# Патент USA US3092799

код для вставкиJune 4, 1963 B. w. LEAKE ETAL 3,092,790 DIRECTIONAL FILTERS Filed May 12, 1960 l 4 Sheets-Sheet l R11 2 i 11/ 4 ,- T __>a1 11/ 3 1 _ 4 _ I14 a44—- I d I 1 x 11/ , ,5 INYENTORS BERNARD W. LEAKE DONALD A. MACDONALD ATTORNEY June 4, 1963 E. w. LEAKE ETAL 3,092,790 DIRECTIONAL FILTERS Filed May 12, 1960 4 Sheets-Sheet 2 INVENTORS BERNARD W LEAKE DONALD A. MACDONAL D i s 1/ W {)Mvum ATTORNEY June 4, 1963 B. w. LEAKE ETAL 3,092,790 DIRECTIONAL FILTERS Filed May 12, 1960 4 Sheets-Sheet 3 so so - INSERTION 4 0.. L085 30" — FREQUENCY 545676510 2 1110987654321‘; FIG. " 50 100 db 70 6050- INSERTION 40 FREQUENCY L055 30 2010- 3 4 5678910 *4 100 u109a7s54321§>12345s7a9 _>FREQUENCY zDUILFI’S FIG. 8 INVENTORS BERNARD W. LEAK E DONALD Av MACDONALD BYM% ATTORNEY ‘ P'Nwwvu. June 4, 1963 B. w. LEAKE ETAL 3,092,790 DIRECTIONAL FILTERS Filed May 12, 1960 4 Sheets-Sheet 4 21 "///////I/II”I/4 ‘III/IIII/M will/a1’. will/1,111,111. F751 10A FIGJOB ,JFO SECOND 51% THIRD i1 :HARMONIC HARP/IONIC | l f | | INSERTION LOSS A R M l m A m 4_ Fcl /// F0 FREQUENCY“ F'ZG'. // [NVENTORS BERNARD W LEA KE DONALD A. MACDONALD BY United States Patent O?ice 1 r 2 3,092,790 effects the ?lter Q. Thus, for a required value of Q, the insertion loss is determined. In the referenced article, the maximum ‘attenuation in . DIRECTIONAL FlLTERS Bernard W. Leaks, , V , and Donald A. Mac donald, Natick, Mass” Vassi'gnors to Raytheon Com the main transmission line occurs at frequencies for which the loop length is an integral number of wave lengths and can be made very high (over 20 db). Therefore, it is one object of the present invention to provide a directional ?lter, the insertion loss of which when introduced into the main transmission line is @ pany, Lexington, Mass, a corporation of Delaware Filed, May 12, 1960, Ser. No. 28,787 ' 3,092,790 Patented June 4, 1963 1 Claims. (Cl.333-10) This invention relates to directional ?lters, and more particularly, to a novel structure combining the proper 10 sentially the same for all frequencies propagated by the ties of a directional coupler and a. loop ?lter for separat main line below some chosen frequency, while still pro ing, for example, harmonic signals from a fundamental viding very .high attenuation in certain frequency bands signal in a main transmission line while at the same above the chosen frequency. time causing no attenuation of said fundamental signal in the line. . Directional couplers have been employed extensively for routing part of a signal from a transmission line, which may be, for example, a waveguide to a cavity wherein a measure of the energy content or frequency of the transmission line signal may be made by a probe. Directional couplers are also, employed to route a frac tion of re?ected signals in a waveguide to a load out side of the waveguide wherein the re?ected signals are absorbed. 15 It is another object of the present invention ‘to provide a directional ?lter employing wave?“ whereby the sharp cuto? characteristics typical 0 *a waveguide con tribute to the ?ltering or frequency isolation. It is another object of the present invention to pro vide a ?ltergfor, removing second harmonics of a funda mental frequency in a transmission line without attenuat ing said fundamental frequency ther ' _ It is another object to provide metional coupler for coupling harmonic signals from a mainrjtransmission Generally, directional couplers consist simply of a four 25 line to an auxiliary line, the coupling between the main and auxiliary lines being cut off to the fundamental of arm junction in which it is possible to group the four said harmonic signals. .1, 7 n arms into ?rst and second pairs of arms such’ that mem It is another object to provide a directional ?lter where by certain frequencies in a transmission line may be coupled therefrom so that insertion loss in the trans to both members of said second pair. For the above mentioned applications of directional couplers, adjacent 30 mission line with regard to said frequencies will exceed coupled arms of different pairs are usually formed by a Various embodiments of the present invention provide transmission line conducting a signal for some useful a directionali?lter including a main transmission line purpose, while the remaining two adjacent coupled arms and an auxiliary line directional coupler coupled to the are employed to sample a portion of the signal from the main and auxiliary lines exhibiting cté?’ at different transmission line in an auxiliary line. In most applica frequencies, and at least one ‘transmission’ line loop con tions it is desirable that the latter two adjacent coupled nected to the auxiliary line, adjacent uncoupled arms of arms feed into matched loads so that no signals feed said directional coupler being designed for cut off at thereto are re?ected back into the transmission line. different frequencies so that harmonics of said funda It has been proposed, in the past, to combine the properties of a directional coupler and a transmission line loop to produce a “directional ?lter." Such a scheme is described, for example, in an article by Oohn and Ooale bers of each pair are decoupled from each other, and both members of said ?rst pair are reciprocally coupled 20 db. I ' ‘ " The presenf'invention employs, for example, a loop waveguide coupled to a main transmission line by a directional coupler, the loop waveguide being designed of a range of frequencies entering a ?rst ‘arm of the sys tern will be transferred to a second arm with little inser to. be cut OH’ to the fundamental frequency in the trans tion loss and will be considerably attenuated (15 to 20 db) in a third arm. Over the frequency range for which 50 opposition to harmonic the directional coupler is designed the fourth arm is, for most practical purposes, isolated and the ?rst arm is non- ' re?ecting. Consequently, the system acts as a “directional ?lter” passing the desired ban-d of frequencies to the sec ond arm and attenuating it in the third arm so the de Some embodiments; of the present idljéfon are loops cascaded together with directional coupler? some of which are cut off to different frequencies to thereby obtain modi?ed directional ?ltering actions. 55 Other features and objects of the present invention will sired band is isolated from the remaining frequencies of be more apparent from the following speci?c description the range. Such directional ?lter systems depend for taken in conjunction with the drawings in which: frequency selection upon the change of electrical length FIGS. la, lb, 1c and 1d illustrate typical directional with frequency of the loop of auxiliary line connecting couplers employed in the past having all arms designed two coupled arms of the directional coupler. The fre 60 for out off at substantially the same frequency; quency sensitivity or “Q” of such a ?lter depends largely FIG. 1e illustrates a symbolic represeggstion of such on the degree of coupling of the directional coupler. The directional couplers to aid in understanding the theory of operation‘; coupling structure suggested in the referenced article ex hibits little frequency dependence; ‘thus ‘the ?lter provides FIG. 2 illustrates a directional ?lter employing a single similar stop and pass bands at essentially regularly spaced 65 loop having a given internal load; frequency intervals. The loss introduced in a. main trans FIG. 3 illustrates a directional ?lter employing two mission line by the insertion of such a ?lter has a mini couplers and is equivalent to the system’ 2, but mum value at frequencies for which the loop length is with the load externally connected to arm 2'; an odd number of half-wavelengths. The loss in the FIGS. 4 and 5 illustrate cascaded directional ?lters 70 transmission line can never be zero, and depends on the coupling factor of the directional coupler, which ‘also 3,092,790 The parameter 6, is determined by the structure itself and depends on the size, shape, location, and number of a different number of loops cascaded in the manners shown in FIGS. 4 and 5; FIG. 7 illustrates a two loop directional ?lter having holes such as shown in FIGS. 10 and id, or the dimen sion d and number of the coupling waveguide sections shown in FIG. 1a. When, for example, the dimension d is very small, 0 approaches zero. However, for pur poses of the present description, assume that 0 is be loops coupled by a single directional coupler; FIG. 8 illustrates a curve of insertion loss versus a function of loop phase for the system shown in FIG. 7; FIG. 9 illustrates a directional coupler having main‘ and auxiliary lines designed for cut off at different fre quencies; FIGS. 10a and tween zero and 90 degrees as established by the dimen sions of the structure. If the arms _2_ and g are connected to a transmission 10 10b illustrate simple forms of a direc line loop having a propagation constant 7 and length I, tional ?lter employing couplers having arms thereof cut off at different frequencies whereby cut off character‘ istics contribute to frequency isolation and ?ltering; and then the wave ‘amplitude at; is expressed as follows: (2) The propagation constant 7 is a complex quantity and quency for a typical directional ?lter showing the effect 15 ‘may be de?ned in terms of its real and imaginary parts FIG. 11 illustrates a curve of insertion‘ loss versus fre of the above-mentioned loop cut off frequency. as follows: Turning ?rst to FIGS. 1a, 1b, 1c and 1d, there are shown typical directional couplers employed in the past. In FIG. 1a the directional coupler includes two wave Referring next to FIG. 2 there is shown a directional FIG. 1a, including two waveguides 6 and 7 coupled to coupling parameter of the coupler shown in FIG. 2 is de noted 6, then the amplitude coupling coefficient between arms 1 and i1 and between arms g and § is cos 0 and the coupling coefficient between arms 1 and g and between _2_ guides 1 and 2 coupled together by three one-quarter 20 coupler having its arms _2_. and § joined by a transmission line of length l, and propagation constant 1 represented by wavelength guides 3, 4 and 5 at points which are sepa impedance 18. If arms 1 and 1 look outward into rated by preferably one-quarter wavelength. In FIG. 1b matched impedances, denoted by the wedge, then b1 and there is shown another directional coupler structurally different but operationally equivalent to the coupler in 25 :14 are zero and, it follows, that b2 and a3 zero. If the gether by openings 8, 9, and 10 disposed one-quarter wavelength apart as shown‘. FIG. 10 shows a directional coupler formed by a coaxial transmission line 11 and a waveguide 12, the center conductor 13 of the coaxial 30 and 1 is — j sin 0 for a suitable choice of reference planes. line being coupled into the guide at points approximately With the above being understood, it follows that the one-quarter wavelength apart. The coupler in FIG. id matrix equation representing amplitude coupling between is similar to the one in FIG. 1c including a coaxial line arms of a ssytem such as shown in FIG. 2 for an ampli tude input, a1, is as follows: 14 and waveguide 15; however, coupling is through open ings 16 and 17 therebetween. 0 0 0 O 0 —J Sin 6 Cos 0 Cos 6 ——J Sin0 at ——J Sin 0 Cos 6 O 0 0 be Cos 0 ——J Sin0 0 0 0 bt (4) ' Equation 4 may be solved to yield the following ex In couplers shown in FIGS. 1c and 1d, adjacent un pressions for b;, and b4. coupled arms are cut off at different frequencies but not 45 (5) b3 in the couplers shown in FIGS. ‘la and 1b. JSinO (11 In FIG. 1e there is shown a symbolic representation of such directional couplers including arms 1, 2, § and 1 which represent interfaces 1, g, g and 1 in the directional couplers shown in FIGS. la, lb, 10 and 1d. The ampli 50 tudes of waves in each of the arms are denoted as a and When operating the system in FIG. 2 as a ?lter, it is desirable that one frequency signal input a, be completely cancelled so that b.,, for this frequency, is zero. From moves away from the coupler, thus 01 and I), represent Equation 6, b., is zero when: the amplitudes of waves in arm 1. If S“ represents the reciprocal amplitude coupling factor between arms 1‘, i, 55 (7) Cos (i=ell or Cos 8:241 [Cos{3l—lSin,6l] so that, for example, the coupling between arms 1 and g b, where ‘wave a moves into the coupler and wave b Since cos 0 is a real value, Equation 7 can only be true is represented as S12, the matrix equation describing the behavior of a directional coupler shown symbolically in when Bl equals 2 N11‘, where N is an integer, and: FIG. 1c is as follows: (1) b1 0 0 S31 S41 11 b2 0 0 S32 S42 (12 S31 S32 0 0 (13 S41 S42 0 0 04 be b4 = 60 When the above conditions are true, namely when 491:2 N11‘ at a given frequency; and when cos 9 equals e—“1, then the amplitude transmission coe?icient or coupling ooe?icient of the loop transmission line joining g and § 65 must ‘be e-“1 and is represented in FIG. 2 by a lumped impedance 18. The lumped impedance 18 of FIG. 2 must necessarily Equation 1 describes amplitude coupling in a direc introduce no re?ection, and reduce the input amplitude tional coupler in which arms 1 and g are decoupled and by a factor e"21 at the output. These properties can be arms § and 1 are decoupled. The amplitude coupling exhibited by, for example, a suitable section of lossy line, coef?cient between arms 1 and 1, and between arms 2 and g, denoted above as S41 and S32, respectively, are 70 or alternatively by some device which in itself has no loss, but reduces the input amplitude by the required factor, equal, and may be written as Cos 0, then the coupling by virtue of the fact that it couples power out of the ring coe?icient between arms 1 and _3_ and between arms 2 into an external load. One device falling into the latter and 1, denoted above as S3, and S42, are expressed as -—i category is a directional coupler with coupling parameter sin 0. This indicates that the two coupling coefficients are in quadrature for suitable choice of reference planes. 75 3,092,790 5 6 02 connected as shown in FIG. 3. To satisfy the loss re and where ¢B is small, it can be assumed that: quirement, it is necessary to satisfy the equation: Cos 93:2“! (l6) But it has already been shown that in the circuit of FIG. 2, the condition necessary to make b4=0 is: Therefore, at the loop resonant frequency in, where ¢=¢o=2 Nr, it is apparent that tan ¢B=¢O—0B. Fur tan ¢n__1 —2——2 tan qbB thermore, remembering that 1-—Cos20=Sm26=l—K2, then Equation 15 reduces to the following: Therefore in order to make the circuits of FIGS. 2 and 3 equivalent, the couplers of FIG. 3 must have the same coupling parameter, 01:02 and maybe identical, provided 10 the connecting transmission line is loss free, and a=j? and arms _2_' and §’ of coupler ()2 look out to matched im Equation, I? may be solved forsin??, which is equiva lent to 1-—K3, and the resulting expression substituted in Equation 12 to yield the following expression for power Referring again to FIG. 3, in the general case where coupling between arms _1_ and g of transmission line 19. 15 61 and 62 are not equal, it can be shown by a rigid analysis pedances. that the amplitude ratio between an output in, and the input a1 and an output b2 and an input a1 are expressed by (18) the following equations: E2 __ 81 (star-4m)“ ‘In,_ :|_,,, ‘“[1+2(1-Cos ¢)(2 1) Now since 1-Cos ¢=2 Sin'~’¢/2 and since Sm°/ 2 is small, 20 it can be assumed, as a close approximation, that (9) a1—1—e“1' Cos 61 Cos 6g and consequently, it can be assumed as a close approxi . (10) mation that: Q___ 6-5 Sin e1 Sin 0, of‘ l—e"" Cos Bi Cos 62 25 It can be shown that Equation 9 reduces to the form of Equation 5 when 62:61:19 and when -y=jB. In a practical ?lter, the transmissionrline loss will not be zero, so it is necessary in order to make b4=0', to satisfy the general equation COS 6122-“ COS (92 which requires that Cos 01=e—“1 Cos 62 Substituting Equation 19 in Equation 18, the following ex pression for power coupling betweenjgms 1 and g of a system such as shown in FIG. 4 consisting of m wave 30 (20) 35 and [=2 N1r This can always be done for normal vaiues of a. Referring again to FIG. 2 and assuming that Cos 6:64‘ the power ratio between signals in arms _1_ and g is repre -- stem shown in that power, coupling from arm 1 to arm a. denoted, [ha/ad’, is as follows: 11 may be expressed as follows: “l shown in 4. Consequently, an equation of substan tially the same form as Equation 20, express the power FIG. 5. 7A rigid analysis of the cascaded‘system such as shown in FIG. 5 consisting of m cascaded loops reveals For simpli?cation let e-‘1=K and [81:96, then Equation ( 1—1K’)2 imiIn 2__[1+2K2(l—C0s a) It can be shown that a cascaded system such as shown in FIG. 5 is equivalent in some respects to the system coupling between arms _1_ and § of 45 (12) guide loops is obtained: ' Obviously, Equations 20 and 21 are equivalent, except the function 0139 in Equation 20 is the reciprocal of a similar function in Equation 21. This function for convenience may be represented by n20 in Equation 39 so that: 50 Obviously 90, is the electrical length in radians of the waveguide loop at any particular frequency; for example, at frequency 1‘. Therefore, if m units such as shown in FIG. 2 ‘are cascaded in the manner shown in FIG. 4, it 55 ' It can be shown that ¢°~¢B is approximately propor can be shown that at a frequency h; where: tional to one half the 3 db bandwidth All; which is centered (13) 20 log 01 =3db at f0 and that ‘so-gs is approximately proportional to one half any other bandwidth Wider than the 3 db bandwidth, for which ¢ is equal to ‘$3, the following equation for rig; 60 for example, Af, also centered at in. Furthermore, the in terms of K and m can vbe derived: constants of proportionality are equal. ‘Therefore it fol (14) lows that bandwidth ratios are equivalent to the phase tan functions as follows: The frequency fl; is the frequency at which insertion 65 loss from ‘arm 1 to any given one of the other arms, for example, arm g, is three decibels. FIG. 4 shows a plurality of m loops coupled to a matched transmission line 19. If the coupling factor Cos 0 for each of the m directional couplers shown in 70 FIG. 4 is approximately unity and, thus, K is approxi mately unity, Equation 14 reduces to the following: (15) In FIG. 6 there is shown a semi-log plot of decibels at 1 to arm 5, in the case of the system in FIG. 4, and from arm 1 to arm §, in thescase of the system in FIG. 5 vs. the phase function 75 u, as de?ned above, for each of the systems shown in ' tenuation or insertion loss from arm 3,092,790 The principle described above with reference to FIGS. those ?gures. The plot consists of a family of curves for 10a and 11 may be applied to a similar device shown in FIG. 10b. In FIG. 1%, a waveguide loop 27 is coupled to a coaxial transmission line 28. The method of cou pling is the same as already described with reference to different values of m and in addition there are curves of frequency vs. db attenuation for m==4 for each of the cascaded systems. The frequency is plotted as frequency units above and below f", the loop resonant frequency. Each frequency unit is equal to M3, the bandwidth at 3 db attenuation. FIG. 1d; however, other suitable directional coupling structures having one pair of coaxial adjacent coupled arms and another pair of waveguide adjacent coupled Turning next to FIG. 7 there is shown another form of arms may be substituted. In order to remove harmonics cascaded loops in which two loops are coupled by a single of a signal at frequency f0 transmitted in coaxial line 28, directional coupler. It can be shown by a rigid analysis 10 the cut off frequency of waveguide loop 27 is preferably of such a structure that the power coupling ratio from between in and the ?rst harmonic of f0 and the funda‘ arm 1 to arm a is expressed ‘by the following: mental resonant frequency of the loop is substantially (26) be 2_ (rim-d0‘ 1“ equal to f0. 15 The above expression for power coupling between arms 1 and §, presumes that 01:03 and that 62 may or may not be equal to 01 and 03. A semi-log plot of decibels attenua tion vs. n, representative of Equation 23, is shown in FIG. 8 and also included therein as a plot of frequency vs. deci The cut olf frequency of coaxial transmission line 28 is zero and, therefore, is not the same as the cut off fre quency waveguide loop 27. However, FIG. 11 is applica ble for showing the characteristics of the device in FIG. 10b requiring only that f0, be moved along the abscissa 20 to the zero frequency. bels attenuation for the system of FIG. 7. Frequency is plotted as frequency units above and below resonant fre quency, f0, and each unit is equal to M3. The techniques describe-d above for designing the loop and main transmission line for cut off at different fre quencies can be applied to any of the directional ?lter devices shown symbolically in FIGS. 2, 3, 4, 5 and 7. Turning next to FIG. 9 there is shown one form of a directional coupler wherein one pair of adjacent coupled 25 For example, in FIG. 3 couplers 61 and 62 might be de signed so that arms _2_ and §, 1' and 1’ and g’ and §’ are arms are designed for cut olf at one frequency and the all designed for cut off at a higher frequency (fez) than other pair of adjacent coupled arms are designed for cut arms 1 and 1. Thus, only harmonics of a fundamental off at another frequency. For example, arms 1 and 3, frequency in arms 1 and 1 are coupled into the loop and formed by a waveguide 20, may be designed for cut off at a frequency fol, whereas arms 1.; and § formed by wave 30 attenuated or applied to non-re?ecting loads coupled to arms 2' and g’ and the fundamental is not attenuated. guide 21 are designed for cut off at higher frequency fez Likewise, in the systems shown in FIGS. 4 and 5, the di (wavelength A02). One novel method of coupling wave rectional couplers and the loops may be designed so as guide 20 to waveguide 21 might, for example, consist of to couple only harmonics of a fundamental frequency in a plurality of stepped coupling guides such as 22, 23 and 24, each somewhat identical in shape and disposed from 35 arms 1 and 1 into the loops for attenuation and cancel lation or application to matched loads. For example, in each other along waveguides 20 and 21 at approximately the attenuation curves of FIG. 6, )‘0 might represent the one-quarter keg intervals as shown in FIG. 9. Directional center frequency of the second harmonic of a funda couplers of this type may be employed for coupling har mental frequency in arms 1 and 1 of the cascaded systems monies only of a fundamental frequency from waveguide 20 to waveguide 21. In such cases, waveguide 21 would 40 shown in FIGS. 4 and 5, and the broken line in FIG. 6 might represent a suitable fez. In such a case, the wave be designed for cut off at a frequency between said fun guide loops are preferably resonant at said fundamental damental and its ?rst harmonic. Turning next to FIG. 10a there is shown a device em ploying a coupler such as shown and described with frequency and also cut off to it. It then follows that said loops are resonant to harmonics of the fundamental reference to FIG. 9, for coupling signals from waveguide 45 but are not cut off to such harmonics. in the system shown in FIG. 7, directional coupler 25 to a waveguide loop 26, the purpose being, for ex 6, might for example, be similar to the coupler shown ample, to Couple at least the ?rst harmonic of a funda in FIG. 9 with the loops in FIG. 7 and couplers 62 and 03 mental frequency, in, in a waveguide 25 to the wave all designed for cut off at a frequency fez shown by the guide loop 26, but not to couple f0 from guide 25 to loop 26. Consequently, the cut off frequency M, of loop 26 broken line in FIG. 8. There is described herein a number of embodiments preferably lies between the fundamental frequency f0 of the present invention employing directional couplers and its ?rst harmonic. FIG. ll is a plot of insertion loss coupling signals from a main transmission line to a single vs. frequency for the divice. The insertion loss from or cascaded transmission line loops whereby selected fre arm 1 to arm 11 peaks at resonant frequencies of the loop denoted ft)’, in", fo'”, etc., thus creating a multitude 55 quency bands are attenuated in the main transmission of narrow frequency hands over which the coupling from arm 1 to arm 1 is a minimum. The fundamental resonant frequency of the loop is below the cut off frequency of waveguide 26 forming the loop and is approximately equal to fo The frequency separation between the bands centered at f0’, 0'’, f0'”, etc., and the width of these bands in creases as frequency increases. Therefore, the frequen line. Embodiments of the present invention make use of waveguide frequency cut off characteristics to accomplish frequency isolation. These characteristics are employed in novel single loop and cascaded loop directional ?lters 60 to achieve various desirable effects and may be applied in other similar systems not shown herein without deviat ing from the spirit or scope of the present invention as set forth in the accompanying claims. What is claimed is: cies f0’, f0", M” are not precisely harmonics of the fre 65 l. A directional ?lter comprising ?rst wave conducting quency f9 transmitted in ' waveguide 25. However, the means, second wave conducting means cut off at a differ bands centered at these frequencies are sufficiently wide to include harmonics of f0 as shown in FIG. 11. Waveguide 25 is preferably designed for cut olf at a frequency fm, which is considerably below f0 and the fundamental resonant frequency of loop 26, and the wave guide loop 26 is designed for out off at fez which falls between f0 and f0’. Consequently, the harmonics of f0 will be coupled from guide 25 to loop 26 and these har monies will be cancelled in guide 25 while f0 will be unaffected. ent frequency than said ?rst for conducting waves over at least one closed path of given coupling coefficient, and a four-arm directional coupler having ?rst and second pairs of arms coupled to said ?rst and second conducting means respectively, the coefficient of coupling between arms of each pair being substantially equal to said given coupling coefficient whereby selected frequencies con ducted in said ?rst conducting means are cancelled. 3,092,790 10 2. A directional ?lter comprising ?rst wave conducting equal to an integral number of wavelengths of different means, second wave conducting means cut o?’ at a higher frequency signals in said transmission line, and separate means directionally coupling each of said loops to said transmission line whereby harmonics of each of said dif ferent frequency signals are cancelled in said transmission line. frequency than said ?rst for conducting waves over at least one closed path of given coupling coe?'icient and a four-arm directional coupler having ?rst and second pairs of arms coupled to said ?rst and second conducting means respectively, the coef?cient of coupling between arms of 6. A directional ?lter coupling sections of a transmis each pair being substantially equal to said given coupling sion line comprising a plurality of waveguide loops each coe?icient whereby selected frequencies conducted in said ?rst conducting means are cancelled. 3. A directional ?lter coupling sections of a transmis designed for cut oil at a higher frequency than said trans 10 sion line comprising a plurality of waveguide loops each designed for cut otf at a di?enent frequency than said transmission line a plurality of directional couplers cou pling said loops together, the electrical lengths of said loops being substantially equal to an integral number of wavelengths of given frequency signals in said transmis mission line, the electrical length of each loop being substantially equal to an integral number of wavelengths of a given frequency signal in said transmission line, a plurality of directional couplers coupling said loops to~ gether thereby cascading said loops and a single direc 15 tional coupler coupling each of said transmission line sec tions to different of said loops whereby harmonics of said given frequency signal are cancelled in said transmis sion line and means directionally coupling said loops to sion line and transmitted from at least one of said plurality said transmission line sections whereby said given fre~ of directional couplers. quency signals are cancelled in said transmission line. 20 7. A directional ?lter coupled to aitransmission line 4. A directional ?lter coupling sections of a transmis comprising a plurality of ‘waveguide loops each designed sion line comprising a plurality of waveguide loops each for cut oil at a higher frequency than said transmission designed for cut oil at a higher frequency than said line, the electrical length of each loop being substantially transmission line a plurality of directional couplers cou~ equal to an integral number of wavelengths of a fre pling said loops together thereby cascading said loops, 25 quency signal in said transmission line, a plurality of the electrical lengths of said loops being substantially pairs of different directional couplers each pair coupled equal to an integral number of wavelengths of given to a different one of said loops, means coupling one frequency signals in said transmission line, and a plurality coupler of each pair to one coupler of another pair thereby of means each directionally coupling diiferent of said cascading said loops, and means coupling the remaining loops to different of said transmission line sections where coupler of one of said pairs to said transmission line. by said given frequency signals are cancelled in said transmission line. References Cited in the ?le of this patent 5. A directional ?lter coupled to a transmisison line UNITED STATES PATENTS comprising a plurality of waveguide loops each designed for cut o? at a higher frequency than said transmission 35 line, the electrical length of each loop being substantially 2,849,689 2,922,123 2,96l,619 Kock _______________ __ Aug. 26, 1958 Cohn ________________ _._ Jan. 19, 1960 Breese ______________ __ Nov. 22, 1960 ‘in i W i

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