US2557686A - Wave guide with electrical end termination - Google Patents

Description

June 19, 1951 h J RADQ 2,557,686

WAVE GUIDE WITH ELECTRICAL END TERMINATION Filed March 27, 1946 INVENTOR. JOHN A. RADO,

Patented June 19, 1951 UNITED STATES TENT OFFICE WAVE GUIDE WITH ELECTRICAL END TEREHNATION Application March 27, 1946, Serial No. 657,397

8 Claims.

This invention relates to wave guides including improved arrangements for delimiting their electrical lengths.

As used broadly in this specification and in the appended claims, the term wave guide applies to a system of conductive surfaces which act as the boundaries of an electric wave and have the ability of directing the propagation of such a wave, much as the rigid walls of a speaking tube guide sound by preventing the sound from spreading freely into space. Wave guides are well known in the art and may take the form of two or more separated conductors in open space, such as a power line or a telephone line. This type may be called transmission lines of the open-wire line variety. Another type of wave guide comprises simply a single hollow conductor capable of propagating the wave through its interior; this type may be called a hollow guide. Still other types include one or more conductors enclosed within but electrically insulated from another conductor, as in the conventional coaxial line. While certain details of design and method of excitation may depend on the type of guide and manner of its use, the term wave guide is used here in its generic sense, including any of the types mentioned, and refers also to the relatively short guides often designated as cavity resonators.

A prior arrangement used to determine the effective length of a Wave guide is a simple shortcircuiting bar or end cap which reflects currents flowing in the guide structure. When the guide is so disposed and excited as to operate in a resonant condition, high currents may flow between such a short-circuiting device and the adjacent portion of the conductive surfaces of the guide. This high current condition introduces troublesome contact problems, especially when the shorting device must be movable along the guide to adjust its length over a range of Values.

Another prior device, used in the types of Wave guides having a hollow cylindrical conductor, comprises a piston-like structure or tuner closed at one end and having a length approximately equal to one-quarter of the operating wave length. Such a piston functions in effect as a quarter-wave end section of the guide and, if r desired, may be movable to produce variations in the electrical length. With this arrangement, contact between the piston and the active part of the guide may be made at points far enough away from the point of current maximum that the currents flowing at the points of contact are appreciably smaller than is the case with a simple shorting device. While this arrangement decreases the deleterious elfects of high or variable contact resistances, experience has shown that some effect remains and may become objectionable in certain applications.

A so-called choke arrangement also has been used to determine the electrical length of a tubular wave guide. In this arrangement a plug fits inside the tubular guide, but is spaced therefrom to provide a channel. If this channel is made of a proper resonant length, the guide may be effectively terminated at the face of the plug. This arrangement differs from. the piston-like structure mentioned above in that the plug does not function as an end section of the guide, the face only of the plug delimiting the electrical length of the guide. When such a plug is used,

the over-all guide structure must have an extra length, necessary to obtain the proper resonant condition in the channel between the plug and the inner wall of the guide structure. Also, with a choke-plug arrangement the deleterious efiects of high or variable contact resistance still may be experienced. Here a contact is established between the inner wall of the guide structure and the supporting element of the plug.

It is an object of this invention, therefore, to provide an arrangement which substantially avoids one or more of the above-mentioned limitations of the described prior arrangements.

It is a further object of this invention to provide a wave guide having an improved arrangement for determining its electrical length.

A more specific object of the invention is to provide a wave guide of adjustable electrical length having improved efiiciency and increased freedom from deleterious effects due to contact resistance.

A still further object is to provide an improved means for supporting a short-circuited pistontype tuner in a wave guide.

In accordance with the invention, a wave guide comprises a conductive structure including at least one longitudinally extending conductive surface defining a wave-propagating space for guiding electromagnetic waves. A short-circuited guide section, having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of the guide, is longitudinally disposed within this propagating space to determine the electrical length of the wave guide. The short-circuited section includes at least one conductive portion adjacent to but spaced from the conductive surface of the guide structure and provides therewith an open-circuited transmission-line section having an electrical length of an odd number of quarter- Wave lengths at the operating frequency. The wave guide also includes means for supporting the open end of the short-circuited section with reference to the last-mentioned conductive surface of the structure comprising a dielectric 3 member positioned within the transmission-line section adjacent the open end of the short-circuited section.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is an axial section through one end portion of a wave guide of the coaxial transmission-line type embodyin the present invention; Fig. 2 is a transverse section through the wave guide of Fig. 1; Fig. 3 is an axial section through one end portion of a hollow guide embodying the invention; Fig. 4 is a plan view of one end portion of a wave guide of the open-Wire transmission-line type embodying the invention; and Fig. 5 is a sectional elevation of the wave guide of Fig. 4.

Referring to Figs. 1 and 2 of the drawing, there is shown an end portion of a wave guide comprising a conductive structure including a plurality of longitudinally extending conductive surfaces on portions Ii], II defining a wavepropagating space I2 for guiding electromagnetic waves. The conductive structure shown is of the coaxial type in which the conductive portion or surface I8 is a hollow cylindrical outer conductor and the conductive portion II is a coaxially aligned hollow inner conductor, establishing the wave-propagatin space I2 between the outer and. inner conductors as in conventional coaxial lines. An end plate I3 aligns and spaces the conductors I and II.

A short-circuited guide section I5, also of the coaxial type, is provided. The section I is disposed within and longitudinally adjustable along the wave-propagating space, in the embodiment illustrated, in order to effect tuning of the guide over a range of operating frequencies. The section includes longitudinally extending conductive portions shown as two hollow cylindrical conductors I6 and H, the inner conductor I1 and outer conductor I6 being coaxially aligned. These conductors are conductively connected at one end by a short-circuiting diaphragm I8 and, in the usual construction, are selected of such length that the section I5 has an effective electrical length approximately equal to one-quarter wave length at the mean operating frequency of the guide structure.

At least one, but preferably both, of the conductors of section I5 are adjacent to but spaced from the corresponding conductors of the guide structure and provide therewith open-circuited transmission-line sections, individually having an approximate electrical length of one-quarter wave length at the mean operatin frequency. Thus, in the illustrated embodiment, the inner conductors II and I! have a separation 29 and constitute a quarter-wave line section that is open-circuited at each end. Similarly, the outer conductors II) and I6 have a spacing 22 and form an open-circuited line section of the same length.

An adjustable short-circuiting section of the type illustrated may be conveniently referred to as a tuning piston or cup that is closed at one end by diaphragm I8 and is open at the opposite end. A means is provided for supporting the open end of piston I 5 with reference to inner conductor I l of the guide structure. This means comprises a dielectric member or ring 25, positioned within the space 20 between conductors II and I1 and, therefore, positioned within the transmission-line section formed by those conductors. Ring 25 further i positioned adjacent the open end of piston I5, and is constrained for movement with the piston by a suitably shaped recess 24 formed by a rib integral with the free end of conductor I! and thus near the open end of the piston. A material molded of, or impregnated with, a phenolic resin is suitable for ring 25. For purposes of assembly, the ring may be segmented and the segments separately fitted into the recess, or the recess may be formed by crimping the free end of conductor I! over a continuous dielectric ring.

A support 21 of generally toroidal shape is located behind the closed end of piston I5 to support that end in coaxial alignment with conductors I0 and II. Support 2'! is preferably of insulating material, and may be of a phenolic material havin high dielectric-loss properties since it is outside of the active propagatin space and need have mechanical strength sufficient to support only one end of piston I5. The support 27 is fastened to short-circuiting diaphragm I8 by means of elements such as screws 34.

Piston I5 and its support 21 are longitudinally adjustable through a driving mechanism including a driving rin 28 positioned within conductor II. Slots 29, 30 are out into conductor II and extend longitudinally for a distance somewhat greater than the desired traverse of piston I5 during adjustments of the guide structure. Keying pins 3|, 32 pass through slots 29, 30 and connect driving ring 28 with support 21. Ring 28 has an internal thread for engaging externally driven lead screw 33. The screw has suitable shoulders bearing against end plate I3 to prevent longitudinal movement. Suitable means (not shown), including a calibrated tuning dial, may be provided for rotating lead screw 33 to effect and indicate a longitudinal displacement of driving ring 28 within conductor I I and a corresponding displacement of piston I5 within the wave-propagatin space I2. If sup port 21 is of an insulating material, the entire piston can be placed at a direct current voltage or low-frequency (power-supply) voltage different from the voltage of the rest of the guide structure. The short-circuiting member, supported according to the invention, may be used to determine a fixed electrical length, in which case the support 21 may be afiixed to the guide conductors and the lead screw omitted.

If, in operation of the wave guide of Figs. 1 and 2, it is desired to tune over a range of operating frequencies, longitudinal adjustments of the piston are made by rotating lead screw 33.- During the adjustment, dielectric ring 25 slides along inner conductor II, and the position of the short-circuited end of the piston, relative to the remote end (not shown) of the guide structure, determines an adjustable active length of the guide. Accordingly, the effective electrical length of the guide is delimited by the diaphragm I8 of the piston I5. During resonant operation, current is reflected at the closed end of the piston, and the reflected wave energy combines with wave energy traveling toward the piston to produce alternate points of low and high peak current spaced along the guide at intervals of onequarter wave length. Thus, standing waves are produced in a well-known manner, with maximum current at short-circuiting diaphragm i8 and minimum current at the open end of the quarter-wave piston. The physical length of the piston corresponding to an electrical length equal to a quarter-wave length is not extremely critical, and it is usually entirely satisfactory to proportion the parts as if for operation at some frequency Within the desired range, approximately the mean frequency.

To obtain effective electrical connection between the conductors of the guide and a shortcircuited guide section of the specified length, it is not necessary that every portion of those conductors have a corresponding adjacent portion in the short-circuited section. Moreover, when the short-circuited section includes conductive portions adjacent to but spaced from a substantial portion of the conductive surfaces of the re- 1 mainder of the wave guide, there ordinarily is sufiicient coupling between those portions of the short-circuited section and the rest of the structure to ensure effective electrical connection at radio frequencies by means of that coupling alone. Under these conditions, no physical contact is necessary at any point between the shortcircuited section and the rest of the guide. Thus, the short circuit is efiectively provided without any deleterious eiiects due to the variable resistance of contacts between the short-circuiting member and the conductive structure of the wave guide.

Regarding the location of dielectric ring 25, it may be noted that power loss in a dielectric material not only depends on the loss properties of the dielectric, but also is proportional to the square of the voltage gradient in the dielectric. At the open end of piston l5, efiective contact is made between the piston and the conductors It] and II where a condition of low peak current but high peak voltage exists in the wave-propagating space. If the open end of the guide section were to be supported by a dielectric member within the Wave-propagating space at this high volt-age point, that member would have to be of an expensive material having low dielectric-loss properties, such as one of the better electrical grades of a polystyrene or polyethylene material, or an inorganic composition such as fused quartz or a steatite material made especially for highfrequency radio applications. Suitability of a given material often depends, of course, on the operating frequencies involved. If used in this manner, any materials exhibiting dielectric losses substantially higher than those of the named materials would impair seriously the operation of the wave guide. In accordance with the invention, however, ring 25 is located within the transmission-line section 26 provided by conductors H and H, to avoid such limitations.

Any wave energy which may enter the trans mission-line section 20 from the propagating space !2 of the guide travels to the far end of the section adjacent the short-circuiting diaphragm it. This far end of section 2!! is opencircuited, and is a point of voltage reflection where no current can flow. Consequently, wave energy traveling down the transmission-line section is reflected at the far end and sets up standing waves within the transmission-line section itself with a point of voltage maximum at its far end and (considering the transmission-line section alone) a point of voltage minimum and current maximum a quarter-wave length along the line section from its far end. Since the line section has a length of one-quarter of the operating wave length, its near end represents a point of voltage minimum. The supporting ring 25 is positioned within line section 26 adjacent its near end so that no appreciable voltage gradient exists in the space in which the ring 25 is positioned. By so positioning the ring, it is effectively protected from the high-voltage field in the wavepropagating space l2 of the guide, shielding the ring from the fields established in the guide. Positioning the ring in recess 24 augments the shielding.

A dielectric member so positioned in a region of low-voltage gradient may be made of a relatively inexpensive material having relatively high dielectric-loss properties without seriously increasing the power losses. Materials having such properties and also suitable mechanical properties include cellulose acetate, resins of the phenolformaldehyde and polymethyl methacrylate types, and certain cross-linked addition polymers identified as allyl resins.

Even using a relatively high-loss material for dielectric member, the wave guide of the invention operates with high efficiency. Moreover, the construction is such as to provide high mechanical strength and rigidity, which may be necessary under conditions of severe vibration. It is notable further that the transmission-line section Ell not only has low voltage across it at the point adjacent the open end of the piston, but also (considering the wave guide as a whole) has low current at that point, since it is at a point of low current in the wave-propagating space l2. Since neither appreciable voltage nor current exists across the section 2!! at the point where it opens into the wave-propagating space 12, very little power is applied to the section by the energy in that space. The transmission-line section 29 is thus only weakly excited. It is apparent that this is also true of the line section 22. It follows that there can be very little leakage of energy from the wave guide at the point where it is terminated by the short-circuited guide section. Since there may nevertheless be some voltage field at the far ends of these transmission-line sections, it may prove desirable to cut away portions of the support 2?, as shown, so that it touches member It only at the central portion of the member. Such a construction substantially shields this support from the fields established in the guide structure.

Fig. 3 shows the application of the invention to a hollow wave guide including a conductive surface comprising at least two portions 48 and M. These portions comprise opposing surfaces of a hollow conductive structure defining a wavepropagating space 42. The hollow structure is provided with an end plate 63, and may have any of the various cross-sectional shapes known to the art as suitable for hollow guides.

A short-circuited section is longitudinally disposed within propagating space G2 to determine the electrical length of the guide. The section 55 includes conductive portions 45 and 57, comprising opposing surfaces of a hollow section preferably similar in cross section to the hollow conductive structure, and situated adjacent to but spaced from the corresponding wall portions to and ti, respectively, of the hollow structure. Section 35 thus provides with the walls of the hollow structure a transmissionline section 58 open-circuited at each end. The short-circuited section &5 has the form of a tuning piston or cup closed at one end by short circuiting disc at. The conductors ii; and ii are selected of such length that piston 35 and transmission-line section so have effective electrical lengths equal to an odd integral multiple of onequarter wave length at the operating frequency of the guide, usually one-quarter wave length, as with the coaxial line of Figs. 1 and 2.

Referring again to Fig. 3, a recess formed by an integral rib A, near the open end of piston 45, positions a dielectric member 55 within transmission-line section 56 for supporting the open end of the piston with reference to the walls of the hollow structure. In the embodiment illustrated, the piston 45 is longitudinally adjustable for tuning the guide, and the recess 54 may be formed in any manner suitable to constrain member 55 for movement with the piston. As in the embodiment illustrated in Figs. 1 and 2, the dielectric member 55 may extend continuously around the piston, or may comprise a number of circumferential segments capable of being slipped into recess 54 before the piston is inserted into the hollow structure. A continuous dielectric ringadvantageously may be used on a cylindrical piston, while the segmented type of dielectric member is applicable particularly to guide structures of rectangular cross section.

An insulator 5'! is aflixed behind the closed end of piston 25. A lead screw 58 is held rotatably captive within a bushing portion of insulator 5?. Screw 58 passes through a tapped bushing 59 afiixed to end plate 43. Rotation of the lead screw by appropriate means (not shown) causes insulator 57 and dielectric member 55 to slide along the hollow structure Ml, ti, and accomplishes longitudinal tuning adjustments of piston 45 along the propagating space 52.

In spite of certain differences in their exact electrical behavior, the parts of the hollow guide of Fig. 3 embodying the present invention operate in a manner similar to the operation of the coaxial line of Figs. 1 and 2.

Figs. 4 and 5 illustrate the invention as applied to a wave guide of the open-wire line type. The wave guide comprises a pair of spaced longitudinally extending conductors S3 and 66 defining a wave-propagating space 62. A spreader 63 aligns and spaces the conductors Eli and 6! at the end portion of the wave guide illustrated.

A short-circuited section 65 is longitudinally disposed within the propagating space 62 to determine the electrical length of the guide. The section 65 includes conductive portions 66 and 61, adjacent to but spaced from substantial portions of the conductors 60 and 6!, respectively. Thus, conductors 6E} and 66 have a separation 10, providing an open-circuited transmission-line section. An identical transmission-line section II is provided by conductors GI and 61. A short-circuiting bar 68 is conductively connected to the conductors 66 and 61 of the section. Dielectric ring segments '15 and are positioned within the transmission-line sections 19 and H, respectively,'adjacent the open end of short-circuited section 65 to furnish support with reference to the conductors 60 and SI. The segments 15 and 16 are constrained near the open end of section 65 by expansions 13 and 14 forming recesses in the conductive portions 66 and 51. The segments are forced to bear against the conductors 60, 6| by connecting the conductors 66, 61 to the short-circuiting bar 68 in such a manner as to provide spring action forcing the conductors 66, 61 apart at the open end of section 65.

To align the short-circuited end of the section, a support 11 is fastened to short-circuiting bar 68 by screws 84. The other end of support 11 is fastened to a centering insulator I8, which has two holes through which the spaced conductors B0 and BI pass. The conductors 66 and. 61 of the section 65 are chosen of suitable resonant lengths, as with the other embodiments of the invention described above. The insulator 18 may be moved longitudinally of the guide to effect tuning adjustments of the piston, operation of the openwire line being quite similar to operation of the coaxial line of Figs. 1 and 2.

While there have been described what are at present considered to be the preferred embodi ments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A wave guide comprising, a conductive struc ture including at least one longitudinally extending conductive surface, said structure defining a wave-propagating space for guiding electromagnetic waves, a short-circuited guide section having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and longitudinally disposed Within said propagating space to determine the electrical length of the wave guide, said short-circuited section including at least one conductive portion adjacent to but spaced from said conductive surface of said structure and providing therewith an open-circuited transmission-line section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said conductive surface of said structure comprising a dielectric ring positioned within said transmission-line section adjacent said open end.

2. A wave guide comprising, a conductive structure including at least one longitudinally extending conductive surface, said structure defining a wave-propagatin space for guiding electromagnetic waves, a short-circuited guide section having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and longitudinally disposed within said propagating space to determine the electrical length of the wave guide, said short-circuited section including at least one conductive portion adjacent to but spaced from said conductive surface of said structure and providing therewith an open-circuited transmission-line section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said conductive surface of said structure comprising a dielectric ring having high dielectricloss properties at said operating frequency and positioned within said transmission-line section adjacent said open end.

3. A wave guide comprising, a conductive structure including at least one longitudinally extending conductive surface, said structure defining a wave-propagating space for guiding electro-magnetic waves, a short-circuited guide section having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and longitudinally adjustable within said propagating space to determine the electrical length of the wave guide, said short-circuited section including at least one conductive portion adjacent to but spaced from said conductive surface of said structure and providing therewith an open-circuited transmission-line section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said conductive surface of said structure comprising a dielectric ring positioned within said transmission-line section adjacent said open end and constrained for movement with said shortcircuited section.

4. A wave guide comprising, a conductive structure including at least one longitudinally extending conductive surface, said structure defining a wave-propagating space for guiding electromagnetic waves, a short-circuited guide section having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and longitudinally adjustable within said propagating space to determine the electrical length of the wave guide, said short-circuited section including at least one conductive portion which has a recess near the open end of said guide section and is positioned adjacent to but spaced from said conductive surface of said structure to provide therewith an open-circuited transmissionline section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said conductive surface of said structure comprising a dielectric ring positioned within said transmission-line section adjacent said open end and constrained by said recess for movement with said short-circuited section.

5. A wave guide comprising, a longitudinally extending hollow conductive structure defining a wave-propagating space for guiding electromagnetic waves, a short-circuited guide section having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and longitudinally disposed within said propagating space to determine the electrical length of the wave guide, said short-circuited section including conductive portions adjacent to but spaced from the walls of said hollow structure and providing therewith an open-circuited transmissionline section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said hollow conductive structure comprising a dielectric member positioned within said transmission-line section adjacent said open end.

6. A wave guide comprising, a pair of spaced longitudinally extending conductors defining a wave-propagating space for guiding electromagnetic waves, a short-circuited guide section having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and longitudinally disposed within said propagating space to determine the electrical length of the wave guide, said short-circuited section including at least one conductive portion adjacent to but spaced from one of said conductors and providing therewith an open-circuited transmissionline section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said one conductor comprising a dielectric ring positioned within said transmission-line section adjacent said open end.

7. A wave guide comprising, a conductive structure of the coaxial type including a hollow cylindrical outer conductor and a coaxially aligned inner conductor defining a wave-propagating space for guiding electromagnetic waves, a shortcircuited guide section of the coaxial type including hollow coaxially aligned inner and outer conductors having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and being longitudinally adjustable within said propagating space to determine the electrical length of the wave guide, said inner conductor of said short-circuited section having a recess near the open end 'of said section and being adjacent to but spaced from said inner conductor of said conductive structure to provide therewith an open-circuited transmissionline section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said inner conductor of said conductive structure comprising a dielectric ring positioned within said transmission-line section adjacent said open end and constrained by said recess for movement with said short-circuited section.

8. A wave guide comprising, a conductive structure of the coaxial type including a hollow cylindrical outer conductor and a coaxially aligned inner conductor defining a wave-propagating space for guiding electromagneticwaves, a shortcircuited guide section of the coaxial type including hollow coaxially aligned inner and outer conductors having an effective electrical length equal to an odd integral multiple of one-quarter wave length at the operating frequency of said guide and being longitudinally adjustable within said propagating space to determine the electrical length of the wave guide, said inner conductor of said short-circuited section having a recess near the open end of said section and being adjacent to but spaced from said inner conductor of said conductive structure to provide therewith an open-circuited transmission-line section having an electrical length of an odd number of said quarter wave lengths, and means for supporting the open end of said short-circuited section with reference to said inner conductor of said conductive structure comprising a ring having high dielectric-loss properties at said operating frequency positioned within said transmission-line section adjacent said open end and constrained by said recess for movement with said short-circuited section.

JOHN A. RADO.

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

UNITED STATES PATENTS Number Name Date 2,226,479 Pupp Dec. 24, 1940 2,342,254 Dallenbach Feb. 22, 1944 2,400,777 Okress May 21, 1946 2,404,086 Okress July 16, 1946 2,408,895 Turner Oct. 8, 1946 2,421,591 Bailey June 3, 1947 2,423,998 Schantz July 15, 1947 2,438,912 Hansen Apr. 6, 1948 2,439,388 Hansen Apr. 13, 1948 2,456,770 Dearing Dec. 21, 1948 2,503,256 Ginzton Apr. 11, 1950

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