US2548881A - High-frequency attenuating apparatus - Google Patents

Description

April 17, 1951 T. M. FERRILL, JR

HIGH-FREQUENCY ATTENUATING APPARATUS Filed Feb. 24, 1945 3 Sheets-Sheet 1 um. mm b m mwml s w JR L) M Rm Rm w O l N mfm MM M Mw w T \N hu April 17, 1951 T M, FERRLLQJR v 2,548,881

HIGH-FREQUENCY ATTENUATING APPARATUS Filed Feb. 24, 1945 3 Sheets-Sheet 2 3kg/KM( ATTORN EY April 17, 1951 T. M. FERRILI., JR 2,548,881

HIGH-FREQUENCY ATTENUATING APPARATS Filed Feb. 24, 1945 '3 sheets-sheet;

BY ATTORNEY Patented Apr. 17, v1951 .l

UNITED STATES PATENT-'OFFICE HIGH-FREQUENCY ATTENUATIN G APPARATUS Thomas M. Ferrill, Jr., Hempstead, N. Y.,assign0r i -f to The Sperry Corporation, a corporation of l Delaware Application February. 24, 1945, Serial No. 579,559 14 claims. (c1. ris-44) The present invention relates to devices for attenuating high frequency electromagnetic energy, and especially to devices adapted for use in energy conducting systems such as concentric transmission lines or wave guides.

Attenuating devices of a type generally similar to that herein contained have been known heretofore and have been provided in a wide variety of forms. Typical of the prior devices is one consisting of one or more high loss dissipator elements arranged, with or without associated dielectric bodies, in a suitable energy conductor to attenuate, by a predetermined amount, the energy transmitted through the conductor from a so-called sending or source end to what Ais known as the receiving or load end. v-For most efcient operation of many types `of attenuating devices,-special provision is made to properly match the impedance of said devices to that of the sending and/or yreceiving portions o f thesystem coupled thereto in order to avoid the production of undesirable wave reflections and standing Waves and the well-known harmful effects attendant thereto.

It is also desirableto provide attenuating devices which are completelysymmetrical with respect to the impedance offered thereby relative to the characteristic impedance of the portions of the system coupled thereto,'as viewed from either end of said device. This kind of symmetry is desirable because completely symmetrical attenuating devices may be coupled in either sense to a source and a load Without effecting impedance mismatch.

Another feature to be desired iny attenuating devices is adjustability, whereby the components of said devices may be altered, at will, to accommodate the requirements of a particular use. Furthermore, for the sake of economy and simplicity. it is desirable that a certain prescribed operating range of attenuation be made available in xed, small-valued steps which may be introduced into or withdrawn from the device with a minimum of effort and disturbance of the existing impedance conditions.

As viewed in one of its broader aspects, a principal feature of this invention lies in the provis ion of 'attenuating devices wherein uniformly spaced dissipator elements are provided, each of said elements having a similar transformer lsection associated therewith to lproduce a sym'- metrical, multi-unit attenuatorU adapted' to match the impedance of high frequency apparatus coupled to said devices.

Still anotherv desirable'feature lies'in the pro;-

vision of impedance transformer sections com-jprised of solid dielectric bodies to which dissipator elements may be attached, said bodies serving the purpose of providing additional mechanical support forsaid elements as well as for providing for the transformation of the char-l acteristic impedance of the portion of the -conductor containing the dielectric body to that of the air-dielectric portion of said conductor. Furthermore, if the dissipator element is-not purely resistive, but instead, is appreciablyreactive as well, theV length'of the solidv dielectric bodies may be changed to compensatefor this react-,anca

Still another featureAV of the present invention is to provide attenuating devices wherein similar air-filled transformer sections are associated with each of `a seriesA of attenuating elements, said air-filled transformer sections being so" con; stituted with Arespect to 'the-dimensions-"thereof that theyv may-be used interchangeablywith solid dielectric-filled transformer sections`` of equivalent electrical length. i

In accordance with the present invention, there is provided an attenuating device which not `only possesses the above specied desirable features, but which is further character-ized by theA fact that a certain, predeterminedamount of attenuation is obtainable by the use 'of separable attenuating units consisting of identical dissipator elements and identical impedance transformer sections. Such units may, at will, be inserted into or withdrawn from the device to introduce or subtract` predetermined, smallvalued fractions of the total attenuationrange; that is, to introduce .or subtract predetermined small increments of attenuation as measured in decibels or similar units. If it is required tov pro,- duce a unit producing an attenuation increment of a value different from the predeterminedvalue adopted, the use of solid dielectric bodiesmay be partially or completely eliminated and the di: mensions of the conductor, which forms part of the attenuating device, may be correspondingly altered to compensate for the change in impedance caused by the change in the amount of-the solid dielectric used. The improved attenuating device-pf the presen t inventionstems largely from the discovery that, by extending the -length of the conductor forming part of the device beyond the terminal dissipator elements therein, through-a distance equal to the interval between a pair of adjacent dissipator elements, it is possible tofconstructa tics, any number of which maybe used together to obtain substantially any desired amount of attenuation.

Accordingly, it is an object of the present invention to provide new and improved attenuating devices wherein special provision is made for matchingthe impedance of the attenuator to the impedance of the portions of the system coupled thereto.

- It is another object of the present invention to provide attenuating devices wherein substantially equal increments of attenuation may be obtained by the addition of identical attenuating units in step-by-step fashion.

It is still another object of the present invention to provide attenuating devices which may be fabricated as a multisection, continuous impedance, symmetrical attenuator having broadbandfrequency characteristics.

Still another object of the present invention lies inthe provision Vof attenuating devices which are mechanically rugged, and at the same time, are of relatively simplied construction and economical vto produce by mass'production methods.

A stillfurther object of the present invention to -provide a symmetrical kattenuating device Whichhas desirable impedancematching proper tiesand is suitable for yielding smallfractions of attenuation.

The presentinvention in another of its aspects relates to novel features of theinstrumentalities described herein .for achieving the principal objects of the inventionand to novel principles employed in those instrumentalities, Whether or not these features and principles are used for the said principal objects or in the same eld.

A further object of theinvention is to provide improved apparatus and instrumentalities embodying novel featuresand principles, adapted for use in realizing the above objects and also adapted for use in other lields.

Other objects and advantages will be apparent from the following detailed specification taken in connection with the accompanying drawing wherein the invention is embodied in concrete form.

In the drawing,

Fig. 1 is a longitudinal cross-sectional view of one form of attenuating device, in accordance with the present invention;

Figs. 2 and 3 are admittance diagrams useful in explaining the theory of operation of the device of Fig. 1; and Figs. 4, V5 and 6 are longitudinal cross-sectional views of modified forms of attenuating devices.

Referring now to the drawing, and particularly Fig. 1, there is disclosed a section of concentric or coaxial transmission line having an inner conductor |I and an outer conductor I3 extending 4between a source of high frequency energy I anda high frequency` load |1. Theconductors II,

I3 are maintained in fixed concentric relation by insulating supports or spacers I9. The line section is adapted to be joined at either end to other lines, such as lines 2 I, 23, and for this purpose, the spacers I9 are so disposed that when the adjoining lines 2|, 23 are placed in abutment with line section II, I3, as shown, the spacers are separated by a distance electrically equivalent to onequarter of the operating wavelength in free space, in order to substantially eliminate disturbing reections due to discontinuities at the dielectric and conducting junctions. To avoid damaging the structure during handling, the inner conductor-I is terminated slightly interiorly of the outer conductor I3 and a smooth joint between the inner conductor II and those of the lines 2|, 23 is made by means of a joining member 25, as here shown. The abutting outer conductors are joined and connected by a snugly fitted outer sleeve 2l positioned so as to overlap the adjacent ends of the outer conductors.

To attenuate the high frequency electromagnetic energy conducted along the line 2| from source I5 toward line I I, I3, a plurality of identical admittance units, here shown as four in number, are provided in the line II, I3. Typical of said units is admittance unit 29 which extends between lines a-a and b--b and consists of a relatively thin dissipator element 3| and an impedance transformer section, here shown as a solid dielectric body 33 of electrical length equal to one-quarter of the operating wavelength, to which element 3| may be permanently attached in any suitable manner. A suitable form for the element 3|- has been found to comprise a re1- atively thin layer of resistive material, such as carbon or other semi-conducting material which may easily be adjoined to or deposited on the solid dielectric body 33. It is to be noted, however, that the present invention is not restricted to this form of dissipator, since many other forms may be used. A suitable form of solid dielectric body may be formed of the insulating material known as polystyrene, althoughhere again any equivalent dielectric material may be utilized without departing from the spirit of the invention. Suitable values of the admittance of dissipator element 3| and the length of body 33 may be determined in a manner to be hereinafter described.

It may be shown that an attenuation of a constant predetermined amount substantially equal to 3.7 decibels is obtained when a single polystyrene body 33 of dielectric constant 5:2,56 is used with a single dissipator element 3| characterized by a ratio of reactance to resistance equal to 0.4. Any desired number of composite admittance units similar to unit 29 may be added, in tandem, in the line II, I3 to produce a corresponding integral multiple of 3.7 decibels attenuation. However, the form of attenuator thus produced is asymmetric, yielding impedance match between attenuator and line coupled thereto only when connected in one energy transmission direction o r sense and yielding a gross mismatch when reversed in position in the same circuit.

To overcome the asymmetry referred to above, I add a primary impedance transformer section, which in the embodiment of Fig. l is a solid dielectric body 35, positioned in the line I I, I3 next adjacent the dissipator element 3| of the unit 29. Body 35 is'entirely similar to body 33 distinguishing therefrom only by the fact that no dissipator element is f'lXedly attached thereto. Also,

additionalunits, such as 31, 39 and 4I vextendausg-ssiing respectively between lines b-b and c--c, c-c and d-d, and cL-d and e-e may be arranged in cascade, as shown, for a purpose to be hereinafter described in detail. Units 31, 39 and 4I are like unit 29 and comprise dissipator elements 43, 41 and 5I, and impedance transformer sections such as solid dielectric bodies 45, 49

and 53. All the dissipator elements employed are of mutually identical characteristics, and are preferably purely resistive, although dissipator elements having some reactance may also be used. All the impedance transformer sections are of mutually identical characteristics andi -these sections may be of any solid, liquid, or

gaseous dielectric material suitable for use in high frequency operation.

Having thus described the basic structural aspects of one form of the present invention, it is proposed in the following analysis to describe the operation thereof in detail. For this purpose, a concrete illustrative example will be considered specifying a particular type of dissipator element which may be used with a particular type of impedance transforming section. It should, however, be clearly understood that the specification herein of particular types of dissipator and impedance transforming section is merely for the purpose of explanation, and that the scope of the invention should not be limited thereby, since other types of dissipator elements and transformer sections may be employed with equally desirable results.

In the following analysis, let it be assumed that the load I1 has an admittance YL, and is coupled to the air-filled transmission line 23 of admittance Ya, and that Ya is equal to YL. In general, the admittance Ya of the line 23 may or may not be matched to the admittance'YL of the load I1, and if there is no match, e.Y g., if Ya is different from YL, one is then confronted with a mismatch at the junction of line 23 and load I1, which may or may not be crucial. In Vsuch a case, however, the attenuating device of line II, I3 is presented with a resultant or joint admittance Yr Whose value is determined by the values of Ys and YL and it is to this resultant admittance Yr that the attenuating device delivers energy from source I5. In the particular instance of Ya equal to YL (so that Ya also equals Yr) clearly, when the admittance-YU of the attenuating device is matched to the line 23, it is also matched to the load I1. In either event, however, whether Ya is equal to or different from YL, the attenuating device may be matched to the admittance Yr presented thereto, from Without, and its construction and/or operation is entirely independent of the condition of match or of mismatch, experienced elsewhere in the system. Hence, the foregoing assumption that Ya is equal to YL should not be construed as a limitation imposed on the present invention, but merely as a further simplification adopted purely to facilitate the analysis of the following example.

Consider, then, the case of an attenuating device having admittance transformer sections of polystyrene characterized by a dielectric constant, eo of 2.56 such as obtains at an operating frequency of 3000 megacycles, corresponding to `a free space wavelength M of centimeters, and dissipator elements preliminarily assumed to be purely resistive. A similar case, but one wherein dissipator elements having generally reactive properties, will be considered next following the analysis of the purely resistive dissipator example.

The operation of the attenuating device described above may be moreclearly; understood through study of the admittance circle diagram shown in Fig. 2. Such circle diagrams are widely used for simple graphical analysis 'of transmission line behavior. Complete detailed descrip-l tions of the nature and use of such diagrams may be found on pages 22 through 33 of the vbook Microwave Transmission of J. C. Slater, first edition, 1942, McGraw-Hill Book Company, New York, and on pages 689 through 693 of an article entitled Graphical solution of voltage and current distribution and impedance of transmission lines by R. C. Paine, in the Proceedings of the Institute of Radio Engineers, volume 32, November 1944. Since the use of the admittance circle diagram is so well established, the repetition here of a full description thereof is believed `to be unnecessary.

The circle diagram may be employed, generally, for analyses of the change of impedance or admittance of a load produced by a transmission line section, with respect to the electrical length of the section and the ratio of the receiving end impedance oradmittance to the characteristic impedance or admittance of the transmission line section. For the present purpose, however, it is convenient to consider the characteristics of the circuit elements in terms of admittanceY, susceptance B and conductance G rather than impedance, reactance and resistance, although precisely the same results may be obtained regardless of which set of elements is considered.

Thus, in Fig. 2, the axis of ordinates represents the susceptance B while the axis of abscissae represents the conductance G, both the susceptance and the conductance being normalized with respect to the characteristic admittance Yo of the transmission line whose characteristics are to be investigated. superimposed on the rectangular coordinate graph there is a rst series of circles having centers on the axis of ordinates, each of these circles intersecting the axis of abscissae at the point (1, 0) and representing constant electrical distance, l, in degrees, along the transmission line. Portions of two such constant l circles are shown at 55, 51. A second series of circles, having centers on the axis of abscissae, are provided mutually orthogonal with said first series of circles and surrounding the point (1, 0) on the axis of abscissae. Each of said second series of circles intersects the axis of abscissae at mutually reciprocal points. Two such circles are shown at 59 and 6I by Way of illustration.

A simple illustrative example will serve to demonstrate the properties of the latter series of circles in relation to the rectangular coordinate system. If the point (l, 0) is taken to represent the characteristic admittance of any transmission line and if the line is terminated in an admittance relative to said characteristic admittance corresponding to the arbitrary point h, different from the said characteristic admittance of the line, then, as the length of the line is varied, the sending end admittance assumes values according to points on the circle 63 on 'which said point h lies. For eachY 'electrical degrees of line added, one complete traversearound circle 63 is made.

Then, for an attenuating device of the char-4 acter hereinabove specified, the ratio,

tem, and representing the conductance of load Vi 'l `(equalto the characteristic admittance of airi'lled line 23) in relation to the characteristic ad mittance Y0 of the solid dielectric lled section of line Il, I3. It is this conductance that is seen looking toward Vthe load Il from the line m-m o'f Fig. .-1.`

To Vdetermine the value of the admittance of the dissipator 'to be employed and the electr-ical length of the dielectric body or admittance transformer section associated therewith, one .needs only to select from the said second series of circles Vthe one which passes through the point on the axis of the abscissae corresponding to Y (37,0, 0) or (0.625, 0) here -labelled q` and its reciprocal here labelled r; For the assumed case, this circle is 'readily observed 'to be circle 63 the upper half of which is shown as a solid line. A vector drawn from the point q as origin and terminating on the circle 63 then has a magnitude equal to the desired value of the dissipator admittance of purely conductive character. The impedance transforming section is then chosen as 90 electrical degrees in length.

In the general case, e. g., when the dissipator i's` characterized by a complex admittance, the vector corresponding to the dissipator admittance fis drawn f-romthe origin q, in a sense determined bythe ratio of susceptance to conductance defining a certain admittance angle, 6, of the dissipator. Such a vector is shown at 64 making an angle 0 with the axis of abscissae defined by and terminating on circle 63 in point 1c. The

length of the Vector qlc is determinative of the desired value of complex dissipator admittance. 'The length of transformer section, expressed in equivalent electrical degrees, required (l) to compensate ior the susceptive component of said dissipator admittance and (2) to additionally effect an admittance transformation sufcient to produce matched admittances, is read :from the samecircle :63 by proceedingin a clockwise direction along said circle 63 from point k to point q. The length of arc thus traversed corresponds to an electrical length, ,81, which .for the illustrative example corresponding to vector qlc is approximately equal to 101. Of this length, about 11 serve to compensate for the susceptive admittance of the dissipator while the remaining 90 filo Vserve to transform *the admittance to produce a matched condition.

In a relatively simple idealized case, such :as the one to be considered in the example immediately following, 'the dissipator is assumed ato be a pure conductance having zero susceptive com'- ponent. In this case, the dissipator vector -is drawn from point q dening an angle 0:0 With respect to Van axis of abscissae and terminating at the point r on said circle 63. Hence, the vector qr is the diameter of the admittance circle 33 and the required admittance of the dissipat'or is thereby determined. The required length "of transformer section is, in this simplified fc'ase,

exactly equal to 90 electrical degrees, as may be seen by proceeding along said circle 63 4from point r 'to point q.

The effect of the quarter-Wavelength dielectric lbody 35 on the admittance seen looking 'into the load from points within lthe body 35 successively removed from the line m-m (Fig. 1) is represented by the semicircle E3 of 'which the origin coincides with the terminus of vector pq (0.625, 0) and which extends, through 90 electrical degrees (180 of arc in thediagram) lto a point r on the abscissa axis corresponding to l 0 625 or 1.6

The vector pr=pq+qr represents the admittance seen looking toward the load Il` from line c'L-a (Fig. l) which, it will be observed, is vfairly large as compared Vwith the admittance in ai'r of the transmission line.

The eiect on the admittance produced'by dissipator Si is to produce a mismatch of admittances with respect to transmission line 2l which is even greater than that caused by body 35. The magnitude of the conductance added by dssipator 3| (here assumed to be purely conductive) is represented in the circle diagram by vector 'Ts whose Amagnitude is determined in the manner hereinabove described and is equal to q1, The admittance transformation eie'cted by the dielectriciilled transformer section 33 is shown by the semicircle 35 extending between the ,terminus 's .of vector ps and a point t on the axis of abs'cissa'e corresponding to a. very small admittance value. Conductive admittance element 43 then adds the influence of its vadmittance which has the eiect of increasing the ladmittance proportionately to the conductance of element 43 and this admittance change is indicated by the vector 'tu also equal to vector qT. The dielectric section 45 transforms the admittance via the sem'icircle 6 1 to a value corresponding to the point c, which, it will be observed, approaches the starting admit;- tance represented by the vector pq. It may 'also be noted, at this point, that the addition to the primary impedance transformer section, compris:- ing the solid dielectric body 35 of the two identical admittance units 23, 3i yields a resulting admit-- tance pli which, relative to the starting admittance pq, represents a mismatch of only approximately 15%. It will be apparent, from ythe following analysis, that the successive addition of lunits 39 and 4l renders the percentage mismatch smaller and smaller, yielding a value which is negligible 'for most practical purposes.

As the point of View shifts farther `vfrom the load, as by the addition of units V39 'and 4i, itis seen that the conductance of element 41 changes the admittance by an amount represented bythe vector cw (also equal to qr). lt is to A'be lunderstood that all the horizontally disposed vectors amasar-l are colinear with the axis of abscissae (since it was assumed, for the present analysis, that all the dissipator elements are purely conductive, having zero suspective component). To avoid confusion, however, vector uw, since it overlies to a considerable extent the vectors qr and tu, is drawn parallel to and slightly displaced from the axis of abscissae.

The dielectric transformer body 49 transforms the admittance, in a manner indicated by the semicircle 69, from the point w to point x, representing an admittance which is a mismatch relative to the starting admittance of approximately 7%. This percentage mismatch may be reduced to approximately 3% by the addition of unit 4I of which the dissipator element 5I changes the admittance from that represented by the point a: toa value represented by the point y, whence the admittance is transformed by body 53 to a value e, very close to starting point q.

It is readily seen from the foregoing 'analysis that as the number of identical units is increased, the percentage mismatch approaches zero, although, for most practical purposes a mismatch of 3% may be considered the equivalent of a perfect match.

That the attenuator of Fig. 1 is symmetrical is obvious by inspection, for it is clear that if the positions of the source I5 and the load I'I were interchanged and the foregoing analysis were applied starting at the end adjacent the new position of the source I5, the resulting circle diagram would be identical with that shown in Fig. 2.

To determine the amount of attenuation introduced by each unit of the attenuator it is only necessary to compare the conductance of load I'I, as seen through the solid dielectric body 35 with the sum of the conductances of the dissipator element and of the load. Thus, theratio of the energy passed to the load I'I to the total energy received per admittance unit may be taken as the attenuation per admittance unit.

In the foregoing example, the conductance of the load is 0.625Y0. As seen through the quarterwave dielectric body transformer section 35 (at a-a.) this conductance looks like a conductance of 1.6Y0 as indicated by the point r of the circle diagram. -For unit 29, the total conductance consisting of the conductance of the element 3| and that of the load I'I is seen from the circle diagram to be equal to 2.6Y0. Hence, the relative attenuation for unit 29 is corresponding to 2.1 decibels. Similarly, the relative attenuation introduced by element 43 is calculated by obtaining the quotient of the conductance of the load as seen from the line b-b and the total conductance corresponding to dissipator element 43. Inspection of the circle diagram reveals that the relative attenuation for element 43 is thus,

10 Furthermore, it will be apparent that, as the number of attenuator units is increased'beyond two, the attenuation per unit approaches a constant value approximately equal to 3.75 decibels. The over-all attenuation for an attenuator section comprising any number of identical units issimply the sum of the relative attenuations of the individual units. Thus, it may be seen that I have heredescribed a symmetrical attenuating device having admittance matching characteristics wherein a desired' attenuation may be obtained by the addition of a predetermined member of identical units, each one of which, beyond an initial preliminary set of two such units, provides substantially equal increments of attenuation.

In the above discussion, attenuators comprising purely resistive dissipators .have been considered. Actually, at very high frequencies, a dissipator comprised of compressed or deposited carbon particles, which is a common attenuating resistive material, is likely to have a complex impedance rather than a pure resistance. At a frequency of 3000 megacycles, corresponding to a free-space wavelength A of 10 centimeters, for example, certain samples of'dissipators have been found to have appreciable capacitative reactance. In actual tests, some have been found to have a ratio of susceptance to conductance of 0.4. When such a dissipator is provided in each of the above-described admittance units, the length of the solid dielectric body transformer section is suitably altered from the effective electrical length, as explained above, in order t0 provide compensation for the susceptive cornponent of the dissipator element.

Fig. 3 shows an admittance circle diagram useful for analysis of the attenuator wherein the dissipators are again of mutually identical characteristics but which now have complex impedances of a magnitude determined as hereinabove described. In the following example, it will again be assumed that the solid dielectric bodies are of polystyrene having a dielectric constant 6:2.56 at A=10 centimeters. However, here the equivalent electrical length of the dielectric body is suitably increased to compensate for the susceptive components of the complex admittance dissipators.

The point q is located, as in the above analysis for ordinate O and abscissa 0.625, defining the vector pq which represents the conductance of load II (again assumed to be equal to the charac-..

teristic admittance of air-filled transmission line 23) in relation to the characteristic admittance, Yo,. of the solid dielectric-filled section of line II, I3. As mentioned above, the dissipators employed in this example are characterized by a ratio,

of susceptance to conductance of 0.4. The admittance angle, 0, of such a dissipator is the angle whose tangent equals 0.4, or angle 0'=21.8.

Thus, as the observational viewpoint recedes from the load end of primary transformer body 35, or, from line m-m (Fig. 1) to the line a-a, the admittance varies as indicated by the arc II (Fig. 3) from an admittance represented by the point q to a complex admittance represented by the point r', spaced an electrical distance,

vector Ts is drawn oriented at, an angle 2,1,.8

withrespect to the axis of abscissae and extending for a distance proportional to the predeterminedcomplex admittanceA of dissipator 3 l (equal.

to' qlc of Fig. 2) to the point s?. This point s',

it will. be noted necessarily lies on the axis of,

abscissae, since as mentioned hereinabove, the length of body 35. is preselected to compensate for the susceptive admittance of dissipator 3|.

Dielectric body 33.of,unit 29 introduces a furtherv admittance transformation Vof 104 electrical degrees, as indicated by arc 13, to point t whence the element 43 and, body 45 of unit 31 carrythe admittance viavector tfu and arc 15, respective- 1y, to a complex admittance represented by pointy tenuationA element 5| and transformer section 55" of unit 4| carry the admittance from point to point e', Whereat theA admittancediffers from that of the air-filled line 23, matched thereto, by a negligible amount.

By means of an analysis similar to the foregoing, it has been found that admittance-matching characteristics of my attenuating device is virtually unaffected by a shift in the operating frequency by as much as 4% above or below the normal operating frequency. For an analysis ofv the effect produced by a shift of, say 4% below 3000 megacycles, one needs only to reduce the electrical length of the solid dielectric trans- `former sections by a proportionate amount and then follow through the investigation as hereinabove described in detail.

Fig.' 4 illustrates a convenient manner of utilizing prefabricated standardized dissipator units and primary transformer sectionswhich are especially adapted for mass production assembling of the attenuator deviceof Fig. 1'. As shown in Fig. 4, a section of hollow cylindrical conductor 19 ofsuitable length vandinside diameter is provided with a tapped opening 8| near one end of its side wall.

A prefabricated primary solid dielectrictransformer section 83 is adapted to'be positioned in said hollow conductor 19, this transformer section 83 comprising acylindrical Vdielectric body 85. of .predetermined equivalent electrical' length andan axially disposed inner conductingrod 81 having a length equal to that of the body 85. and preferably heat molded in said vbody to insure uniform contact therewith. The transformer section 83 may be removably retained within said hollow conductor 19 by means of a set screw 89 in the tapped opening 8|.,- For detachably connecting the inner conducting rod 81 to a joining member such as that shown at 25 in Brig, l, the end of said rod 81 is drilled axially to provide socket 9| in which the turned-down end of joining member 25 is adapted to .be snugly received.

A prefabricated admittanceunit 93 is adapted to be positioned adjacent said primary transformer section 83 in the .hollow conductor. 19, said.

admittance unit 9.3.comprising a cylindrical dielectric body 95. of predetermined equivalent electrical length and .a dissipatoraelement 91: having line.

predetermined admittance characteristics and. iixedly attached to one end ofjsaid bodyv 9,5. inner, conducting; rod` 99,V is, coaxially disposed;

Within said dielectricbody and said dissipator element 91, one end of said rod 99 being flushed with the free end of said dielectric body 95,` while the other end of the rod 99 extends somewhat)` beyond the free end of the dissipatorV element 91. The rod 99 is preferably heatl molded inthe composite. dielectric. body and dissipator; element to insure uniform contact therewith. For detach-` ably connecting admittance unit 93 tothe primary transformer section 83, the extended end` of conductingV rod 99 adjacentthe dissipator ele.-`

ment 91 is turned down to provide a pin |9| which is adapted to lit snuglyin a socket 9| provided at the end-of rod 81 adjacent unit 93. The otherI end of rod 99 drilled to receive a similar pin of an adjoining unit or of ajoining member. As many additional units'similar tounit 93-may be provided, as desired, to produce a symmetrical attenuator having a desired amount of attenuation, as hereinbefore described inconnection with the device of Fig. 1.

While the admittance transformer sectionskof.

said line section being joined at either end tol other lines, such as lines |01, |09, for the purpose. of. attenuating, by a predetermined amount, the

high frequency energy transfer between said lines |01 and |09, and of. effecting. good admittance match between said. line section |03, |05 and saidJ adjoining lines |01, |09.

As described above,l in connection with the admittance circle diagrams of Figs. 2 and 3, explanatory of the operation of the device of Fig. l, thev points q (Fig. 2) and q' (Fig. 3)- are determined in view ofthe value of the ratiov of the admittance of theair-,lled line to the admittance of the solidV dielectric-filled portion of the line. It is known that the admittance Yo o f the solid dielectric-filled portion of the lineis a functionof the natureY of the dielectric ma'- terial within theline and. also of. the ratio of, the diameters of theconductors comprising the.

Thus, for a. given value, Ya, of the admittance ofthe air-filled linefor load, the valueof Yojrequired to 'satisfy vthe conditionv set aboveV may be obtained by changing the dielectric.ma.

terial in the line or by alcorresponding change in the diametral ratio of theiconductors of said line, or both. Hence, in the device of Fig. 5, airlled primary transformer` section replaces` solid dielectric-filled primary. transformer. secV tion.35' (Fig. l) andadmittanceunits, suchas. unit I3, vco'rnprisirigfairf-f-'filled transformer. sec?" tion H5, anddissipator element ||1 Areplace thel attenuatorunitssuch A'asunit-'29,0f`Fig. 1. To` compensate for,- the change.y admittance .produced by substituting. air for` the, solid. dielectric.. in the line section |03, |05, theratio.,ofgthedif;

13 ameter of the outer conductor |03 to the diam'- eter of the inner conductor |05 is correspondingly decreased relative to the diametral ratios of the conductors of lines |01, |09. As in the case of the solid dielectric-filled attenuator, as

many additional air-nlled admittance units, as

desired, may be employed to produce a device of substantially anyA desiredamount of attenuation without impairing the symmetry characteristics thereof.

It will be understood that, while the attenuator formed of line section |03, |05 is shown in Fig. 5 as integral with or permanently connected to lines |01, |09, said section |03, |05, may if desired, be detachably connected to the lines |01, |09 as by the joining members 25 and the outer sleeves 21 of Fig. 1. Obviously, the outer sleeves 21 would, in this case, be formed to conform with the altered dimensions of conductor. |03. Fig. 6 is a fragmentary showing .illustrating an attenuating device of similar characteristics to that of Fig. 5, wherein the altered diametral ratio is achieved by increasing the diameter of the inner conductor throughout the length of theattenuatcr while `maintaining the outer conductor diameter unchanged. Thus, as'shown, the attenuator section comprises an -outer conductor |9 and an inner conductor |2| suitably positioned coaxially with respect to said outer conductor ||9. To increase the effective diameter of the inner conductor throughout the length of the attenuator, thereby decreasing the diametral ratio of outer to inner conductor, I provide tubular conducting members |23 slideable over the inner conductor |2| and adapted Vto be positioned between adjacent pairs of distioned when said member is slipped down over the conductor |2|. Thereafter, a dissipator disc |25 is slipped into position and retained therein by the following tubular member |23, as shown. Successive sets'comprising tubular'member |23 and dissipator disc |25 may be added, as desired,-

to construct an attenuator of substantially any attenuation.

It will be obvious from theforegoing vdescriptions that attenuating devices of similar-general characteristics may be constructed by combining the teachings of Figs. 1 and 4 and Figs. 5 and 6. That is to say, virtuallyany desired` attenuation characteristics may be obtained in an attenuating device by selecting a dielectric material (solid or gaseous) of a given dielectric constant e, and by so altering the diametral ratio either by changing the. diametergof the outer conductor or by changing the diameter of the inner conductor, or both, asto yield an admittance characteristic, Yo, according to said desired attenuation.

Although the above description of the attenuating devices was based on the employment.- of coaxial transmissionlines for the attenuator and the conducting means coupled thereto, it is In the foregoingf'descrip'tion, the term Lhigliy 14 frequency conductor; has been used in a manner which is intended to denote, generally, hollow or dielectric-filled coaxial line conductors and wave guides and parallel wire transmission lines. Hence, the appended claims should'be read and interpreted in the light of the broader meaning of the term.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made' without departing from the scope thereof, itis intended that all matter contained in the abovev description or shown in the accompanying drawings shall bey interpreted as illustrative and not in a limiting sense.

, What is claimed is:

1. An electromagnetic wave Aenergy conducting system comprising conductor means adapted e to propagate high-frequency Wave'energy therealong, said conductor means having ajcharacteristic admittance; means for attenuating the energy being propagated by said conductor means, said attenuating means comprising a plurality of disc-shaped admittance 'elements spaced along -said conductor means at equal electrical intervals of the orderfof one-quarter the wavelength 0f the propagated energy, each of said disc-shaped elements being disposed in shunt with said conductor means, and means for transforming said characteristic admittance of said conductor means along with said admittance elements to a selected value, said vtrans,- forming means including a plurality of solid dielectric bodies, each of said solid dielectric bodies being disposed between each pair ovfgopposed faces of adjacent disc-shaped admittance a elements and completely filling the space therebetween; and 4two additional solid ldielectric bodies positioned adjacent each of the terminal `faces of the end disc-shaped admittance elements respectively. 1

2. In an electromagnetic wave energy conducting system comprising conductorl means adapted to propagate Wave energy therealong to a load and having a characteristic admittance; the combination with said conductor' meansof means for attenuating the energybeing delivered to said load, ysaid means comprising at least three dissipator elementsr disposed in parallel spaced relation along said conductor means at equal electrical intervals substantially equal to onequarter wavelength of the propagated waves, and means for matching said characteristic admittance to said load, said admittanceV matching means comprising a solid dielectric body disposed between each of the opposed faces of adjacent dissipator elements and completely lling the space therebetween; and an additionalsolid dielectric body positioned adjacent each of the out- Ward faces of the end dissipator elements.

3. In an electromagnetic wave energy conducting system comprising a concentric transmission line terminated in a load; the combination'withv outer conductors of'said concentric transmission line, and an adjacent solid dielectric admittancetransforming body, each of said elements being disposed along said line at equal electrical intervals..v of the orderof one-quarter AWavelengthmf the delivered Wave energy, each of said admittance-transforming bodies being disposed between each pair of opposed faces of adjacent discshaped dissipative elements and completely rilling the space therebetween; and an additional solid dielectric body positioned adjacent the outward face of the dissipative element nearest said load, said additional solid dielectric body extending along said line a distance equal to said intervals beyond the end one of said disc-shaped dissipative element.

4. A transmission line attenuator for delivering energy to a load having a iirst admittance, comprising a transmission line section coupled to said load and having a characteristic admittance different from said rst admittance, and a plurality of dissipative elements in shunt with said transmission line section` spaced at equal electrical intervals therealong of the order of one quarter wave length of the delivered energy Wave, the magnitudes of the impedances of said dissipative elements beingA substantially equal, said transmission line section extending by a distance equal to said intervals beyond the end ones of said dissipative elements for presenting an attenuator input admittance equal to said iirst admittance.

5. A concentric transmission line attenuator for delivering electromagnetic wave energy to a load having a first admittance, comprising a transmission line section coupled to said load and having a characteristic admittance diiferent from said rst admittance, and at least three equal dissipator elements in shunt with said transmission line section spaced therealong at equal electrical intervals of the order of onequarter of the wavelength of said wave energy, said transmission line section extending by a distance equal to said electrical intervals beyond the end ones of said dissipator elements for presenting an attenuator input admittance equal to said first admittance.

6. A concentric transmission line attenuator fordelivering electromagnetic wave energy to a load having a rst admittance, comprising a transmission line section coupled to said 'load and having a characteristic admittance different from said iirst admittance, and means for presenting an attenuator input admittance equal to said iirst admittance comprising a plurality of admittance units supported in juxtaposed relation with each other and in shunt with said transmission line section, each of said units including a dissipative part and an admittance transforming part associated therewith, said admittance transforming part having a length substantially equal to one quarter Wavelength of said Wave energy, said transmission line section extending by a distance `equal to the length of one of said admittance transforming parts beyond 'the end one of said dissipative parts nearest said load.

7. The attenuator defined in claim -6 wherein said dissipative part is an energy-absorbing disc. and said admittance transforming part lis a solid dielectric body attached to said.y disc.

8. The attenuator dened in lclaim 6 wherein saidtransmission line section is coupled to a load by way of a further transmission Vline section having concentric inner'andV outer conductors, and said dissipative partis an kenergy-absorbing disc and said admittance transforming vpart is an vair-.filled sectionof diametral ratio less by ,a predetermined amount than the .diametral ratio ofsaid further transmission line section.

- 9. An energy conducting systemfcomprisng :a

first transmission line section having a first characteristic admittance, a second transmission line secton coupled to said iirst line section and having a second and uniform characteristic admittance, and attenuating means in said second transmission line section for dissipating a predetermined fraction of high frequency energy conducted through said second line section to- Ward said first line section and for presenting to said iirst line section'an admittance substantially equal to said first characteristic admittance, said attenuating means comprising `at least three equal dissipator elements having appreciable admittance effectively in shunt With the admittance of said second line section and spaced at equal electrical intervals therealong substantially equal to one-quarter of the wavelength of the high frequency energy conducted therethrough, admittance transforming means for transforming the admittance of an adjacent dissipator element and disposed between each pair of adjacent dissipator elements and completely iilling the space therebetween, and additional admittance transforming means adjoining the terminal'ones of said elements, said additional admittance transforming means extending a distance equal to said intervals beyond the end ones of said dissipator elements, whereby a substantially matched admittance condition is provided between said first characteristic admittance and the input of said second'transmission line section.

10. An energy conducting system comprising a rst coaxial transmission line section having a rst characteristic admittance, a second coaxial transmission line section coupled to said rst coaxial line section and having a second characteristic admittance, and attenuating means in said second line section for dissipating a predetermined fraction of high frequency energy conducted through said second line section toward i said rst line section and for presenting to said first line section an admittance substantially equal to said iirst characteristic admittance, said attenuating means comprising a plurality of dissipator elements having appreciable admittance effectively in shunt with the admittance of said second line section and spaced at equal electrical intervals therealong of the order of one quarter of the Wavelength of the high frequency energy conducted therethrough, said second line section extending by a distance equal to said intervals beyond the end ones of said dissipator elements and having a diametral ratio of outer conductor diameter rto inner conductor diameter different from that of Vsaid first line section for transforming the admittance seen at the input of said second transmission line section to a value substantially equal to said first characteristic admittance.

11. The system defined in claim l0 wherein the diameter of the outer conductor of said second line section is substantially less than the diameter of the outer conductor of said rst line section and the space between successive dissipator elements is air-iilled.

l2. The system deiinedin claim l() wherein the diameter of lthe inner conductor of said second line section is substantially greater than `the diameter of the inner conductor of said first line section and the space between successive dissipator elements is air-filled. Y

13. symmetrical high frequency wave energy attenuator apparatus comprising a coaxial transmission ;line section, at least three equal dissipative elements spaced at equal quarter-wavelength intervals along saidtransmission line section, dielectric means between adjacent pairs of said elements and completely lling the space therebetween, and additional dielectric means disposed exteriorly of the terminal ones of said elements and extending beyond said terminal elements by a distance equal to said intervals.

14. In a preformed attenuator device adapted to be coupled to a load and including a hollow conductor, a plurality of dissipator elements, a like plurality plus one of admittance-transformer dielectric bodies of electrical length substantially equal to one-quarter of the operating wavelength positioned in said conductor in sequence therealong, with each of said dissipator elements located between successive dielectric bodies, a plurality of conducting members, each of said conducting members being centrally disposed within a respective one of said dielectric bodies, and means associated With said conducting members REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,063,396 Seibt June `3, 1913 2,060,042 Cowan Nov. 10, 1936 2,088,749 King Aug. 3, 1937 2,151,157 Scheikunoi Mar. 21, 1939 2,305,456 Okabe Dec. 15, 1942

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