etc., and the axial lengths (l) and the radii (r and r correspond to the definitions used above. The designation r is employed for the inner radius of an annular element.

Component 1 (inches) 12 (inches) 7 (inches) r (inches) 121 (inches) While there have been described above and illustrated in the drawings various forms of filters for providing sharp cutoff and high attenuation over a broad band of frequencies above the cutoff frequency, it will be appre ciated that the invention is not limited thereto. Filters may also be constructed in other types of transmission lines, such as strip transmission line and planar transmission line. Accordingly, the invention should be taken to include all modifications, variations and alternative forms falling within the scope of the appended claims.

What is claimed is:

1. A low pass coaxial Waveguide filter for high power wave transmission systems utilizing relatively large size waveguide structures, including the combination of an outer conductor, the outer conductor being concentric with a central axis, a number of conductive rings, each of the conductive rings being coupled to the outer conductor at an inner periphery of the conductor and lying in a plane transverse to the central axis, the rings protruding inwardly towards the central axis and each being spaced along the central axis a selected distance from the adjacent rings, and an integral central conductor lying along the central axis, the central conductor having a cross sectional configuration at each point therealong which is concentric relative to the central axis and defined by successive sections, with alternate sections having relatively small diameters and relatively longer lengths, and then relatively larger diameters and shorter lengths, the lengths of the larger diameter sections corresponding to the lengths of the rings coupled to the outer conductor, and the differences in the diameters of the successive sections of the central conductor being selected with respect to the rings which are coupled to the outer conductor such that currents traveling along the inner and outer conductors traverse paths of relatively equal lengths.

2. A coaxial waveguide filter for electromagnetic wave energy having an input and an output and comprising a hollow outer conductor, a first plurality of conductive elements coupled to the inner surface of said outer conductor, and an inner conductor mounted concentrically within said outer conductor, said inner conductor comprising a second plurality of conductive elements connected together by a central conductor of reduced diameter which serves as an inductance at the frequency of operation, corresponding ones of said first and second pluralities of elements being located in the same transverse plane in opposed facing relationship to form shunt capacitances, the axial dimensions of said first and second pluralities of conductive elements being substantially equal and defining, together with the radial dimensions of said elements, current paths from the input to the output along the inner and outer conductors of relatively equal lengths, whereby said currents are relatively in phase at opposing points on the inner and outer conductors.

3. A coaxial waveguide filter for electromagnetic wave energy having an input and an output and comprising a hollow outer conductor, a plurality of conductive rings coupled to the inner surface of said outer conductor, and an inner conductor mounted concentrically within said outer conductor, said inner conductor comprising a plu rality of conductive discs connected together by a central conductor of reduced diameter which serves as an inductance at the frequency of operation, corresponding ones of said discs and said rings being located in the same transverse plane in opposed facing relationship to form shunt capacitances, the axial dimensions of said conductive discs and rings being substantially equal and defining, together with the radial dimensions of said discs and said rings, current paths from the input to the output along the inner and outer conductors of relatively equal lengths, whereby said currents are relatively in phase at opposing points on the inner and outer conductors.

References Cited in the file of this patent UNITED STATES PATENTS 2,030,179 Potter Feb. 11, 1936 2,438,913 Hansen Apr. 6, 1948 2,527,608 Willoughby Oct. 31, 1950 2,543,721 Collard Feb. 27, 1951 2,557,567 Rumsey et al June 19, 1951 2,603,707 r Jaynes July 15, 1952 2,641,646 Thomas June 9, 1953 2,944,233 Fong July 5, 1960 FOREIGN PATENTS 944,576 France Apr. 8, 1949

Claims (1)

1. A LOW PASS COAXIAL WAVEGUIDE FILTER FOR HIGH POWER WAVE TRANSMISSION SYSTEMS UTILIZING RELATIVELY LARGE SIZE WAVEGUIDE STRUCTURES, INCLUDING THE COMBINATION OF AN OUTER CONDUCTOR, THE OUTER CONDUCTOR BEING CONCENTRIC WITH A CENTRAL AXIS, A NUMBER OF CONDUCTIVE RINGS, EACH OF THE CONDUCTIVE RINGS BEING COUPLED TO THE OUTER CONDUCTOR AT AN INNER PERIPHERY OF THE CONDUCTOR AND LYING IN A PLANE TRANSVERSE TO THE CENTRAL AXIS, THE RINGS PROTRUDING INWARDLY TOWARDS THE CENTRAL AXIS AND EACH BEING SPACED ALONG THE CENTRAL AXIS A SELECTED DISTANCE FROM THE ADJACENT RINGS, AND AN INTEGRAL CENTRAL CONDUCTOR LYING ALONG THE CENTRAL AXIS, THE CENTRAL CONDUCTOR HAVING A CROSS SECTIONAL CONFIGURATION AT EACH POINT THEREALONG WHICH IS CONCENTRIC RELATIVE TO THE CENTRAL AXIS AND DEFINED

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