US2634331A - Wave attenuator - Google Patents

Description

-pril 7, 1953 H. 'HONDA 2,634,331

WAVE ATTENUATOR Filed May 19, 1950 3 Sheets-Sheet 2 JNVENTOR. HHJ/mc #DDD/7 April 7, 1953 H. HQNDA 2,634,331

v v WAVE ATTENUATOR Filed May 19, 1950"v 3 Sheets-Sheet 5 INVENTOR. HHN/77 HOND/9 Patented Apr. 7, 1953 WAVE ATTENUATOR Hajime Honda, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application May 19, 1950, Serial No. 162,947

3 Claims.

fThe present invention relates to variable attenuators and, more particularly, to apparatus florv attenuating, to any desired degree, electrojective can be achieved by including a variablel attenuator in the connection between the magnetron and the antenna. Although attenuators are known to the art which are partially satisfactory for this purpose, all of them suffer from certain defects. Certain of them, for example, are unsatisfactory in that they are reliably operative only over a very narrow frequency range. Thus, whenever such prior art attenuators were used in conjunction with a magnetron oscillator, they would provide the desired variation in the power derived from the magnetron only if its frequency of operation remained substantially fixed. Whenever that frequency changed slightly,I by reason of aging of the magnetron or any one of numerous other unavoidable and unpredictable influences, the reliable operation of thev attenuator was disturbed. This type of disturbance is. of course, extremely undesirable, since it renders calibration of the attenuator meaningless. The responsibility for this erratic behavior of prior art attenuators may be attributed,to a large extent, to the fact that these prior art attenuators had, for their principal structural components, a number of simple waveguide .junctions which are inherently highly frequency sensitive or, to use a more common term, narrow band.

c Accordingly, it is a principal object of the invention to provide apparatus for variably attenuating high frequency energy supplied thereto.

It is another object of the invention to provyidea variable high frequency attenuator whose attenuation characteristics are relatively independent of variations in'signal frequency.

f ItisA a still further object of the invention to provide apparatus adapted for connection between two adjoining portions of a section of waveguide, and which is operative to vary the amount of energy supplied through it from one ofthe pQrtions to the other portion.

2 It is a feature of apparatus constructed in accordance with my invention that the degree to which it attenuates high frequency energy transmitted therethrough is controllably and continuously variable to permit the transfer of any desired fraction of the energy supplied to it while4 absorbing the remainder. Furthermore, the adjustment required to effect a desired attenuation is unaffected by relatively wide variations inl the frequency of the transmitted energy.

practicing my invention, there may bel utilized certain conventional four-branch waveguide junctions which are sov constructed and arranged that signals injected into a first branchare transmitted substantially exclusively to second and third branches, said signals being transmitted to said second and third branches respectively in substantially equal magnitudes andin substantially a rst predetermined phase re-V lationship, and which are constructed and arranged so that signals injected into said second and third branches respectively in substantially equal magnitudes and substantially a second predetermined phase relationship are transmitted substantially exclusively to a Afourth branch. Typical of such waveguide junctions are those known respectively as the magic T and the hybrid ring, eitherof which may be utilized in embodiments of the invention. i

- Briefly, in accordance with the invention, there are coupled to the second and third branches respectively, of such a junction, means for refleeting substantially equal fractions of signals transmitted to said second and third branches respectively from said first branch. Also there are included, in at least one of said second and third branches, means for relatively shifting thev phases of signals transmitted to said second and third branches by an amount such as to cause said reected fractions of said signals to be reinjected into said second and third branches respectively in substantially said second predetermined phase relationship. Further, in accordance with the invention, there are disposed in said second and third branches respectively, means for absorbing fractions of signals trans` mitted to said branches which are not reflectedv by said reflecting means.

In such an arrangement, by virtue of the inbalance of the energy being absorbed by the absorbing means disposed respectively in the second and third branches. In further accordance with the invention, the means coupled to the second and third branches respectively for reflecting substantially equal fractions of signals transmitted to said branches from the rst branch may be made controllable to vary the magnitudes lofl the fractions reected, whereby the amount of energy transmitted from the rst, or input, branch to the fourth, or output, branch may be Varied. f.

It is a feature of apparatus constructed as hereinbefore outlined, that all the energy whichy is not supplied to the output branch of the juno-- tion is absorbed in the energy absorbing means disposed in the second and third branches, so

that no reflection of energy into input' brancli' takes place regardless of how the reflecting means may be adjusted. Thus there is prevented the fermaton, in the input l branch.. of. v.Standing waves. Whichmight resultin deleteriously altering vthe characteristics ofthe attenuator as well as L the SystemV to which it is connected.

,'Ijhe detailed description of the `construction and Operation of `apparatus in accordance with my invention to which I shall now proceed will be.

more readily understood'with reference tothe accompanying drawings, in which: f

Figure 1 is across-sectional view of. a variable attenuator constructed in accordance with my invention and incorporating a waveguide. junction of the type commonly known as a hybrid Figure 2 is av view, in perspective, of the attenuator of the vform illustrated in Figure 1 together with means for adjusting certain of its components to vary the attenuation which it produces; and

Figure 3 is a view of a variable attenuator, also I3,l I4 and. I5, extendV radially outward from' spaced points along the outer circumference of waveguide loop Il. As is usual in hybrid ring junctions, the connection between each branch guide and the annular guide takes the. form of a conventional E-plane junction, in which the lon-y gitudinal axes of all of the branches lie. in. acommon plane and each branch is oriented with respect. toits longitudinal axis so that the electric eld vectors of waves propagated therein are parallel to that plane. In they arrangement as thus described, al1 of the waveguide sections mayv be of rectangular cross-section and of the` usual dimensions for the frequencies with which it is to be' employed... The section plane, it .will be noted, istaken parallel to the directions: of the.

electric eldvectors of `waves propagated in the several sections.

'As indicated by appropriate dimensional desig-l nations in the diagram, the spacing between branches I2 and I3between branches I3 and I4, and between branches Ill and I5, measured, along the annular waveguide I l from junction to junction are eachmade equal to a quarter-wavelength `zation means.

at the operating frequency. The spacing between branches I2 and I'5, on the other hand, is made equal to three-quarters of a wavelength, measured in either direction around the annular guide Il. The free end of branch guide I2 is provided with a conventional choke-type waveguide coupling I6 which adapts it for connection to a source of input signals (not shown). A similar coupling I1 is provided at the free end of branch guide I4 for connection to signal utili- A tapered plug I8, of energy absorbent material, is fitted into the free endof branch' guide I3, and a similar plug I9 is fitted into thefree end .ot .branch guide I5. These two plugs function in .accordance with principles well known in the priori art to provide substantially reectionless terminations for their respective branch guides. At' a convenient distance d from annular 'guide I I, there is joined, to branch guide I3, a waveguide section 20, containing a plunger 2| :adaptedvforlongitudinalmovement therein. The juIlQtion between guide section wandbranch guide I3 taires the-formof an. E-plane.

junction, as hereinbefore defined. ,A eccomiV waveguide section 22, containing a plunger adapted for longitudinal adjustment theren,fis joined to branch guide. .igby ,means of another; E-plane junction at a distance froinannular guide II which exceeds the aforementioned .dis-` tance z, -by an odd integral number nl of quarterewavelengths of signals to be transmitted through..

the attenuator. It, will bevrecognized Ithat wayef guide sections 2i) and-2,2, together. Wit-h their're. Spective plungers 2.1 and 23,1constitute-tuning;

stubs, whereby the impedancegpresented by;

positions in their respective' tuning stubs. Meansfor moving the'two plungers; simultaneously .and-- for maintaining. suc-h correspondence will be dis.A cussed hereinafter.. z

Considering now the: operation of the structure. as illustrated, in response to input signalso` pre-4 determined frequency injected into branch' I2;v itv will be observed that. these," signals will be propagated in both directions around. annular' guide I I. By reason of the4 spacin'gs of ther various branch guides, as. hereinbeior'e. mentioned and asvshown in the iigure, the signals thus pro! pagated will arrive at branch I3-.having their? respective electric elds in phase. opposition.. The same relationship will exist with respect tosignals arriving from-opposite directions atbranchil'; Accordingly, and by reason of the existence ci" these phase relationships, the'isigna'lsit at :these points willy be. caused to propagate into these branches respectively, It will beobserved, howfe ever, that, owing to the one-half wavelength spacing between branches I3. and I5, the signals thus propagated into these branches, respectively, will differ mutually in phase by Further it. wil1 be observed that signals injected into branch I2 and propagated in opposite. directions around the annular section will arrive at branch I4 with their electric fields in the same phase, so that none of these signals will be propagated into branch I4.

@Signals propagated into branch guides I3 an I5 would eventually reach the absorbent plugs which terminate these branches and be absorbed thereby in their entirety, were it not for the presence of the tuning stubs 2B and 22 coupled to guides I3 and i5 respectively. As aforementioned, each of the tuning stubs may function to modify the effective impedance of the branch with which it is associated to an extent determined by the position of its plunger, and, de-

pending upon the extent to which the impedanceof each branch is thus modified, a certain fraction of signal energy entering the branch will be reflected and caused to return toward annular guide II. Since, in accordance withthe invention, the plungers of both tuning stubs are adjusted in the same manner, the impedances of both branches I3 and I5 will be modified to the same extent so as to cause equal fractions offsignals injected into both branches to be reflected. Only the remaining unreflected fractions of the signals thus propagated .through the branch guides are permitted to pass on to, and be dissipated in the terminating plugs I8 and I9. Since, as has been pointed out, the signals initially supplied to the Avariable attenuator through branch guide I2 vare, equally divided between branch guides I3 and I5 by virtue of their relative displacements from the input branch guide along annular guide II, and since, furthermore, equal fractions of these divided signals are reiiected by the tuning stubs into the respective branch guides, the absolute magnitudes of the signals which are thus reflected from each of branchguides I3 and I5 back into annularguide II are equal. Specifically, when the plunger surfaces which confront branch guides I3 and I5, respectively, are llush with the inner walls of these guides, none of the signals initially propagated into either of these guides will be reflected, so that the attenuator output will be zero, and its attenuation a maximum. Maximum reection of signals in branch guides I3 and I5, and consequent minimum attenuation of signals supplied to the attenuator, takes place when the guideconfronting surface of each plunger is at a quarter-wavelength mean distance from the associated branch guide. Plunger adjustments between these limits, of course, produce intermediate-attenuation values.

n Next it will be observed that signals supplied to branch guide I3 from annular guide II and reected by reason of the action of tuning stub 2B, will undergo a phase displacement equal to the distance 2d, measured in wavelengths, which they traverse, .between their introduction intobranch guide I3 .and reissue therefrom. Signals reected in branch guide I5 by reason of the action of tuning lstub 22, on the other hand, will undergo a phase displacement of 'where n is an odd integer, between their appliof this, and by reason of the fact that the signals,

propagated into branches I3 and I5 respectively from annular guide II. differ inphase by 180,.

annular guide II, reflected signals derived from branch guide I3 will arrive at the-junction of.. branch guide vI2 and annular guide I I in the samev phase as reflected signals derived' from branch guide I5, with the result that none of .the re-1 flected signals will be propagated into branch I2; thereby preventing any reflection of signals into` the sourc-e from which the signals are originally On the other. hand, reflected signals derived from each o1= supplied to the variable attenuator.

branch `guides I3 and I5 will arrive at thejunction of bran-ch guide I4 with annular guide II in phase opposition and will therefore be propagated: into branch guide I4.

From the foregoing, it will appropriate adjustment of the two tuning plung-` ers provid-ed in my variable attenuator, anydesired fraction ofsignals supplied to the ,attenus ator through input branch guide I2 may be transmitted through the attenuator and'will issue therefrom through output branch guide I4, l'the remaining fraction being dissipated yin the energy absorbent plugs which terminate branch guides I3'and I5. Since, as-has been shown, the elec-4 trical characteristics of my Variable lattenuator are such as to inhibit reilections of energy supplied thereto through branch guide I2 back' intothesame branch guide, one of the most vexing` problems of prior art devices, namely the high;

input standing wave ratio, is solved.

While the embodiment of Fig. 1 has been described with reference to the casein which branch I2 operates as the input to the attenuator, and branch I4 as the output therefrom, it is to be noted that similar results will obtain if branch I4 is used as the input branch and branch lI2 as the output branch. While it is not deemed necessary to discuss the mode of operation under such circumstances in detail, since it is essentially` similar to that which obtains in the case described, it maybe noted that signals, injected into branch I4 will be propagated in oppositedf rections around annular vguide II. The oppositely propagated components will arrive out of phase at the junctions between branches` I3 and I5 and annular guide II respectively and vwill therefore be propagated into those bran-ches. In this instance, however., the signals propagated into branches I3 and l5 respectively will bepin phase rather than out of phase. By .reason of the difference in length of the two branches, however, the respective signals will be returned,V to the annular guide II in phase opposition. Because of this, the components, which they cause to be propagated in opposite directions in guide I I, will arrive at the junction between branch I2 and guide II in phase opposition so las to cause them to be propagated through branch I2; but will arrive at the junction between branch I 4 and annular guide II in phase so as to prevent their entry into branch I4. Apparently, therefore, it is immaterial into which of branches -I2 and I4 the signals to be attenuated are injected. Y

It is, of course, important to the achievement of the stated objects of my invention,vthat thew signals reflected in cach .of branch guidesIA By`vir'` tue of the spacing of guides I3 and I5 from the. other two branch guides I2 and I4 joined to the,

and' :15, by the Aaction ofv the. tuning plungers, be .as nearly equal as possible, for only such equality can insure complete cancellation of reflected signals at the junction of input branch guide I2 and annular guide II, thereby guaranteeing reiiectionless yattenuation of signals supplied to the attenuator. vSince the magnitude of the fraction of energy reflected in each of branches I3 and I5 is dependent upon the adjustment of the plunger of the tuning stub coupled to that section, Ait is accordingly essential that positioning of these two plungers within their respective stubs be as nearly the same .as possible. One suitable mechanism for maintaining this relationship, while simultaneously varyingV the. positions of the plungers in the respective tuning stubs, is illustrated in Fig. 2 in association ywith an 'attenuator of the form just described with reference to Fig. 1.

While Fig. 2 is a perspective view, .rather than asectional one like Fig. 1, it will be apparent that" the attenuator shown in Fig. 2 is identical in form to that shown in Fig. 1.v Accordingly no further discussion of the attenuator proper is deemed necessary, and ycorresponding reference numerals will be .employed in referring to certain elements of the attenuator of Fig. 2 which are identical to those of Fig. 1. To enect simultaneous and equivalent adjustment of the plungers of tuning stubs 20 and .22, there are provided pinions V26 and 29 which, respectively, engage racks `formed in the driving rods 26 and 21 anlxed to the .plungers of tuning stubs 20 and 22 respectively as shown in Fig. 1. Pinions. 23 and 29 are mounted for rotation about ktheir respective axes .on brackets kSI1 and 3| suitably aixedto opposite sides of tuning .stubs '20 and 22 respectively. `Both pinions 128 and 29 are driven by a common spur gear 32 rigidly aiixed to a shaft 32a. A knob 33 is aixed to the end of shaft 32a to provide for manual rotation thereof.v It is noted that the relationship, between the racks on driving rods 26 and 21 and the pinions 28 and 29 respectively engaging said racks.. is such that, when shaft 32a, is rotated in a clockwise direction, the plungers of both tuning stubs 2i) andA 22 will be driven inward simultaneously toward the branch sections I3 andr I5, while, if shaft 32a. isv rotated in a. counter-clockwise direction, both plungers will move in the reverse direction. In either case both plungers will move the same distance if both ,racks and both pinions are identical. For convenience in adjusting the plungers: to yield the desired. value of `attenuation, a suitable scale 311 may be provided for cooperation with the pointer on knob 33, which scale may be. calibrated in terms. of any desired units of attenuation.

It will be apparent that the adjusting mechanism just described with reference to Fig. 2 does not constitute an essential feature of the invention, and that manyv other mechanical arrangements for producing equivalent results may be employed, which will readily occur to those skilled in the art..

As. hereinbefore indicated, the hybrid ring type of waveguide junction is not the only one adapted for use in embodiments of' my invention, but other junctions possessing similar properties may be substituted therefor, such, for example, as the magic T. This is likewise a fourbranch junction characterized in that signals injected into a first branchthereof are transmitted substantially exclusively to second and third branches, lsaid signals being transmitted to said second and third branches Vrespectively in substantially equal magnitudes and. in substantially a rst predetermined phase relationship, and also characterized in that signals injected into saidy second and third branches respectively in substantially equal magnitudes and in substantially a second predetermined phase relationship are transmitted substantially exclusively to Athe fourth branch. Figure 3 illustratesv an alternative embodiment of my invention incorporating .such ya magic T junction.

Referring now to Fig. 3 the magic T junction comprises a section of rectangular waveguide 35 to which are joined two other sections of waveguide 3G and 31. The axes of the latter sections are disposed perpendicularly j to the axes of the first section and perpendicularly `to eachother, the two sections being electrically coupled to the iirst section through openings in adjacent sides. cf the first section. The portions of waveguide. section. 35 on either side oiv the junction constitute separate branches oi the four-branch junction `and are designated 35av and 35h respectively. As is generally recognized. the resultant four-branch junction comprises the combination. of an E-plane T-junction with an H-plane T-junction.

The free .end of branch guide 36 is provided with a. conventional choke-type waveguide coupling 33 which. adapts it for connection to a source of input signals (not shown). A similar coupling, 39, is provided at. the free vend ofA branch. guide 31 for connection to a signal utilization device. A pair of tapered plugs 40 and 4I, of energy absorbent material, are tted, respectively, into each of the free ends of branch guides 35a and 35h. These two plugs are similar in purpose to plugs i8 and I9 of Figure 1 and function, in accordance with principles Well known in the prior art, to provide substantially reilectionless termination for energy propagating towards the free ends of branch guide` 35a and 35?). At a convenient distance d measured along branch guide 35a from the junction of guides 35, 36 and 31, there is joined, to branch guide 35a, a tuning stub ft2', comprising a waveguide section 13 and a plunger 44 provided with a driving rod 16, which is of the same form as those shown in Fig. l. Similarly,v at a distance measured along branch guide 35h from the junction of guides 3.5, 36 and 31 which exceeds the distance d by an odd integral number n of quarter-wavelengths, there is joined, to branch guide 35h, a tuning stub '41, comprising a waveguide section 48. containing aplunger 43 provided with a .driving rod I. As discussed at length in the description of the embodimentillustrated in Figuresv 1 and 2,v movement of plungers 11d and 49 within their respective tuning stubs is eiective to vary, throughout a wide rangev of values, the impedance presented by branch guides 35a and 35h to energy propagated therein outwardly from the junction of guides 35, 36, and 31.

Signals to be attenuated may be supplied to the arrangement through branch gui-de 36.-v By virtue of the characteristics of the magic T junction, which are well known, these signals dividey equally between, and propagate; respectively into branch guides 35a and 35h, while substantially none of the input signal is propagated into output branch guide 31. The signals thus propagated into branch guides 35a and 35h will be' mutually out of phase. In the absence of tun4 r9 lng stubs 42 and 41, signals propagated through these branch-guides toward-thefree ends thereof would eventuallylreach Wave absorbent terminal plugs 40 and 4I and would be absorbed thereby in their entirety. lThe same eifect is achieved by positioningvboth `' plungers 44 and 49 in such a. manner that their branch;L guide-confronting i'a'ces I"are :flush with .the inner. `wall surfaces Iof I'these bran'chguides. .When these tuning' plung- 'e'xs arie thusadjusted yto permitzunimpeded' passage of' signals through the waveguide-.branches with which they are associated and towards their signal absorbent terminal plugs, the attenuator as a Whole is in its maximum-attenuation condition, all of the signals supplied thereto being dissipated therein, and no portion thereof being supplied either to the output of the attenuator, or reflector into the input branch of the attenuator. Adjustment of tuning plungers 44 and 49 to different positions within their respective waveguide sections causes the impedance presented to signals propagating through branch guides 35a and 35h to vary, with the result that varying fractions of the signals propagating therethrough in the direction of the respective terminal plugs will be reflected towards the principal waveguide junction, instead of being permitted to proceed to the terminal plugs where they would be absorbed. Signals reflected in branch guide 35h will undergo, between introduction into and issuance from branch guide 3519, a, phase shift Which exceeds the phase shift undergone by signals similarly reflected in branch guide 35a by an odd number of one-half wavelengths owing to the different positioning of stub 41 with respect to branch guide 35h, and since the signals propagated into branch guides 35a and 35h were initially out of phase, as hereinbefore stated, their reflected fractions will be in phase upon reissuing therefrom and will, consequently, combine and propagate into output branch guide 31, while substantially none of these reflected fractions of energy will be able to reenter input guide 36, Where they might produce deleterious standing waves. This inability of reflected fractions of signals from branch guides 35a and 35h to propagate into input guide 36 is due to the inherent operation of the magic T waveguide junction and is predicated upon the substantial equality in amplitude as Well as in phase of the reflected fractions of signals, which issue from branch guides 35a and 35h. In order to insure this amplitude equality of reflected fractions of energy, it is necessary that both tuning plungers be adjusted simultaneously so that both plungers will, at all times, be at the same mean distances from the waveguide branches associated with their respective tuning stubs. As in the embodiment of Figures l and 2, this identity, of plunger positioning may be achieved in any desired manner, as, for example, by an adjusting mechanism of the sort illustrated in, and described in detail with reference to Figure 2 of the drawings. Again as in the embodiment of Figures 1 and 2, the plunger position for which all of the signal propagating through branch guides 35a and 35h are reflected, and for which the overall attenuation of the attenuator is substantially zero, is that at which the branch guideconfronting surface of each plunger is at a mean distance of one-quarter wavelength from its corresponding branch guide.

As in the embodiment of Fig. l, the signal input and the signal output connections may be interchanged without in any way affecting the yOperation of the device inaccordance with the invention.

pend'edclaims.

1'.An electromagnetic wave attenuator coni- -prisingzv a waveguide junction having iirst, secr "ond, third andfourth branches,=said junction .be-

ing constructedand arrangedso that 'signals injected into 'said 'first branch are transmitted substantially exclusively to said second and third branches, said signals being transmitted to said second and third 4branches respectively in substantially equal magnitudes and in substantially a first predetermined phase relationship, and said junction being constructed and arranged so that signals injected into said second and third branches respectively in equal magnitudes and in substantially a second predetermined phase relationship are transmitted substantially exclusively to said fourth branch; a pair of tuning stubs respectively coupled to said second and third branches and arranged to reflect substantially equal fractions of signals transmitted to said second and third branches respectively from said rst branch, said tuning stubs being coupled to their respective branches at points whose effective electrical distances from said first branch differ by an amount such as to cause said reflected fractions of said signals to be substantially in said second predetermined phase relationship, and said tuning stubs being provided with means for varying their respective electrical lengths simultaneously to Vary the magnitudes of said reflected fractions of signals transmitted to said second and third branches respectively from said first branch While maintaining the magnitudes of said fractions substantially equal; and means disposed in said second and third branches respectively for absorbing fractions of signals transmitted by said branches which are not reflected by said tuning stubs.

2. An electromagnetic wave attenuator comprising: a hybrid ring waveguide junction including a closed waveguide loop and rst, second, third and fourth branches coupled to said loop at points so spaced along said loop that signals injected into said first branch are transmitted substantially exclusively to said second and third branches in substantially equal magnitudes and in substantially opposite phase relationship, and that signals injected into said second and third branches respectively in substantially equal magnitudes and in such phase relationship as to arrive at said loop waveguide in substantially equal phase relationship are transmitted substantially exclusively to said fourth branch; a pair of tuning stubs respectively coupled to said second and third branches and arranged to reflect substantially equal fractions of signals transmitted to said second and third branches respectively, said tuning stubs being coupled to their respective branches at points whose effective electrical distances from said loop waveguide diifer by substantially a quarter wavelength at said operating frequency, and said tuning stubs being provided with means for varying their respective electrical lengths simultaneously to vary the magnitudes of said reflected fractions of signals while maintaining the magnitudes of said fractions substantially equal; and means disposed in said second and third branches for absorbing Yfractions of signals transmitted by said branches which are not vre- Y REFERENCES CITED- 1366158@ by Said tunng'sflbsl The following references' are of recordr in the 3. An electromagnetic wave attenuatorl acfue. of this. patent; v i curdi-'ng to, cIaim- 1 iri which each said tuning 5 stub is comprised of a waveguide section, joined UNITED STATES to the respective branch by an E-plane junction, Number Name Date containing a conductive plunger whose 2,436,828 Ring Mar. 2, 1948 `spacing from said respective: branch is mechanf- 2,451,997 George Jan.. 4'.. 1949 i'eaIl-yl adjustable to vary the said fractionof 10 234'321-,256 Vaughan Cet. 11r'1949 Signals reected by each said' tuning stub. 2,498,548 Howard Feb. 2.1, `1.950

HONDA.

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