# Патент USA US3035244

код для вставкиMay 15, -1962 3,035,236 H. J. RIBLET MICROWAVE FILTERS Filed Aug. l5, 1958 €92595|5 FI G. 4 9495 ,a /6 9475 f’ FIG.3 „'if le?9“ " 'E FIG. | T JNVENTOR. HENRY J. RIBLET ATTORNEY L4 United States Patent O er 3,035,236 CC Patented May 15, 1962 1 2 3,035,236 FIG. 5 is a lengthwise sectional view through a wave guide filter having the selectivity of the filter of FIG. 3 and capable of handling high power levels, but more com MICROWAVE FILTERS Henry J. Riblet, 35 Edmunds Road, Wellesley, Mass. Filed Aug. 15, 1958, Ser. No. 755,304 8 Claims. (Cl. S33-73) The present invention relates in general to direct coupled or quarter-wave coupled microwave filters and more particularly concerns novel high-Q narrow-band microwave filters capable of transmitting selected spectral components of incident microwave energy at very high power levels while minimizing the response of the filtering pactly arranged. ’ With reference now to the drawing and more particu larly FIG. l thereof, there is illustrated a lengthwise sec tional view through a filter section constructed according to the invention. Since the arrangement of irises within a rectangular waveguide is well-known, sectional views 10 are employed to best illustrate the principles of Áthe in sections to frequencies outside the narrow band. lt is well known in the microwave filter art that many vention by showing the spacing between adjacent refiect ing elements within the waveguide. Thus, the waveguide section 11 may typically be a rectangular waveguide dimensioned to support the propagation of microwave of the techniques developed in connection with the syn 15 energy of the frequencies selectively transmitted by the I’thesis of lumped parameter linear passive networks may filter. Each end of the section 11 includes a reñecting be adapted for synthesizing microwave filters formed of a element such as inductive irises 112 and »13. It is im cascade of large, generally lossless, reflecting elements portant to note that the separation between irises 12 and regularly spaced in a uniform waveguide. General syn 13 is an integral multiple of substantially one half the thesis procedures for “quarter-wave coupled” and “direct 20 guide wavelength of microwave energy at the center fre coupled” filters are set forth in Microwave Transmission quency of the uîlter formed by section 11 cascaded with Circuits, vol. 9 of the M.I.T. Radiation Laboratory Series, other sections. While 4the other sections may be any in pp. 661-706. For both types of filters, synthesis pro tegral multiple of half this guide wavelength, section 11 e cedures are described based on the use of a ladder net work prototype having a prescribed insertion loss func tion. A closely related synthesis procedure for the design of direct coupled filters is also disclosed in a paper by S. B. Cohn entitled Direct-Coupled Resonator Filters in is two or more times the half guide wavelength. The irises 12 and 13 are characterized by coefficients r2 and r1, respectively, which relate current and voltage on the load and source side of each iris. These relations are set forth in detail in the discussion below concern the “Proceedings of the I.R.E.,” vol. 45, pp. 187-196, ing the mode of operation. for February 1957. 30 A view of iris 12 is better seen in FIG. 2 which shows Generally, prior high-Q Ifilters have consisted of a num a sectional View of section 11 through section 2_2 of ber of reflecting elements spaced within the waveguide FIG. l. From FIGS. 1 and 2, it is seen that the sec by one-fourth or one-half the guide wavelength of micro tion 11 includes top and bottom walls 14 and 15 and wave energy at the filter center frequency. While the side walls `16 and 17. The particular dimensions of desired filter selectivity is readily obtained with such 35 the irises are determined to provide the desired filtering l structures, at high power levels the potentials developed characteristics by techniques described in the publica in the waveguide portions between the reflecting elements tions cited above or any other suitable method. may exceed the breakdown potential and arcing may The following analysis will be helpful in understanding occur. how separating the reflecting elements by an integral Accordingly, the present invention contemplates and 40 multiple of half guide wavelengths, where the integral has as a primary object materially increasing the power multiple is at least two, preserves .the selectivity charac handling capabilities of high-Q waveguide filters while still teristics while increasing the -power handling capabilities. providing the desired degree of selectivity. It is convenient to designate current and voltage rela Still another object of the invention is to provide high-Q tions in the vicinity of the reflecting elements at a fixed waveguide filters in -accordance with the preceding ob 45 time with the subscripts L and S designating the load ject of the minimum size required to selectively transmit and source side of the elements, respectively. Assum a predetermined segment of the frequency spectrum of ing that the power flows from Ileft to right in the struc input microwave energy at a given power level. ture of FIG. 1 4and labeling parameters in the Vicinity According to the invention, the power handling capa of irises 13 and 12 with subscripts 1 and 2 respectively, bilities are increased by spacing the reflecting elements the following equations relate the currents, i, to the an integral number of half wavelengths apart, at least two adjacent reflecting elements being no less than a volt-ages, v. wavelength apart. Space is conserved by arranging the spacing between adjacent elements so that the maximum potential developed in each cavity formed between ad 55 jacent refiecting elements is just under the breakdown potential of the cavity in View of the power level and bandwidth of incident microwave energy. Other features, objects and advantages of the invention will be better understood from the following specification 60 when read in connection with the accompanying drawing in which: FIG. 1 is a lengthwise sectional view through a Wave guide filter section constructed according to Vthe invention; FIG. 2 is a view along section 2_2 of FIG. l to better 65 illustrate a typical reflecting element; FIG. 3 is a lengthwise sectional View through a repre sentative embodiment of a waveguide filter constructed according to the invention; T2 where _- i à@ p-J S n M and r is a positive real factor corresponding to the cou pling reactance X-2 referred to in the portion of vol. 9 FIG. 4 is a graphical representation of the square of 70 of the M.I.T. Radiation Laboratory Series cited above. the maximum voltage as a function of frequency in the L/Àgo is substantially lone-half at »the center frequency different sections of the filter of FIG. 3 ; and of the ñlter including the section 11 when K=1, L being 3,035,236 the distance between reflecting elements 12 and 13. The the r’s are determined by a suitable synthesis method. 'When these coefiicients are determined, irises or other guide wavelength `at center frequency is designated Ago. Solution of the above equations may be effected by suitable reflecting elements are designed by well-'known matrix manipulation to express vgS/i-ZS as a function of VIL/im. This relationship establishes the frequency char acteristics of `filter section »11 and is found by performing the following multiplication of matrices. Wl tml Wl j/t/Eo p1 ¿NEG 4 ing lhigh power filters. First, the number and value of analytical Iand/or experimental techniques to provide corresponding values for the VSWR at the plane in the waveguide where each is located. r.The square of the maximum voltage in the cavities between reflecting ele ments spaced by a half wavelength may then be deter 10 mined by analytical, graphical or experimental means. From a `knowledge of the prescribed power handling capabilities of the filter, it may be determined in which It is convenient to designate these three matrices -as R2, W21 and R1, respectively, corresponding to the char cavities the maximum voltage squared should be re duced in order to avoid breakdown. The elements sep tion coupling reflecting element 12 yto reflecting element 15 -arated by these cavities are then moved one or more acteristics of refiecting element 12, the waveguide sec 13 and reflecting element 13, respectively. additional half wavelengths apart. '_The increased sep The fre quency-sensitive characteristics of section 11 are then -aration between only some elements will not appreciably fully characterized by affect the frequency response characteristics of preceding sections. [Ri'Wzi’Rzl This will be better understood from the fol- lowing considerations. This matrix product is The relation between the current on the source side of iris 13 in section 11 to develop a given voltage on the load side is 25 It is desired to increase the power handling capabilities of ñlter section 11 without altering the value of this Thus, the current on the source side of iris 13 has been reduced by a factor of vk. Itis well known in the micro wave art that a current maximum occurs at an inductive iris and that a voltage maximum occurs substantially a quarter wavelength away from the current maximum and 30 will be seen that this is accomplished by separating the product matrix. From the analysis which follows, it reflecting elements by k half guide wavelengths where k is an integer greater than one. As a practical matter, small values of p are of interest. Since QTL p__-j sin Äg is proportional to the value of the current maximum. Therefore, the maximum voltage in the cavity is reduced by the same factor \/k. Since the peak power is pro portional to the square of the maximum voltage, it fol lows that the power transmitted through the section may be increased by a factor k in a k half wavelength section over that which may be transmitted through a section only a half wavelength long before the breakdown po tential of the cavity is exceeded. With reference to FIG. 3, there is shown a lengthwise 40 sectional view of a ñlter having four cavities 31-34 separating iìve inductive irises 41-45 by one and one the guide half wavelength at center frequency; for small half guide wavelengths at the filter center frequency. values rof p, the sine may be approximated by the dif The square of the maximum voltage of each cavity was ference between an appropriate integral multiple of 11 experimentally determined, the irises being dimensioned and its argument. Thus, and L is k times to provide a filter exhibiting Tchebyshev characteristics within the pass band. These measurements are graph ically represented on a common frequency scale about the filter center frequency in FIG. 4 with the amplitude In conventional filters, k=l. It can be shown that increasing the length of section normalized. The curves are labelled with the number 11 by a factor k correspond-s to substituting kp for p in 50 of the cavity having the plotted maximum Voltage squared. the matrix representations of characteristics of the filter Examination of these curves shows that the two inner section. The Itransfer characteristics of the section are cavities 32 and 33 have a maximum voltage squared then represented by the product matrix [R2] [W21]k[R1]. below two over the frequency band from 9450 to 959@ To show an example illustrating the propriety of re megacycles while the two end cavities 31 and 34 have placing p by kp when k half wavelength sections are 55 a maximum voltage squared below one over the entire between adjacent irises, consider `the case where k=-2. range. lf it is desired to provide a filter operative over this frequency band of minimum length to transmit a Then [w21]2 is given by prescribed power, the inner sections should be twice as long as the end sections. If inner cavities a guide wavelength long are capable 60 which reduces to of transmitting the desired power levels, then the end [il] lì? cavities need only be a half guide wavelength. A length wise sectional view of such a filter is shown in FlG. 5 with elements designated by the reference numeral primed If p is replaced by a factor kp, the magnitude of the 65 of a corresponding element shown in FIG. 3. This em functional relationships characterized by this matrix re bodiment of the invention provides the same degree of mains unchanged if r1 and r2 are each decreased by a «factor k. The matrix product remains selectively as the filter of PEG. 3 but minimizes the size of the filter in view of the power level requirements. The information in FIG. 4 is also helpful in deter 70 mining the proper lengths of sections when operation 0 over a wider bandwidth is required. For example, con sider operation over `a bandwidth from 9425 to 9525 megacycles. The maximum voltages squared of the cavi ties 31-34 are below one, 2.2, 3.7 and 3.3, respectively. ing the description which follows of a method for mali 76 The ratio of lengths would then be 1:2.5:4:4. ln terms The preceding discussion should facilitate understand 3,035,236 6 of guide half wavelengths, the integer k for the cavities in waveguide between adjacent sections coacting with said order from the source end to the load end would be 2, sections to provide a filter exhibiting a predetermined selec tivity characteristic whereby only a narrow band of said 5, 8 and 8. In general, the pattern for selecting the cavity lengths in the optimum manner involves choos spectral components is transmitted therethrough, each of ing the length of the cavity having the highest maximum said sections having a length substantially equal to an in voltage squared within the desired band to be the small est integral multiple of a half guide wavelength which tegral multiple k of half the guide wavelength at said center frequency, k being at least two for an intermediate one of said sections, the reflection coeñicient of those of said reflecting elements adjacent to the ends of said one section being reduced by said factor k over the value re results in transmission of the desired power levels with out exceeding the breakdown potential of the cavity. The remaining cavities are then made a lesser or equal integral number of half guide wavelengths long, the par ticular integer also being the minimum which results in quired to provide said predetermined selectivity character istic with said one section being only half said guide transmission of the desired power levels without exceed wavelength long. ing the breakdown potentials of the respective cavities. 5. A direct-coupled resonant cavity microwave ñlter re In a representative embodiment of the type illustrated 15 sponsive to microwave energy having spectral components in FIG. 5, wherein the waveguide section was 2.85 cm. near a predetermined center microwave frequency com wide by 1.25 cm. across the narrow dimension, the length prising, a plurality of cascaded sections of waveguide hav of inner cavities 32’ and 33’ was 3.75 cm. and that of ing a uniform cross-section and being dimensioned to nor end cavities 31’ and 34', 1.875 cm. The width of the mally propagate said energy, reflecting elements in said centered conducting plates defining inner inductive irises 20 waveguide between adjacent sections coacting with said 42', 43’ and 44’ was 1.6 cm. and the width of the centered sections to provide a filter exhibiting a predetermined selec tivity characteristic whereby only a narrow band of said conducting plates defining inductive end irises 41’ and 45’ was 0.8 cm. This filter selectively transmitted 80 peak kilowatts of power, its bandwidth being 60 mega cycles between the one db points. lt is apparent that those skilled in the art may now make numerous departures from the speciñc embodi ments and techniques described herein without departing from the inventive concepts. Consequently, the invention 25 spectral components is transmitted therethrough, each of said sections having a length substantially equal to that integral multiple k of half the guide Wavelength at said center frequency resulting in approximately equal values of the square of maximum voltage being developed in each section, k being at least two for one of the intermedi ate sections, the reñection coeñìcient of those of said re is to be construed as limited only by the spirit and scope 30 tlecting elements adjacent to ends of each section being reduced by the associated factor k over the value required What is claimed is: to provide said predetermined selectivity characteristic 1. A direct-coupled resonant cavity microwave ñlter with all said sections being only half said guide wave responsive to microwave energy having spectral com length long. ponents near a predetermined center microwave frequency 35 6. A direct-coupled resonant cavity microwave filter for comprising, a waveguide section of uniform cross-section selectively transmitting a narrow band of frequencies about having a cutoff frequency below the frequency of said a predetermined center frequency at high power levels spectral components, and a plurality of reñective reactive comprising, a plurality of cascaded reliecting elements, of the appended claims. elements in respective planes within said section, said waveguide sections of uniform cross-section planes being spaced by an integral multiple of the guide 40 respective separating adjacent ones of said reñecting elements and half wavelength of said energy at said center frequency, coacting therewith to provide a predetermined frequency at least two of the intermediate adjacent retiective ele selectivity, said reñecting elements projecting into the ments being separated by at least one guide wavelength waveguide formed by said cascaded sections, each of said of said energy at said center frequency. sections being dimensioned to normally propagate micro 2. A direct-coupled resonant cavity microwave filter 45 wave energy within said narrow band of frequencies and responsive to microwave energy having spectral compo having a length substantially equal to half the guide wave nents near a predetermined center microwave frequency length at said center frequency multiplied by an integer comprising, a waveguide section of uniform cross-section k, at least one pair of intermediate adjacent reflecting ele having a cutoff frequency below the frequency of said ments being separated by a long section of waveguide hav spectral components, and a plurality of irises in respec 50 ing a length such that said integer k is greater than one, tive planes within said section, said planes being spaced the reflection coeiiicients of said one pair of reñecting by an integral multiple of the guide half wavelength of elements being reduced by the multiplicative integer k said energy at said center frequency, at least two inter of said long section over the value required to provide mediate adjacent planes being separated by at least one said predetermined frequency selectivity with said long guide wavelength of said energy at said center frequency. 55 section being only half said guide wavelength long. 3. A direct-coupled resonant cavity microwave ñlter re 7. A microwave filter in accordance with claim 6 where in each section which would develop a maximum voltage therein greater than its breakdown potential when trans prising, a plurality of cascaded sections of waveguide di mitting said high power levels if only half said guide wave mensioned to normally propagate said energy, all of said 60 length long is the minimum value of k guide half wave sponsive to microwave energy having spectral components near a predetermined center microwave frequency com sections being uniform in cross-section retiecting elements in said waveguide between adjacent sections coacting with said sections to selectively transmit only a narrow band lengths long required to reduce said maximum voltage to a value below said breakdown potential when transmitting said high power levels while said filter exhibits said pre determined selectivity. of said spectral components, each of said sections having a length substantially equal to an integral multiple of half 65 8. A microwave iilter in accordance with claim 7 where the guide wavelength at said center frequency, at least in said retiecting elements are inductive irises. one of the intermediate sections being at least one guide wavelength long. References Cited in the ñle of this patent 4. A direct-coupled resonant cavity microwave filter UNITED STATES PATENTS responsive to microwave energy having spectral compo- 70 nents near a predetermined center microwave frequency comprising, a plurality of cascaded sections of waveguide having a uniform cross-section and being dimensioned to normally propagate said energy, reñecting elements in said 2,546,742 2,623,120 Gutton et al ___________ __ Mar. 27, 1951 Zobel _______________ __ Dec. 23, 1952 2,859,418 Vogelman ____________ __ Nov. 4, 1958

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