US2538771A

US2538771A - High-frequency attenuator

Info

Publication number: US2538771A
Application number: US547774A
Authority: US
Inventors: Feenberg Eugene
Original assignee: Sperry Corp
Current assignee: Sperry Corp
Priority date: 1944-08-02
Filing date: 1944-08-02
Publication date: 1951-01-23
Anticipated expiration: 1968-01-23
Also published as: GB605198A

Description

1951 E. FEENBERG HIGH-FREQUENCY ATTENUATOR 4 Sheets-Sheet 1 Filed Aug. 2, 1944 LOAD LOAD

FEGB

INVENTOR. v EUGENE FEENBERG ATTORNEY Jan. 23, 3951 E. FEENBERG HIGH-FREQUENCY ATTENUATOR 4 Sheets-Sheet 2 Filed Aug. 2, 1944 FIG.6

FIGH

FEGJZ FIGQ INVENTOR. EUGENE FEENBERG FIG. I3

ATTORNEY Jan. 23, 1951 E. FEENBERG 2,53,??1

HIGH-FREQUENCY ATTENUATOR Filed Aug. 2, 1944 4 Sheets-Sheet 5 FIGS INVENTOR. EUQENE FEEN ERG ATTORN EY Jan. 23, 1951 E. FEENBERG 5 HIGH-FREQUENCY ATTENUATOR Filed Aug. 2, 1944 4 Sheets-Sheet 4 INVENTOR. EUGENE FEENBERG ATTORNEY Patented Jan. 23, 1951 HIGH-FREQUENCY ATTENUATOR Eugene Feenberg, New York, N. 1 assignor to The Sperry Corporation, a corporation of Belaware Application August 2, 1944, Serial No. 547,774

' '7 Claims.

The present invention relates to dev ces for attenuating high frequency electromagnetic energy, and especially to devices adapted for use within high frequency conductors such as coaxial transmission lines or wave guides.

In many high frequency electrical systems, it is necessary or desirable to attenuate by a predetermined amount the energy transmitted from one point to another. Usually such energy transmission is accomplished through a self-enclosed conductor, such as a hollow-pipe waveguide or a coaxial line, and it is often desirabe that attenuatng apparatus be especially designed to be inserted entirely within or made a part of such a high frequency conductor.

Usually considerable attention is given to the design of the self-enclosed high frequency conductors of the above-described types, as Well as to the parts of a system linked together thereby, to achieve a condition of complete impedance match throughout the high frequency conductor. Accordingly, it is particularly desirable that an attenuator for use Within such a conductor be so designed as to present a desired input impedance when terminated in a proper y matched load.

In some cases, the attenuating device may be electrica ly asymmetrical, and care must be exercised in the orientation of the device with respect to the source and load ends of the transmission line to maintain a desired impedance match throughout. In other cases, it is desirable that a symmetrical attenuating device be provided so that the source and load ends of the transmission line incorporating the attenuating device may be interchanged or the attenuator orientation reversed without introducing a condition of impedance mis-match.

It is also desirable to provide an attenuator structure which is easy to manufacture and install, and which is sufliciently rugged mechanically to insure that the attenuating and impedance-matching characteristics of the device remain constant; The practicability of attenuator elements for use within a high frequency energy conductor depends to a great extent on the ease of mechanical adjustment of the dissipator sec" tions therein. If very th n discs of semi-conductive material are required as the dissipators of a coaxial line attenuator,-for example, and if these discs must be separated by air-dielectric trans- 'mission line sections of carefully predetermned length, it is often found that very tedious operations are required to achieve the proper positioning of the dissipator elements Within the line. Also, rough handling of the transmission line so constructed, or prolonged vibration thereof, may result in a shift of position of such dissipator discs or even in breakage of th discs, changing the characteristics of the attenuator and thus seriously impairing its performance.

In some forms of the present invention asolid dielectric body is inserted within the high-frequency energy conductor to fill a section of the energy conductor of appreciable length. The dissipator element is attached to th s solid dielectric body, and is mechanically supported and rigidlyheld in correct position by the dielectric body. 'The dissipator element either may be a preformed thin disc of compressed and bonded carbon particles, cemented to the end of the dielectr c body, or it may be 'a'layer of carbon-bearing paint applied to the end of the dieectric body and dried thereon. Other semi-conductive materials besides carbon may be used in the dissipator element, if desired." The sol d dielectric body is of an appreciable length usually comparable with or greater than its diameter, and. being formed of a size adapted to fit snugly within the outer 'metallic portion of the hollow high frequency energy conductor, it serves not only to support the dissipator element but also to'align it-normal to the air s of the energy conductor.

The solid dielectric body for insertion Within the high frequency energy conductor serves not on y as a support for the dissipator element; 'as described above, but also as means for changing the characteristic admittance of the transmission line sect on filled by the dielectric body from the characteristic admittance with air-dielectric. Thus, the solid dielectric body is made to serve simultaneously as an important structural member and also as a vital admittance transformer member within the attenuator embodments of the present invention. The admittance-transforming function of the dielectric body is particularly useful in rendering the input impedance of .the attenuator substantial y equal to the output for load impedance to which h gh frequency energy is delivered through the attenuator.

Furthermore, if the dissipator. element is not purely resistive, but instead is characterized by appreciable reactance in addition to its resstance, the length of the solid dielectric body may be ad- 'justed to compensate for this reactance, rendering the attenuator input impedance purely resistive. Accordingly, it is an object of the presentinventon to provide improved high frequency attenuating devices incorporating special provision jor matching the impedance of the attenuator to 3 the impedances of the other devices connected thereto.

It is another object of the present invention to provide improved high frequency attenuating devices which are relatively easy to construct.

It is a further object of the present invention to provide improved high frequency attenuating devices which are mechanically rugged, so that constant characteristics of the devices may be maintained.

It is still another object of the present invention to provide improved attenuating devices which easily may be inserted directly within concentric line transmission systems.

It is still a further object of the present invention to provide improved attenuating devices which may be inserted within hollow pipe waveguide high frequency energy conductors.

It is yet a further object of the present invention to provide an improved attenuating device wherein a solid dielectric body is provided for energy dissipation distributed therethrough.

Other objects and advantages will become apparent from the specification taken in connection with the accompanying drawings wherein the invention is embodied in concrete form.

In the drawings:

Fig. 1 shows a longitudinal view, partly in sec tion, of one form of attenuating device especially useful where a fixed small value of attenuation is desired, and where asymmetry of the attenuating device is permissible;

Fig. 2 shows an equivalent circuit diagram of the system of Fig. 1, showing the manner in which the attenuator of Fig. 1 is inserted in a transmission line between a source of high frequency energy and a load matched to said transmission line without altering the impedance presented to said source;

Fig. 3 is an admittance diagram useful in explaining the theory of operation of the device of Fig. 1;

Fig. 4 shows a longitudinal cross-sectional view of an attenuating device characterized by symmetrical arrangement of elements, and further characterized by an impedance transformation to a relatively high input admittance;

Fig. 5 is an admittance diagram useful in explaining the theory of operation of the device of Fig. 4;

Fig. 6 shows a longitudinal cross-sectional view of a form of attenuating device employing three dissipative elements separated by solid dielectric beads Or bodies; the attenuating device of Fig. 6 being characterized by symmetry and further characterized by input admittance equal to the load admittance;

Fig. '7 is an admittance diagram useful in explaining the theory of operation of the device of Fig. 6;

Fig. 8 shows a longitudinal cross-sectional view of a further form of attenuating device wherein the energy dissipating material may be dispersed throughout a solid dielectric body, or a normally high loss dielectric material may be employed, so that the dissipation is distributed throughout the solid dielectric body of the attenuating device;

Fig. 9 shows a longitudinal cross-sectional view of an attenuator simiar to that of Fig. 1, but

view of an attenuating device similar to tha of 4 I Fig. 6 wherein a reduced inner conductor diameter is employed throughout the axial extent of the solid dielectric bodies;

Fig. 11 shows a longitudinal cross-sectional view of an attenuating device somewhat similar to that of Fig. 6, wherein the outer conductor diameter is increased throughout the axial extent of the solid dielectric bodies;

Fig. 12 shows an attenuating device generally similar to that shown in Fig. 8, but with the inner conductor diameter decreased and the outer conductor diameter increased throughout the axial extent of the high-loss solid dielectric material;

Fig. 13 shows an attenuator generally similar to that of Fig. 1 adapted to a circular crosssection hollow pipe waveguide; and

Fig. 14 is a cross-sectional view taken along the line Hii 3 of Fig. 13.

Referring now to the drawings:

Fig. 1 shows a concentric or coaxial transmission line having an inner conductor 3 and outer conductor 4 extending between a source of high frequency energy 5 and a high frequency load 6. The inner conductor 3 and outer conduct-or 4 of the concentric transmission line may be generaly separated by air dielectric, with pairs of thin solid dielectric discs 7, i inserted at suitable points along said line for providing rigid mechanical positioning of inner conductor 3 with respect to outer conductor 4. The two spacers of each pair are separated by substantially a quarter wavelength at the operating frequency of source 5, so as to provide cancellation of reflections produced by the spacers of each pair. The characteristic impedance of the airdielectric transmission line 3, 4 is matched to the impedance of load 6.

A simple attenuator may be made within a section it of transmission line 3, 4, as shown in Fig. l. A dielectric body 8 and a dissipator disc 9 are positioned within the hollow outer conductor A of the concentric transmission line. The dissipatcr disc 9 is composed of suitable semi-conductive material to provide a dissipative current conduction path shunting the load 6. Part of the energy delivered through the concentric transmission line 3, 4 from source 5 is dissipated in the semi-conductive disc 9, while the remaining energy from source 5 is delivered to load 6.

The dielectric body 8 in this embodiment of the present invention performs two important functions in relation to dissipative disc or layer 9. First, the dielectric body 8 having an appreciable length L, serves as an excellentmechanical means for rigidly aligning semi-conductive disc 9 perpendiculary within the concentric line 3, l. Also, the dielectric body 8 in juxtaposition to the semi-conductive surface layer 9 cooperates with inner and outer conductors 3 and to form a solid dielectric filled transmission line section it having a difierent characteristic impedance from that of the air-filled transmission line section l2 extending to source 5 and the air-filled section i3 extending to the load 6. This solid dielectric-filledtransmission line section thus serves as an impedance transformer, the effect of which may be varied over a wide range by variation of its length L.

By virtue of this impedance transformer section, an appreciable loss may be inserted by the attenuator without introduction of an impedance mismatch at the input end of the attenuator adjel i nt the transmission line section l2. Sup- 5 pose, for example, that the impedance of load 6 is 100 ohms resistance and that the characteristic impedance Z of the air-dielectric sections I2 and I3 of transmission line 3, 4 is 100 ohms. Suppose further that 6 decibels relative attenuation is desired to be provided by the dissipator disc or layer 9, so that one-fourth of the input power is to be transferred to the load. Then the layer 9 should be a 33 -ohm resistance element. Acting in shunt with the 100-ohm resistance represented by the load 6 coup ed through transmission line section It, this 33 -ohm dissipator element 9 dissipates'% of the energy arriving from sourcefi, while one-fourth of this energy passes onward to load 6, as required.

Thus, the dissipator element 9 could function alone as a B-decibelattenuator, if it were mechanically self-supporting, and if the impedance presented to the tranmission line section l2 were immaterial. The impedance presented to the transmission line section l2 extending to source from the dissipator would be or '25 ohms resistance. Thus a serious condition of impedance mismatch and standing waves would be introduced in this air-dielectric section of transmission line having 100 ohms characteristic impedance. The impedance presented to source 5 might then vary over a 16:1 range, being any value between 25 ohms and 400 ohms, and might be part y reactive instead of purely resistive, depending on the length of the transmission line section l2 between the source and the dissipator.

The highly undesirable impedance mismatch produced by the introduction of dissipator 9 alone within the transmission line 3, 4 is entirely overcome in the present invention by the use of a dielectric body 8 of chosen characteristics and dimensions. The length L of the body 8 should be determined in accordance with the equation where A is the free-space wavelength corresponding to the frequency of source 5, and e is the dielectric con tant of the body 8 and is determined by the relation for 6 decibels attenuation,

and

A A A Thus, for a complete attenuator. according to the above example for 6'. decibels attenuation and a completely matched condition at a source frequency of 750 megacycles, per second, for example, :40 cm., and 13:5 cm. with a dielectric constant of 4. In this example, the characteristic per second impedance of transmission line section II is 50 ohms, as given by taking the quotient of the characteristic impedanceof an air-filled section, 100 ohms, divided by \/e. Also the effective electrical length of the solid dielectric body 8 in the above example is as given by Fig. 2 is an equivalent circuit'diagram through which the operation of the foregoing transmission system and attenuator may be more clearly understood. An alternating-current source '5' supplies energy to. a -ohm resistive load 6' through sections 12 and 13' of transmission line ,3, 4' having a characteristic impedance of 100 ohms. A 33 -ohm resistor 9is connected across the transmission line 3, 4' so that the resultant impedance provided by the load 5 through transmission line section I3 and the dissipator 9 connected across the line 3, 4' is 25 ohms.

A transformer 3 having half as many turns in its secondary winding as in its primary winding for an impedance transformation ratio of 4:1 is introduced into the circuit with its secondary winding connected to the dissipator 9 and its primary connected to transmission line section i2. The impedance then presented by the primary winding of transformer 8' to the source 5' and the transmission line section 12' is 100 ohms resistance. Therefore, the system impedances are properly matched, and no standing waves are present along the transmission line In this equivalent circuit diagram, it is readily seen that if the positions of the source 5' and the load 6 were interchanged, the attenuator con- 'sisting of dissipator 9' and transformer 8' would notbe suited for maintaining a condition of impedance match throughout the system. The 100-ohm load would then be connected across the 100-ohm winding of transformer 8, and

would produce a 25-ohm impedance in the op- Through study of the equivalent circuit dia-- gram of Fig. 2 which illustrates the cooperation of the dissipator 9 and the solid-dielectric filled impedance transformer section of transmission line adiacent dissipator 9, the operation of the coaxial-line attenuator may be clearly understood. It is thus made apparent that the attenuator of Fig. 1 must be properly oriented with respect to the load and source ends of the system in which it is employed, since impedance matching is effected for energy transmitted in only one direction. 1

In the above discussion, a purely resistive dis- -'sipator 9 is assumed. Actually, at very high frequencies, a dissipative disc or layer made up of compressed or deposited carbon particles is likely 'to have a complex impedance rather than a pure At a frequency of 3000 megacycles 0:10 cm.), for example, certain samples of dissipator layers have beenfound' to have appreciable capacitive reactance. For such a dissipator, the length L of the solid dielectric body 8 may be varied from the 90 effective elec-' trical length as used in the above example, to provide compensation for the reactive component of impedance of dissipator 9.

resistance.

Fig. 3 shows an admittance -circle diagram useful for analysis of the more complicated matched-impedance attenuator wherein dissipator 5i has a complex impedance. Such circle diagrams are widely useful for simple graphical analysis of transmission line behavior. If a section of transmission line of uniform characteristic impedance is terminated in a load of equal impedance, a completely matched condition is achieved and the input impedance presented by the transmission line section is uniform and independent of its length. If on the other hand the transmission line section is terminated in an impedance diiferent from its characteristic impedance, then the input impedance of the transmission line section varies between two limits as the length of the transmission line section varies. One of these limits is the termination impedance, and the other is given as the quotient of the square of the characteristic impedance of the section divided by the termination impedance. The input impedance alternates between these limits at space intervals of 90 electrical phase difference. and at intermediate lengths, varies through regions of complex impedance, first capacitive and then inductive.

A circle diagram of the type shown in Figs. 3, 5 and 7 may be used for analysis of the variation of impedance (or admittance) of a transmission line section with respect to the length of the section and the ratio of the termination impedance or admittance) to the characteristic impedance (or admittance) of the transmission line section. In such a diagram, as used for admittance analysis, the abscissae represent conductance values and the ordinates represent susceptance values. For study of these admittance components, the rectangular coordinate graph is provided. A. series of circles having centers on the axis of abscissae are superimposed on the rectangular coordinate graph. Each such circle passes through points of reciprocal abscissae on the axis of abscissae.

For example, a circle l5 passes through reciprecal points (0.6610) and (15,0). This circle represents the variation of input admittance input conductance and input susceptance-with respect to the variation of length of a transmission line section of admittance Y and termination of conductance G:% Yo or G= Y0. The input admittance of such a line section varies clockwise along curve as the length of the line section between the termination and the input point considered is increased. For an air dielectric line, the input impedance varies between the above two limits over a length of M4, making a complete cycle of variation around circle l5 from one limit to the other and return in a length of A/Z, where A is the free-space wavelength of the energy transmitted through the trans-mission line. Graph paper suitable for use in such analyses is supplied by the Keuffel and Esser Co. of New York, under the type designation 369-14. In order to make use of such a diagram for the present purpose it is convenient to consider the characteristics of the circuit elements in terms of admittance Y, susceptance B, and conductance G rather than impedance, reactance and resistance. The ordinate axis of Fig. 3 represents the ratio B/Yo of susceptance B to the characteristic admittance of solid-dielectric filled line section H, while the abscissa axis represents the ratio G/Yn of con ductance G to characteristic admittance Y0.

Furthermore, the computation by use of the circle admittance diagram 'for a desired attenuator unit is simplified if a predetermined dielectric material is first chosen for the body 8. The example illustrated in Fig. '3 is based on a body 8 of polystyrene, for which the dielectric constant 5:2.56, at A=l0 centimeters. The dissipator 9 in this case is characterized by a ratio of reactance to resistance of 0.4, defining an admittance angle of 213. The point b in Fig. 3 is then located for ordinate 0 and 1 1 abscissae -0.62a The vector ab then represents conductance Gr. of load 6 (equal to the characteristic admittance of air-filled transmission line sections l2 and lit), in relation to the characteristic admittance Y0 of the section II of transmission line of conductor dimensions equal to those of sections l2 and 13 but of solid dielectric 8 characterized by the dielectric constant 6:2.56.

The dissipator element 9 is efiectively in shunt with the load Gr. presented through the transmission line section I3, and .so a vector representing the admittance YD of the dissipator 9 must .be added directly to the vector ab to produce the resultant admittance of load plus dissipator. The magnitude of the vector YD is to be determined from the circle diagram of Fig. 3, but the line be along which the vector must lie is known to have a slope of 21.8". Now, a circle l4 having a diameter 1), 1) extending between the point I) of coordinates (0.625, 0) and the point I) of coordinates is drawn in the diagram of Fig. 3. The intersection 0 of the circle I4 and the line he is then located as the terminus of the vector bc representing the admittance YD of dissipator S for which the polystyrene-filled transmission line section H is capab.e of providing an admittance at the input end thereof equal to the conductive admittance G1. of load 6.

'The phase angle calibration circles passing through the point (1.0, 0) and having centers on the axis of ordinates then provide a measure of the efiective electrical length of the polystyrene body 8. This length is seen in Fig. 3 to be 104 of which a portion of 14 length is effective to compensate for the capacitive susceptance BD of dissipator 9 and the remaining efiective length section serves as an admittance transformer as described .above for rendering the admittance at the input end of attenuator section ll equal to the load conductance GL.

Thus, the diagram of Fig. 3 includes two admittance vectors GL and YD added to define a point 0 having the rectangular coordinates (1.46, 0.33) representing the sum of the shunt admittances of load 6 and dissipator 9 in relation to the characteristic admittance 1.0, 0 of the polystyrene-filled section of the transmission line 3,, 4. From the point e an arc of the circle l4 passing through 11' and 1), points of reciprocal abscissae, proceeds in a clockwise direction to terminate at point b, repr senting a transformation efiected by line section I I from the large comp x admittance ac at the dissipator element 9 to the desired conductive input admittance ab at the input end of transmission line section I I.

With the phase angle of admittance of dissipator 9 nxed, and with polystyrene chosen as the material for solid dielectric bodyB for insection in the transmission line of uniform diameter conductors 3 and 4, a unique solution is thus obtained for the length of body 8 and the admittance of dissipator element 9'. By comparison of the conductance G1. of load 6 with the sum of the conductive component GD of the admittance YD of dissipator 9 (shown as bd in Fig. 3) added to GL, the ratio of the energy passed to load 6 to the total energy received is obtained, and thus the relative attenuation of the transmission line section II may be determined. For the above example, this found to be corresponding to 3.7 decibels attenuation in section II.

It is readily seen that any desired number of composite attenuator. units similar to that comprising the dielectric body 8 and the attached attenuator element 9 may be used in cascade within the transmission line 3, 4 for any integral multiple of 3.7 decibels attenuation, without departure from the desired condition of input admittance equal to the conductance G1. of load 6.

Fig. 4 shows a high frequency energy transmission system embodying a symmetrical attenuator unit which presents a much lower input impedance than the load impedance to which energy is supplied through the attentuator. This system comprises a load 6" supplied with high frequency energy by source 5" through coaxial transmissionline 3", 4". If the characteristic admittance of the air-dielectric section l2" of the transmission line 3", d" is much greater than the conductance of load 6" and the equal characteristic admittance of the air-dielectric section 13" of transmission line 3", 4", then the solid dielectric body 8" may be used to support a dissipator 9" at the load end and an equal dissipator 9" at the source end. The dielectric body 8" may be made of such length as to serve only for compensating for the susceptance components introduced by dissipators 9 and 9",

or the body 8" may be made sufficiently longer to provide an admittance transformation effect as well.

The admittance diagram of Fig. 5 illu trates the above two alternative ways in which the attenuator of Fig. 4 may be made'to operate. As before, polystyrene is takenas the material for the solid dielectric'body, 8". From the dielectric constant 5:2.56, \/e=1.6, and thus the reciprocal abscissa points b and b on this diagram are fixed for the diameters of conductors 3" and 4" in the polystyrene dielectric section II" equal to those in the air-filled section l3" extending to load 6", The circle [4", and the location and ext nt of vector bc, are illustrated for a dissipator 9" equal to that illustrated by the generally similar admittance diagram of Fig. 3.

If a very large input admittance is to be presented by the attenuator section I l" of the transmission line 3", 4", the effective electrical length of the polystyrene body 8" may be made 2X14, as indicated by are cd of Fig. 4, the essential condition being that point d shall have the same conductance as point e, and equal magnitude sus- Ceptance of opposite sign. Then the admittance 10 added by dissipator 9 is represented by vector de, resulting in an input admittance equal to GL+2GD, where G'L is the conductance of load 6" and GD is equal to the conductive component of each of the equal dissipators 9" and 9".

If a Somewhat smaller input admittance thanthat defined above is desired, the cfiective electrical length of the solid dielectric body 8" may be made 14+57, or 71, as illustrated by the arc odd inFig. 5, the essential condition being here that point d have susceptance of equal magnitude to that of point d. At 3000 megacycles per second, corresponding to l\=10 centimeters, and with \/e=1.6 for polystyrene, the length of the body 8" is thus given as With this length of the body 8", dotted vector db' of Fig. 5 representing the admittance of dissipator 9, added to the admittance produced through the transformation effect within the solid-dielectric transmission line section, results in an input admittance ab, which is equal to EGL, which for polystyrene dielectric material 8 is equal to 2.56GL.

It should. be noted that for the structure of Fig. 4, unlike that of Fig. 1, there is no special significance in'terminating the vector be on the circle Hi passing through the point 1), since the input admittance to be presented by the attenua tor to the transmission line section 52' extend- "L or l.23:cm.

' ing to the source 5" is not to be made equal to less admittance than shown in Fig. 5 by the equal vectors be, de, and d'b, then such dissipators may be represented by the vectors be, d"e', and d"'b. The eifective electrical length of a body 8" made of polystyrene for use with these loweradmittance attenuators may then be 2 15 or 30 for an input admittance ae'. Similarly, the effective electrical length of body 8" may be 15+58, or 73 for an input admittance ab". The circle M' passing through points 0', d" and d represents the transformation through the dielectric body 8" effected for dissipators of the admittance represented by vector be. The points d and d are located on circle l i as the points having ordinates of equal magnitude but opposite sign to the point e at which vector bc' terminates. A similar analysis may be carried out for dissipators of greater admittance than represented by vector be.

The section 12" of the transmission line 3, 4" extending from the input end of the attenuator section II" to the source 5" is shown as having an appreciably greater inner conductor diameter than the other portions of the transmission line. Such a sect on l2" thus would have much greater characteristic admittance than the characteristic admittance of section l3" and the conductance of load 6". Accordingly, the-much 3 increased input admittance presented by attenuator section I i" to the transmission line section the;latter, so that av completely matched transmission system is provided. The characteristic admittance of section 12" of the transmission line also may' be increased by the use of an outer conductor having reduced diameter, or by use of a dielectric filling of dielectric constant appreciably'greater than unity.

In the system of Fig. l, a construction of the concentric line 3?, 5'5" is shown whereby attenuator. sections may be removed or added as desired.v For this purpose, a plug-in transmission line junction is provided in section 12 of the transmission line 3", and a second, similar junction is provided within section E3" of the line. To form such a plug-in junction, the outer conductor i" ends in a plane perpendicular to the axis or" the transmission line, and the inner conductor 3" ends at two: points substantially /3 from the planes defined by the ends of the outer conductor i". Two. thin dielectric discs I1 and. ll are fitted into outer conductor 4" and are provided withcentrally located holes for.

supporting the ends thus formed of inner conductor 3?. Thus, when two transmission line ends are connected together, the thin dielectric discs are separated by substantially i/d, so that the reflections from the two discs are cancelled.

The ends or inner conductor 3 are drilled centrally. to provide sockets for connecting to and supporting an inner, conductor junction member H8. The latter member comprises a main body of diameter equal to the diameter or" the inner conductor ends joined thereby, with theendsof the junction member i9 turned down to fit snugly into the drilled ends of the inner conductor portions. An outer sleeve 23 is provided to fit snugly over the abutting ends of outer conductor 4", to complete the junction.

By the construction of transmission line at tenuator sections having such ends adapted for plug-in connections the removal, addition, or interchange of attenuator units may be greatly facilitated.

As explained in detail above, the asymmetrical attenuator section of Fig. 1 provides a matched impedance condition for transmission in one directiononly, while that of Fig. l, although symmetrical, fails to provide an input impedanceequal to its output impedance. A transmission line attenuator section shown in Fig. 6 embodies a symmetrical combination of parts, and at the same time provides an input. admittance equal to the terminating admittance.

A portion of a coaxial transmission line having a uniform diameter inner conductor 23 and a; uniform diameter outer conductor 24 is shown in Fig. 6. Three successive dissipator elements 2.5, 26 and 2? are positioned at intervals along a portion of

transmission line

23, 25, separated andv mechanically supported by solid

dielectric bodies

28 and 29.

The admittance diagram of Fig. 7 illustrates the manner in which the dielectric bodies 2'3 and 29 cooperate with the

dissipators

25, 26 and 21 to produce the desirable attenuator characteristics of symmetry and over-all impedance match discussed above. The vector ab, as in the preceding admittance diagrams, represents the characteristic admittance of air-filled portions of. the

coaxial line

23, 24. A relatively low admittance element 2? acts in shunt with the transmission line 23, 2A which is assumed to be terminated in its characteristic admittance.

The inclination of the vector bc corresponds to the phase angle above discussed for certain measured samples. of dissipator, elements; for flea quencies of, the order of 3000 megacycles. The sumof the admittances ab-and bo represented by the location; of the point 0 in Fig. '7, COI'I'-r spends toa slightly capacitive admittance some.- What smaller than the characteristic admittance of the portions of

transmission line

23, 24 filled by

bodies

28 and 29 of. a olid dielectric mate.-.

rial such as polystyrene, for. example. Thesolid dielectric body 29 may be made in a length. corresponding to approximately-v effective. electrical length, and. thus the admittance trans.- formation through the section of transmission 1ine23, 25 filled by the, solid dielectric body 29 may be represented. the arccd. The physical length of the body 29 may becomputed. as.

in 360 y;

transmission line 23, 24- at the sending end of.

the solid dielectricbody. 291. It is noted'that an arc, cf of substantially 9'0? extent, representing the further impedancetransformation provided.

by the section oftransmission.

line

23, 24 filled by. solid dielectricv body 28, provides an ad'- mittance (represented by the point which is slightly inductive and which is somewhat smaller than the termination admittance ab. The ad'- mittance of the, input end dissipator 25 equal to' the admittance of the termination end dissipator 27, is then represented. by the vector, fb,,wh'icli shows clearly thatthe inputimp'edance of the. properly. terminated attenuator section of] the coaxial line 23', 2A is equal to the terminating impedance. thereof, which, of cours'e, is equal, to th characteristic admittance of I the air-filled

portionsof transmission line

23,24.

Actually, toexpeditean acceptable solutionfor. the dimensions and dissipator admittance values of, Fig. 6, the size of. the'enddissipators 25' and 2'! may be arbitrarily chosen and both vectors be and fb; may be located on the admittance difagram. The circles SI and 3'2" passing through points and 0, respectively, then may be located; and finally, a straight edge may be moved; parallel to the vectors fl) and be until arcs cd and e as measured by the effective electrical phase. angle calibrations thereof, are found to be equal. This locates the vector (16, and enables one to complete the circle diagram and" to graphically determine the admittance value which should characterize the middle dissipator' 26;

The

solid'dielectric bodies

28 and 29, being of approximately 90 electrical length as shown by the circle diagram ofFig. '7', should be out tolengths determined by 90 in 360 V? mits of providing any desired attenuation value" within a wide range without even the requirement of any change of dielectric material of

bodies

28 and 29.

A further symmetrical attenuator embodiment which presents an input admittance equal to the load or termination admittance is shown in Fig. 8. This attenuator arrangement is closely related to that of Fig. 6, being comparable in size to the above attenuator. However, the dissipation in the attenuator of Fig. 8 is continuously distributed throughout the length of the dielectric body. For this purpose, the dielectric body 35 which is positioned intermediate the inner. conductor 33 and the outer conductor 3d of

coaxial transmission line

33, 35 is purposely made of a material characterized by very high energy loss at the frequency of the energy to be transmitted. The material 35 may be made up by incorporating carbon particles or other high-loss material within a plastic base. Alternatively, a material which is ordinarily considered a good insulator at low frequencies but which is relatively inefficient at the high frequencies may be employed for the construction of dielectric body 35. The length of the body 35 preferably should be such as to have an effective electrical length of 180 or an integral multiple of 180, since such a length results in a complete cycle of impedance change, and the input impedance is therefore equal to the termination impedance. Thus, the physical length of the body 35 should be made approximately equal to where e is the real component of the dielectric constant of the high-loss material 35, and n is an integer.

In all of the foregoing attenuator embodiments ofthe present invention, the transmission line conductors have been shown as being of uniform diameter throughout the solid dielectric bodies and also throughout the air-filled transmission line portions 13 and I3" extending to the load. Actually, such an arrangement is entirely satisfactory if the material chosen for the solid

dielectric bodies

8, 8", 28, 29 and 35 is of a dielectric constant and other characteristics permitting a desired resultant attenuation characteris tie in the device.

In the attenuator of Fig. l, for example, as explained in connection with the admittance diagram of Fig. 3, the location of the point I; on the admittance diagram is determined as the point the characteristic admittance which would be produced with uniform diameter conductors, but yet different from the characteristic admittance Yo of the air-filled sections I2 and 13 of the transmission line 3, 6.

Fig. 9 shows an attenuator generally similar to that of Fig. 1, but different in the respect that the inner conductor c3 of the attenuator of Fig.9 is" enlarged to a greater, diameter over a portion 33 Xtending throughout the axial length of the solid dielectric body 58, which, as before, is fitted snugly into the uniform diameter outer conductor The solid dielectric body it in the attenuator of Fig. 9 serves to increase the characteristic admittance of the transmission line section filled thereby, just as does the solid dielectric body 8 much higher characteristic admittance than the .n Fig. 1. it.

section H of the attenuator shown will readily be seen from a study of the admittance diagram of Fig. 3 that such a further increase of characteristic admittance of the solid dielectric-filled transmission line section would result in a trace of an are similar to ob along a much greater diameter circle than the circle 26.. This would be accompanied by a relatively smaller vector ab and a much larger dissipator admittance be, and accordingly, a much higher relative attenuation would be provided in the structure of Fig. 9. Thus, even if only one kind of dielectric material is available or suitable for the construction of an attenuator of the general class shown in Figs. 1 and 9, variation of the conductor diameters may be employed to provide an attenuator of any desired characteristics within an exceedingly Wide range.

A change of conductor diameter may be made in a symmetrical attenuator of the general type shown in Fig. 6. By such a change of conductor" diameter, which may be accomplished either as shown in Fig. 10 through a reduction in size of the inner conductor, for example, or as shown in Fig. 11 through the use of an outer conductor of increased diameter, the characteristic admittance Yo of solid dielectric-filled portions of the transmission line may be made substantially equal to the characteristic admittance Yo of the air-filled transmission line portions external of the attenuator unit.

The embodiments of present inventionshown inFigs. 10 and 11 thus may be made to operate electrically just as though very thin discs or wafers of dissipating material 25, 2e and 2? (or 25", 28" and 21') were inserted within a coaxial transmission line and spaced therein in a self-supporting manner, without the dielectric bodies 28, 29 (or 28", 29"). In such an attenuato-r structure as that shown in Fig. 19, the admittance of the middle dissipator should then be substantially double that of each of the end dissipators 25 and 21. Similarly, the admittance of the middle dissipator 26" in the attenuator of Fig. 11 should be substantially double the admittance of the end dissipators 25" and Thus, in the attenuator arrangements of Figs. 10 and 11, an attenuating system is provided which electrically is nearly equivalent to that shown in Fig. 4 of ,U. S. patent application Serial No. 452,319, filed July 25, 1942, now Patent No. 2,514,544, issued July 11, 1950 in the name of William W. Hansen. Mechanically, however, the" attenuator arrangements of Figs. 10 and 11 are greatly enhanced by the rigid support for the dissipators 26', 21' and 25", 26', 2?" afforded by theaxially extensive solid dielectric bodies. which fill the intervening space between successive dissipator elements. As in the Hansen application, the response for varying frequency may be still more improved by adding more dissipators,. spaced by the same distances as the three. elements of Fig. 10 and having admittances substantially proportional to the binomial coefficients. of the expansion of abinomial of order of one less than the total number of dissipator elements.

The attenuator versions of Figs. 9, 10, and 11 all are. characterized by a changeoi diameter. of. either the inner conductor or. the outer conductor throughout the axial extent of solid dielectric. bodies inserted into the line and made a part of the respective attenuators. If desired, the diameter of the inner conductor of a coaxial transmission line may be reduced throughout. the length of a solid dielectric body, and the cooperating outer conductor diameter may be increased over the same portion of the transmission line. Such a modificationv of the structure of Fig. 8 is shown in Fig. 12. Thus, a large increase in the ratio of the diameters of the outer conductor and the inner conductor conveniently may be provided without an extremely large change of. diameter of either conductor.

If the ratio of the logarithm of the diametral. ratio throughout theextent of the solid dielectric body to the logarithm of the diametral ratio of conductors 34' and 33. extending beyond the.

solid dielectric body 35 is equal to \/e, then the characteristic impedance of the transmission line portion filled by the solid dielectric body 35 will be equal to the characteristic impedance of the air-filled portion of transmission line 33, 34.v In this event, the structure of Fig. 12, while able to function generally like the distributedloss attenuator of Fig. 8, may be made of sub.- stantially any length desired without the introduction of an impedance mismatch between the termination or load and the input impedance.

Substantially all of the attenuator structuresv shown in Figs. 1, 4, 6. and 8 through 12 are suitable for incorporation within a hollow-pipe waveguide, if desired.

For example, Fig. 13 shows a hollow-pipe wave guide in which is inserted a solid dielectric body 58 and a dissipative element. 59, substantially similar to the corresponding. elements of Fig. 1 except that no passage is provided through. the solid dielectricbody 58 and. the dissipator. 59 for a central conductor. As in the previous,

examples, the solid dielectric body 53 serves both.

as a mechanical support for the dissipator element 59 and also as an impedance transformer. within the wave guide section occupied thereby. A similar adaptation to use within a. hollow-pipe. wave guide may be made of any of the other em.-

bodiments of the present invention which have.

16 At. the same time, the soliddielectric, body may introduce a discontinuity of characteristic admittance within. the; high frequency conductor section occupied thereby, which thus'may serve as an admittance transformer for producing a desired input admittance as measured at the input end of the attenuator when the output end thereof is properly terminated in a suitable load. Furthermore, if dissipator elements having appreciablesusceptance as Well as. conductive admittance are employed, the solid dielectric bodies.

referred to above may be made ofsuch characteristics and dimensions. as to compensate fully for the susceptance introduced by the dissipative-elements, so that a. conductive input admittance may be provided as. desired at the highirequency input end'of the. attenuator.

'Since many changes could be made. in the above construction and many apparently widely different embodiments of thisinventioncould.

be made Withoutdeparting from the scope thereof, it is intended that all matter contained in the above description or shownv in. the accompanying drawings. shall be interpreted asv illustrative and not in a limiting sense.

What is claimedis:

l. Attenuator apparatus for conducting high frequency energy from a first section of coaxial line to a second section thereof comprising a relatively thin dissipator elementv extending across the space within said coaxial line. adjacentsaid second section and having appreciable admittance effectively in shunt with the admittanceoi said second section, and means for transforming the resultant effective shunt admittance of said second section and said dissipator element into a desired input admittance comprisinga dielectric body within saidhollowconductor adjoining said dissipator element to provide support there- I for andextending along said coaxial line toward asecond section of. said conductor and for presenting to said first conductor section an admittance substantially equal to the admittance of said second section, comprising a first relatively thin dissipator element extending across the space within said hollow energy conductor adjacent said first section thereof, a second relatively thin dissipator. element extending across the space within said hollow energy conductor adjacent said second section thereof, said first and second dissipator elements having equal admittancesappreciable in relation to. the characteristic admittanceof said hollow high frequency energy conductor,. a third relatively thin dissipator element extending across the space within said hollow energy conductor and positioned mide way between said firstand second dissipator elements, said third dissipator element having an admittance substantially twice the admittance value characterizing each of said first and second dissipator elements, afirst dielectric body 17 filling the space within said hollow high frequency conductor between said first dissipator element and said third dissipator element, a second dielectric body filling the space within said hollow high frequency energy conductor between said third dissipator element and said second dissipator element, said first dielectric body providing mechanical support for said first dissipator element, said second dielectric body providing mechanical support for said second dissipator element, and said first and second dielectric bodies jointly providing mechanical sup port for said third dissipator element positioned therebetween, said first dielectric body and said second dielectric body being of appreciable length compared to the quotient of one-fourth of the wavelength of said high frequency energy divided by the dielectric constant of said first and second dielectric bodies.

3. Symmetrical attenuator apparatus as set forth in claim 2, wherein said hollow high frequency energy conductor comprises the outer conductor of a coaxial transmission line.

ment extending across the space within said coaxial line and positioned midway between said first and second dissipator elements, said first, second and third dissipator elements having appreciable admittance in shuntwith the admittance of said coaxial line; a first dielectric body Within said coaxial line extending from said first dissipator element to said third dissipator element; and a second dielectric body within said coaxial line extending from said third dissipator,

element to said second dissipator element, said first dielectric body providing mechanical support for said first dissipator element, said second dielectric body providing mechanical support for positioned therebetween, each of said dielectric bodies being of a length of the order of V where l is the wavelength and e is the dielectric constant of said dielectric bodies, and the diameter of one electric current conductor of said coaxial transmission line throughout the extent of said first and second dielectric bodies being different from the diameter of said conductor in said second section of said transmission line.

'5. An attenuator for high frequency energy comprising a first section of transmission line; a second section of transmission line; and a third section of transmission line interconnecting said first and second sections of transmission line and including more than two dissipator members across said transmission line whose admittances correspond to the binomial coefiicients of order one less than the number of said dissipator members, said third section also including solid dielectric bodies completely filling the spaces between successive dissipator elements, whereby said dielectric bodies support said dissipator elements.

6. Apparatus as in claim 5 in which the plurality of dissipator elements are equally spaced. 7. Apparatus as in claim 5 in which said third section of transmission line has a characteristic impedance substantially equal to the characteristic impedance of said first section of coaxial line.

said second dissipator element, said first and second dielectric bodies jointly providing mechanical support for said third dissipator element EUGENE FEENBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS Number Name Date 1,957,538 Jensen May 8, 1934 2,088,749 King Aug. 3, 1937 2,151,157 Schelkunofi Mar. 21, 1939 2,197,122 Bowen Apr. 16, 1940 2,407,267 Ginzton Sept. 10, 1946 2,407,911 Tonks Sept. 17, 1946 2,408,745 Espley Oct. 8, 1946 2,409,599 Tiley Oct. 15, 1946 2,430,130 Linder Nov. 4, 1947 2,434,560 Gunter Jan. 13, 1948 2,437,482 Salisbury Mar. 9, 1948 2,443,109 Linder June 8, 1948 p Certificate of Correction Patent No. 2,538,? 71 January 23, 1951 EUGENE FEENBERG It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 7 4, for read A; column 7, line 32, before the Word or insert an opening parenthesis; column 9, line 16, after this insert is;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed thi 10th day of July, A. D. 1951.

ERNEST F. KLINGE,

Assistant Oommz'ssz'oner of Patents.