Jan. 14,1947. w.'P. MASON- “ ' WAVE 2,414,115 ' FILTER ' ‘ Filed Aug. 19. 1943 FIG / EL 84T7E/N0UA” P5; J L) . >2 ‘rm :2 . 4 ' FRE QUE/VG Y ‘ ' " I I I ’ ‘ INVENTOR T‘ ' By may/150M @444 KM A TTORNEV Patented Jan. 14, 1947 2,414,115 UNITED STATES PATENT OFFICE‘-” 2,414,115“ ‘ WAVE FILTER- l ‘ Warren P. Mason, West Orange, N. .L, assignor‘to‘ Bell Telephone Laboratories, Incorporated, New York, N . Y., a. corporation of New York _‘ Application August‘19, 1943, Serial No. 499,222 10 Claims. (c1.17s_44) This invention relates to wave ?lters and more particularly to those employing sections of trans mission line as component impedance elements. . The principal object of the invention is to‘sup press one or more bands of high frequencies while suppression ?lter in accordance with the inven tion connected to a coaxial transmission line I having a cylindrical'outer conductor 2 and an inner conductor 3. The ?lter comprises two sim freely transmitting other frequencies. Other ob ilar tandem-connected sections of transmission jects are to decrease the attenuation distortion, sharpen the cut-olfs, increase the height of the attenuation peaks and increase the power carry line 4 and 5, branching off from the line I, and a third section of line 6 connected at the junc-l tion of the sections 4 and 5. In order to save ing capacity of wave ?lters intended for use at 10 drawing space, portions of the sections 4 and 5 high frequencies. , . > ‘ ' have been out out. As shown, each line section is of the coaxial type, comprising a hollow cylin The ?lter in accordance with the invention drical outer conductor and, concentric therewith, comprises sections of transmission line and .ca a cylindrical inner conductor. The sections 4, 5 pacitors arranged in the form. of a bridged-T structure having two equal series arms, an inter-= 15 and 6 have outer conductors 1, 8 and 9, respec tively, and inner conductors ‘III, II and 12, re posed shunt ‘branch and a bridging branch. The spectively. The cylinder 9 is closed at its upper series arms ‘are constituted by two sections of end by a circular metallic end plate I3. The transmission line connected in tandem. ‘The cylinders 1 and 8 ‘are arranged side by side, physi~ cally in parallel, and extend upward from the band-suppression characteristic is desired, the 20 plate. 13 to their points of connection with the line I. ‘The inner conductors I 0 and H extend bridging branch includes a capacitor. The line‘ through the apertures I4 and 15,. respectively, sections are preferably of the'coaxial type. The in. the plate 13 and terminate in the circular sections used in the series arms are preferably metallic condenser plate 11. The plate I3 is sep arranged physically in parallel,‘ with their ends close together, so that only ‘short connections “ arated ifrornthe plate I‘! by a disk I9 of dielectric material which also has‘holes“ to permit passage will be required .for the bridging branch. Cer tain of. the capacitors may be built within the . ‘ therethrough of the conductors, l0 and-ll. Theinner conductor l2 terminates at its upper line sections to ensure adequate shielding and short connections. By proper design of the line 30 end in a circular metallic. condenser plate 20 which is separated fromv the plate I‘! by a disk sections the dissipation therein may be kept low,‘ resulting in a ?lter having small distortion within ‘ H of dielectric material. Extending downward from the plate 20 is a cylindrical condenser plate the transmission. band, sharp cut-oils and high. 22 separated from the outer conductor 9 by a attenuation peaks. If sufficient separation is cylinder 23 of dielectric material. The line sec provided between, inner and outer conductors; a tion 6 is short-circuite‘d at its lower end by means large amount of power may be transmitted with- ' of the annular metallic member 24‘ which may out voltage breakdown. ‘ be slid in or out to adjust the electrical length .The nature of the invention will be more fully of the section. The portion of the inner con understood from the following detailed descrip tion and by reference to the accompanying draw 40 ductor 3 between the points of connection of the conductors l0 and‘! l is removed and a capacitor ing in which like ‘reference characters refer ‘to shunt branch may also include one or more sec tions of line and one or more capacitors. If a similar or corresponding parts and in which: i Fig. 1 is a longitudinal cross-section of a band suppression wave ?lter in accordance with the invention; 1 ‘ Fig. 2 is a simpli?ed equivalent 5 electrical cir cuit used in designing the ?lter; ‘7 of capacitance“ C1, ‘located ‘within the line I, is connected between the upper ends of the conduc tors l0 ‘a‘ndll; Input energy maybe applied at one end of the line‘! at the points 25 ‘and Hand the output taken off at the points 21 and 28. "Fig.1?! shows a‘simpli?ed equivalent electrical characteristic obtainable with the ?lter; ‘ ‘Fig. 4 is another equivalent circuit used in one - 4 c'ircuit'for the structure of Fig. 1. “The network‘ is of the bridged-T type, comprising two equal s‘eries‘arms, an‘ interposed shunt branch‘ and a‘ step of the design; and bridging branch. Each of the series arms is an Fig. 5 is a more exact equivalent electrical circuit for the-?lter of Fig. 1. Taking up the ?gures in more detail, Fig. 1 is inductance L1, the bridging branch'is the capaci tance‘ C1 and the shunt branchis constituted byv a capacitance ‘Cg connected in parallel with ‘an Fig. 3 gives a, typical attenuation-frequency a‘ longitudinal cross-sectional view of ‘a band 55 arm-comprising an ‘inductance L; in seriesvwith a ‘capacitance C3.“ The ?lter i‘s-of the band-sup‘ 2,414,115 pression type, having a lower cut-off at the fre quency iii, an upper cut-oil at in and a peak of attenuation at an intermediate frequency f..,_ For maximum attenuation fa, should be the geometric mean of is and fa. A typical attenua 4 approximate values of these elements are given by the following formulas: CA é A farads (10) farads (11) 3.6>< 10-12 log 5i tion-frequency characteristic, assuming negligible CB-é dissipation in the component elements, is shown in Fig. 3. I B 3.6>< 10-12 log; a—: ‘ The'design formulas for a ?lter of the type shown in Fig. 2 are given on page 284 of appli L1i21l10'B log. 3i henries cant’s book entitled “Electromechanical Trans ducers and Wave Filters,” published by D. Van. (12) Nostrand Company, Incorporated. Using the Lee-.2810‘9 log. 3: lienries (13) notation of Fig. 2, the component elements have. 15 the following values: in which A is the length in centimeters of each of the line sections 4 and 5, a1 and in are, re spectively, the inner diameter of the outer con C‘_47TZol:fBf°° b ' oo farads ductor 1 or 8 and the outer diameter of the inner conductor H] or II, B is the length in centimeters of the-line sections 6 from the lower 02 = W farads (2) end of the cylinder 22 to the short-circuiting member 24 and az and 222 are, respectively, the inner diameter of "the outer conductor 9 and the C3 -— TrZobfifZ (3:82 _fA2)_(b2f:2 __fB2) farads (3) 25 outer diameter of the inner conductor 12. The capacitance C1 is the one connected between the Ll = Q%}%; henries (4) upper ends of the inner conductors i0 and II in Fig. 1', C4 is the capacitance between the plates l1' and 20, C6 is the capacitance between the L: — 47,1,” foo (fBz _fA2) (who? __fB2) henries (5) 30 plates 13 and I1 and C7 is the capacitance be tween the cylinder 22 and the outer conductor 9. in which It will be noted that the capacitance CB at the right end of L3 is shorted out by the member i 2 002-2 -'2> 2+ m2__2 2 24. The shunt capacitances CA at the ends of the ?lter are usually small enough to be neglect ed. When this is done, the circuit of Fig. 5 and Z0 is the image impedance of the ?lter at is seen to have the same con?guration as that zero frequency. shown in Fig. 4. The two circuits will be In some cases the inductance L2 will be too equivalent when large to be furnished conveniently by a section ___L___ . bz\/§\/%+\/b7gzi?z—flt—4 (e) of line. This di?iculty may be overcome, how ever, by adding a redundant capacitance to the 40 2C'A+CG:C21 (14) and shunt branch. First, the capacitance C2 is split into two capacitances C21 and C22, the sum of which is equal to C2. Then the portion of the A suggested procedure for designing the ?lter of Fig. 1 is as follows: First, the cut-off fre shunt branch comprising L2, C3 and C22 is trans formed into-the equivalent form of a capacitance attenuation J‘... are chosen. quencies IA.‘ and f1; and the frequency of peak Then the values of C; connected in series with an anti-resonant loop the capacitances C1, C2 and Ca and the induct made up of a capacitance C5 and an inductance ances, L1 and L2 are computed from Equations L3. The formulas to be used for this transfor 50 1 to 5. Next, the. capacitance C2 is split into C21 and’ C22 and the values of the elements C4, C5 and mation are the following; . L3 found from Equations 7, S and 9. Now, the required length A of each of the line sections 4 and. 5 is found from EquationlZ, using any con 55 venient ratio of 171/111. However, to provide a minimum longitudinal cross-sectional area for the sections I and 2 this ratio should'be 9.2. The diameter 121 of ' the outer conductor will, in any event, be made large enough so that the dissi 60 pation is kept within allowable limits and the The resulting network is shown in Fig. 4. It remains now only to explain how the me chanical structure of Fig. 1 may be designed separation between the.conductors such as ‘I and H1 is sufficient to withstand, without breakdown, the voltage to be appliedto the. ?lter. Next, the value of each, of’ the capacitances, CA is found to be electrically equivalent to the circuit shown in Fig. 4. An equivalent electrical circuit for the 65 from Equation 10. The value of the capacitance Cs may now be- found from Equation 14. The ?lter of Fig. 1 is shown in Fig. 5. Since each of area of the plate; it, the spacing between the the. line sections 4, 5 and 6 will ordinarily have plates l3 and I1 and the dielectric constant of a length which is only a small fraction of a wave the separator "9" are proportioned to give the re length at the frequency f m, it may be represented by an inductance having at each end a shunt 70 quired capacitance C6. The spacing between the plates I"! and ‘29 and the dielectric constant of capacitance. Each of the. sections 4 and 5 is the separator 2!- are proportioned to provide the represented by an inductance L1 and the two required capacitance C4. The capacitance C1 is equal shunt capacitancesv CA, CA and the portion furnished by any suitable type of capacitor, of, the section 6 of length B by the. inductance La and the equal shuntcapacitances CB, CE. The 75 Next, the required, partial length B of; the line 2,414,115 section 6 is found from Equation 13. The ratio of 172/112 may be taken as 9.2, for the reason given above, or as any other appropriate value. The diameter 122 of the outer conductor 9 may con veniently be chosen as approximately 2221. The value of the capacitance CB is found from Equa tion 11 and the value of C7 from Equation 15. The length D of the cylinder 22, the spacing be tween the cylinder 22 and the outer conductor 9 and the dielectric constant of the separator 23 are proportioned to provide the required capaci tance 07. There will be a small amount of in ductance associated with the cylinder 22 but if 6 said one line section near said ?rst-mentioned condenser plate and electrically connected to the inner conductor of said one line section and a cylindrical condenser plate located within said one line section and electrically connected to said second condenser plate. 4. A ?lter in accordance with claim 1 which in cludes a capacitor connected between the ends of said other two line sections remote from said end plate. 5. A ?lter in accordance with claim 1 in which said other two line sections form the series arms of a T network, its length D is kept short enough this inductance may be neglected. 6. A ?lter in accordance with claim 1 in which 15 said other two line sections form the series arms Although the embodiment disclosed herein is of a bridged-T network. a band-suppression ?lter,‘ it will be apparent to 7. A‘wave ?lter comprising three sections of those skilled in the art that other types such, for coaxial transmission line, an end plate common example, as low-pass, band-pass or high-pass, to all three of said sections, and a condenser plate falling within the scope of the invention, may be provided by suitably modifying the component 20 located within one of said sections near said end plate and electrically connected to the inner con impedance branches in accordance with well ductors of the other two of said sections. known ?lter theory. What is claimed is: 1. A wave ?lter comprising three sections of coaxial transmission line, an end plate and a con denser plate, one of said line sections extending in one direction from said end plate, the other two of said line sections extending in the opposite direction from said end plate and said condenser plate being located Within said one line section near said end plate and electrically connected to the inner conductors of said other line sections. 2. A ?lter in accordance with claim 1 which includes a second condenser plate located within said one line section near said ?rst-mentioned 8. A ?lter in accordance with claim 7 which in cludes a second condenser plate located within said one line section near said ?rst-mentioned condenser plate and electrically connected to the inner conductor of said one line section. 9. A ?lter in accordance with claim 7 which includes a second condenser plate located within said one line section near said ?rst-mentioned condenser plate and electrically connected to the inner conductor of said one line section and a cylindrical condenser plate located Within said one line section and electrically connected to said second condenser plate. 10. A ?lter in accordance with claim 7 which condenser plate and electrically connected to the includes a capacitor connected between the ends inner conductor of said one line section. of said other two line sections remote from said 3. A ?lter in accordance with claim 1 which end plate. includes a second condenser plate located within 40 WARREN P. MASON.