Oct. 13, 1964 H. J. RIBLET 3,153,208

WAVEGUIDE FILTER HAVING NONIDENTICAL SECTIONS RESONANT AT SAME FUNDAMENTAL FREQUENCY AND DIFFERENT HARMONIC FREQUENCIES Filed May 6, 1960 2 Sheets-Sheet 1' Z2 Z :l "'Zn INVENTOR.

HENRY J. RIBLET M A TORNEY Oct. 13, 1964 H BLET Filed May 6. 1960 ATTENUATION (DB) ATTENUATION (DB) s 9 I I0 u l2 FREQUENCY (KMC) FIG. 4

a 9 IO M l2 I3 14 l5 l6 l7 l8 FREQUENCY (KMc) F I G. 5 INVENTOR.

HENRY J. RIBLET 2 I A'FTORNEY United States Patent WAVEGUIDE FELTER HAVING NONIDENTICAL SECTIONS RESQNANT AT SAME FUNDAMEN- TAL FREQUENCY AND DIFFERENT ONIC FREQUENCIES Henry J. Riblet, 35 Edmunds Road,Wellesley, Mass.

Filed May 6, 1960, Ser. Dim-27,389 3 Claims. (Cl. 333-73)- This invention relates in general to microwave filters employing sections of Waveguide and more particularly pertains to a novel high-Q narrow band microwave filter in which all the waveguide sections are resonant at a single fundamental frequency but are not simultaneously lresonant at any higher frequency.

entitled Direct Coupled Resonator Filters? in the Proceedings of the IRE, February 1957 volume 45, pages 187496.

Generally, prior high- Q filters have consisted of a num-' ber of reflecting elements spaced within a Waveguide of uniform cross section by one-half the wavelength in the guide of microwave energy at the center frequency of the.

filter. Because all the cavities formed in the uniform waveguide between reflecting elements are equal to or are multiples of A /2 in length (X being the wavelength in the guide), all the cavities resonate at the same fundamentalfrequency and at all higher frequencies at which a single one of the cavities is resonant. In'most uses of resonator filters, it is distinctly undesirable to have the filter pass all the higher frequencies at which the cavities are resonant. More commonly such filters are employed to pass only a continuous band of frequencies centered about the fundamental frequency. Accordingly, the present invention contemplates and has as a primary object the construction of high-Q waveguide filter'in which the cavities of the filter are all resonant at a single fundamental frequency but are not simultaneously resonant at any higher frequency.

Another object of the invention is to provide a high-Q waveguide filter having a plurality of cavities, the design of the filter being such that individual cavities can be altered to optimise high power and insertion loss characteristics.

In accordance with the invention, a high-Q waveguide filter is constructed of direct coupled waveguide sections which in general have different cut-off frequencies and characteristic impedances (viz., a non-uniform waveguide), and reactive elements arranged in the sections in a manner such that each section is resonant at the same fundamental frequency, As the guide wavelengths are different in the different sections, the sections are not all simultaneously resonant at any higher frequency.

The construction of an embodiment of the invention and its manner of operation can be better understood by a perusal of the following exposition when considered in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a direct coupled filter employing reflective discontinuities;

3,153,208 Patented Oct. 13., 1964 ICE FIG. 2 depicts various types of reactive impedance elements;

FIG. 3 is a perspective view of a filter constructed according to the invention;

FIG. 4 is a graph showing the frequency response characteristic of a conventional direct-coupled resonant cavity filter; I

FIG. 5 is a graph showing the improved frequency response characteristic of a direct-coupled resonant cavity filter embodying the invention.

In a monograph entitled A Unified Discussion of High Q Waveguide Filter Design Theory, by Henry J. Riblet, printed in the IRE Transactions of Microwave Theory and Techniques, vol. MTT-6, Number 4, October 19-58, incorporated herein by reference, there is discussed the V general design of filters which consist of a cascade of prototype having a prescribed insertion loss function. A closely related 'synthesis procedure for the design of directcoupled filters is also disclosed in a paper by S. B. Cohn lossless reflecting elements spaced in a regular manner in a uniform waveguide. The following theoretical exposition, which is an extension of the theory developed in that monograph, explains the manner in which the design of a filter having non-uniform waveguide sections can be readily obtained once the design of the conventional uniform waveguide filter is available.

Consider the cascade of reflecting discontinuities separated by lengths of transmission line of different characteristic impedances and cut-off wavelengths shown in FIG. 1. The reflecting discontinuities are indicated as irises 1, 2, 3, 4 and the transmission line as a waveguide partitioned by the irises into sections 5, 6, 7. The wave guide sections each have a different characteristic impedance Z Z or Z and a different cut-off wavelength A or A Further, it is assumed that the reflections from the irises are largely reactive. Now, if suitable resonant line lengths l l 1 which in general are different, are associated with the reactive reflecting elements 1, 2, 3, 4, then the elements can be replaced, for a single frequency at least, by admittance inverters. The matrix representation of the network then is COS 01 j Bill 012 0 j j j 51 11 02 j cos 0 V 1 1 VI; 0

cos 02 7' sin 0 Z 0 l I O y sin 0 J cos 0 2 2 n+i where A in which I, is the section line length A is the wavelength in the guide section for each frequency in the pass band under consideration For narrow band widths this approaches Now for each sin 0 there is a t, such that w=t sin 0,

where w is a normalized frequency variable which for all is is given by the above equation. The expression for the skirt insertion loss is specified) by rewriting that equation as where )igli and t are the guide wavelengths in each section at which the skirt insertion loss for the entire filter is specified, those guide wavelengths, in general, being different for each section because the sections are of different cross-sections.

The matrix product can then be rewritten as and this can be written in ladder network form as J t1Z1 J I t tgZ1Z2 arejknown from classical synthesis procedures and are given by p 1 p p Therefore,

are precisely the voltage standing wave ratios introduced by the reflecting elements in the matched lines of characteristic impedance between which they are placed, .assuming all impedances arenormalized to the input and output transmission lines. The same design procedure used for the uniform line in the monograph can be used for lines of differing characteristic impedance if p is defined not as the inversion factor, but as the voltage standing wave ratio of the isolated reflecting elements and the ts in the general formulas for direct-coupled filters given on page 363 of the monograph are replaced by the t s associated with'the neighboring waveguide section, While capacitive reflective elements can be used, it is preferred that inductive 'irises be employed as coupljhg,65 elements between the waveguide sections due to their better power handling capabilities, relative ease of fabri cation, and lesser mechanical criticality. Some practical forms which the inductance may take are depicted in FIG. 2. The round hole inductive iris of FIG. 2A affords the highest unloaded Q for a resonant cavity, the center post inductive iris shown in FIG..2B.oifers the lowest unloaded Q, while the symmetrical vane iris (FIG. 2C) and the symmetrical post inductance afford intermediate values of unloaded .Q 'lhe-selectioh of one of those types in preference to the others for a given application is generally based upon a compromise between ease of fabrication and acceptable performance of the filter.

Referring now to FIG. 3, there is depicted a perspective view of a filter embodying the invention in which portions of the waveguide have been broken away to show some of the reflective irises. The filter comprises rectangular waveguide sections 10, 11, 12, 13, 14, 15, adjacent waveguide sections being directly coupled. Input waveguide section 10 and output section 15 are identical in cross-section and each of those sections is provided with a flange 16, 17 for coupling to the linein'whi'ch the filter is to be inserted. The waveguide sections 11, 12, 13, 14 are of varied crosssectional dimensions and of dififerent lengths. The sections 10 and 15 have the same characteristic impedance, designated Z and the characteristic impedance of the sections 11, 12, 13, 1'4 are designated by the symbols Z Z Z Z respectively. The sections 11, 12, 13, 14 are of lengths I I I I such that those sections are resonant at the center frequency of the band intended to be passed by the filter. The terms center frequency and fundamental frequency as used herein being synonymous. The input section 10 is coupled to the section 11 by an inductive iris 18 of the symmetrical vane type illustrated in FIG. 2A. The iris size is empirically chosen to produce a voltage standing wave ratio in one of the sections 11 in accordance with the calculated value for 'p when the other section is terminated in its characteristic impedance. For example assuming the input signal to be directed into 0 the flanged end of the section 10 and the section 11 to be terminated in its characteristic impedance, then the iris 18 must be of such dimensions as to cause the voltage standing wave ratio measured in the section 10 to yield the value determined for p As the value of p establishes the amount of energy coupled through the iris 18 from one section to the other and since the coupling action of the iris is reciprocal, the section 10 may be terminated in its characteristic impedance, the signal may be fed in through section 11, and the voltage standing wave ratio may be measured 'in section 11. After the dimensions of iris 18 have been ascertained, the same procedure is used to ascertain the size of'the next iris, viz., the iris 19 coupling the section 11 to section '12. Assuming the iris 19 to be of the center post type shown in FIGZB, the dimensions of that iris are chosen so as to produce a voltage standing wave ratio in one of the sections 11 or 1-2 in accordance with the calculated value for p when the other of those two sections .is terminated in its characteristic impedance. The size of the iris 20 at the next junction and of the other ir-ises in the filter are determined in the same manner using the calculated values of p p etc.

It is evident that a procedure for designing a directcoupl'ed waveguide filteris here described in which each junction between two sections of waveguides of different cross-sections is independently examined in determining the dimensions of the iris coupling the two sections. The procedure, in essence, is not much more complicated than the procedure for designing a direct-coupled cavity filter of the uniform waveguide type. The improvement in performance which can be obtained with a filter of the type shown in FIG. 3 and the conventional uniform waveguide type filter is shown by a comparison of the graphs of FIGS. 4 and 5. Referring first to the graph of FIG. 4 depicting thepenformance of a conventional direct-coupled resonant cavity filter designed to pass a narrow band of frequencies in the vicinity of 9.5 kilomegacycles (k-xnc.), it can be seen that the filter also passes a relative wide band of higher frequencies lying within the range of 15.05 to 15.20, considering any frequency attenuated less than 3 db to be passed by the filter. In contrast, the FIG. 5 graph, depicted the performance of a direct-coupled filter constructed in accordance with the invention and designed to pass the same frequencies as the conven- "tional filter, shows all higher frequencies are attenuated at least 6 db and that the range of higher frequencies at be obtained with the best rectangular waveguide cavity.

This results from the extremely high unloaded Qs ob tained in right circular cylindrical cavities. Where a cylindrical section is employed in the filter as a resonantcavity, it is treated in the same manner as a rectangular waveguide resonant cavity. i

, What is claimed is: i

1. In a microwave filter of the type havinga plurality I of directly coupled waveguide sections and reflective re .ractive elements disposed in the waveguide causing each 1 section to be resonant at the same fundamental frequency,

the improvement of an arrangement in which the sections are not all simultaneously resonant at frequencies higher than the fundamental, the improvement residing in employing directly coupled waveguide sections that have diiferent cut-oh" frequencies and differerent characteristic impedances, and the length of each section being an integral half wavelength of wave energy in that section vibrating at the fundamental frequency whereby difiierent sections are of difierent physical lengths.

2. In a microwave filter of the type having a plurality I of directly coupledwaveguide sections and reflective reactive elements disposed in the waveguide causing each section to be resonant at the same fundamental frequency,

the improvement of an arrangement in which the sections are not all simultaneously resonant at frequen- *cies above the fundamental, the improvement resid- 7 ing in employing as the directly coupled waveguide sections contiguoussections that have different characteristic impedances and different cut-off frequencies,'and the length of each section being an integral' half wavelength of Wave energy in that section vibrating at the fundamental frequency whereby'diffening sections are of different physical lengths. 3. Ina microwave filter of the type having a plurality of directly coupled waveguide sections and elements dis:

posed in the waveguide'forming reflecting reactive irises which cause each section to be resonant at the same fundamentaltlrequency, V l

the improvement of an arrangement in which the sections are not all simultaneously resonant at frequencies higher than the fundamental, the improvement residing in employing directlytcoupled waveguide sections that have different cut-01f frequencies and different characteristic impedances, the iris forming elements being at the junctions of the sections, and the length of each section being an integral half Wavelength of wave energy in that section vibrating atthe fundamental frequency, whereby differing sections are of different physical lengths. 1

References Cited in the file of this patent' UNITED STATES PATENTS 2,106,768 Southworth Feb. 1, 1938 2,432,093 Fox Dec. 9, 1947 2,524,268 v McCarthy Oct 3, 1950 2,540,488 Mumford Feb. 6, 1951 2,546,742 Gutton Mar. 27, 1951 2,737,630 Miller Mar. 6, 1956 $2,968,771 Deloach Ian. 17, 1961