US2940057A - Selective mode filters - Google Patents

Description

Filed Nov. 1, 1957 MQQDOW IN [/5 N TOP 5. E. MILLER wrgm.

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ATTO RNEV United il atent Office 2,940,057 Patented .June 7, 1960 SELECTIVE M03313 PETERS Stewart E. Miller, Middletown, Ni, assignor to Bell Telephone Laboratories Incorporated, New York, N.Y., a corporation of New York Filed Nov. 1, 1957, Ser. No. 693,973 9 Claims. c1. ss3 1s This invention relates to selective mode filters for use with cylindrical waveguides transmitting circular electric wave energy.

In the transmission of electromagnetic wave energy through a hollow conductive pipe or other waveguide, it 'is well known that the energy can propagate in one or more transmission modes, or characteristic field configura- Ltions, depending upon the cross-sectional size and shape of theparticular guide and the operating frequency, and that the larger the cross-section of the guide is made the greater is the number of modes in which the energy can propagate at a given operating frequency. Very generally .it is desired to confine propagation of the energy to one particular mode, chosen because its propagation characlteristic-s are favorable for the particular application involved. If the desired mode happens to be the so-called dominant mode,it is feasible to restrict the cross-sectional "dimensions of the guide so that no modes other than the dominant modecan be sustained therein. This expedient is not available however, if the desired mode is not the dominant-mode or if a guide of large cross-section is prescribed in order, for example, that advantage may be taken of its relatively low attenuation. This is particularly true-of systems employing the TE circular electric mode. As is well known, .the propagation of microwave energy in theform of the TB mode in circular waveguides is ideally suited for the long distance transmission of high frequency wide band signals since the attenuation characftcristiczof this transmission mode, unlike that of other modes, decreases with increasing frequency. However, since the TE mode is not the dominant mode supported in-a circular waveguide, energy may be lost to other modes that are alsocapable oftransmission therein.

.'In an ideal waveguide which is perfectly straight, uniform and conducting, the propagation of T E waves there- .through is undisturbed, but slight imperfections in the guide and especiallycurvature of the waveguide axis may excite waves of other modes and produce serious losses. These losses are attributed mainly to the fact that the bending of the guide produces a coupling between the desired TE mode and other undesired modes, mainly the TM mode.

Recognizing that the coupling between these modes may-be likened to the coupling between traveling waves ontcoupled transmission lines in that an exchange of energy willtake place between the waves when they travel together in media in which they have the same propagation constants, the prior art has provided a large number of devices for modifying the propagation constants of the transmission path with respect to one of the modes.

In.the copending application of J. R. Pierce, Serial No. 416,315, filed .Mar. 15, 1954, now U.S. Patent 2,848,695, issued Aug. 19, 1958, it is disclosed that a helical conductor of diameter greater than 1.2 free space wave- :lengths will propagate a properly excited circular electric IE mode with 'a-ditferent phase constant than the TM mode.- ,This provides a substantial decoupling between nate any spurious non-circular modes.

these modes. In my copending application, Serial No. 416,316, filed Mar. 15, 1954, now US. Patent 2,848,696, issued Aug. 19, 1958, it was shown that if the respective TE and TM modes propagated along the resulting helical transmission path are exposed in a special way to electrically dissipative or lossy material so that they have substantially different attenuation constants, the coupling between the two modes may be made arbitrarily small. The attenuation constant of the TM mode may be substantially increased, for example, without increasing that of the TE mode by surrounding the helical guide with a cylinder-like casing of dissipative material having an inside surface contiguous with the outermost part of the helical conductor. Thus, the longitudinal currentsof the TM mode must pass through the lossy materiaLgiving to that wave a large attenuation constant while the component of the circumferential currents of the TE mode which follows the helix conductor is unaffected.

Where the transmission path is constructed substantially as outlined above, the conversion of TE wave energy to other spurious modes may be made negligibly small. However, it is often not feasible to provide helical waveguiding means for the circular electric mode. It may, instead -be necessary or desirable to use solid-cylindrical pipe as the transmission medium. Where this is so, it will be essential to interpose at regular intervals, mode filtering means for the purpose of removing those spurious modes that will be inevitably generated in any practical system thus minimizing the likelihood of conversion-reconversion distortion efiects. It would appear reasonable to construct such a mode filter in the form of a segment of helical'waveguide as described above to elimi- However, it has been found that merely to insert segments of helical waveguide between portions of solid wall guide had the effect of generating additional spurious modes by virtue of the conversion of the TM mode to 'I'M TE and TE modes. This conversion takes place at the junction of the solid and the helical waveguide and is a result of the change in transmission characteristics experienced by the TM mode at this point. While the additional modes thus generated will also "be dissipated in the mode filter as well as the TM mode, it has been found that to achieve the desired spurious mode level at the outputof the filter, a filter of longer length is needed than would be needed if the TM mode was the only spurious mode present.

It is, therefore, an object of thisinvention to couple, with minimum mode conversion, non-circular electric mode Wave energy in a multimode system from one transmission medium to another having different transmission constants.

It is an additional object of this invention to produce such coupling over a broad frequency band.

It is a further object of this invention to produce such coupling without interfering with the desired TE circu lar electric mode.

in accordance with a specific embodiment of the selec 'tive mode filter of the present invention, to be described is tight wound at one end, but in which the pitch gradualby increases until the spacing between turns is approximately one-half wavelength of the lowest frequency to be transmitted. At this point the second helix is discontinued. The dissipative jacket comprises a cylinder-like casing having aninside surface contiguous with the outermost part of the helical conductors. Thus, the longitudinal currents of the TM mode, which must pass 7 affected.

' lossy material, are gradually exposed to the lossy material, thereby causing this wave to experience a 7 smoothly increasing attenuation constant, while the comconversion is minimized. The filter length is calculated w to reduce the spurious mode level the systernto a specified level and is then discontinued, returning to the solid wall guide. Since the transmission characteristics a of both the filter and the solid wall guide are the same for the circular electriemodes, there is no need for any 'matchingi'mea n s ateither end of the filter for these modes. If the system is limited to transmissions in only one direction, it is sufficient to have only'one taperedhelical member at the input end of the filter. If however, wave energy is'to be transmitted in both diIections sirniIar tapered helical'members must be provided atboth ends of the filter. I 7

These and other objects, the nature of the present invention,and'its various features and advantages, will appear 'ni'ore'fully upon more detailed consideration of the specific illustrative embodiment shown in the accompanying drawings and analyzed in the following detailed description of these drawings.

In the drawings: 1 a Fig. 1 is a perspective view of a wave transmission system employing a helical filter in accordance With the principles of the invention; and

' Fig 2 is apartrally cut away view of the helical filter shown in 'Fig. 1. g I

Referring'more particularly to 'Fig. l, there is shown an electromagnetic wave transmission system comprising a source 11 of TE circular electric microwave energy connected to load 12 by means of solid wall waveguide transmission path 13 of substantially circular cross-section. Load 12 is-adapted to utilize TE circular electric mode wave energy, while transmission path 13 represents thetype of transmission line having bends,"joinits,slight physical imperfections, and deliberately inserted components, any of which tend tointroduce mode conversion effects therealong'. As is well known, these efiectsconvert portions of the TEg wave into'n'on-circular spurious waves, particularly the TM mode. i To prevent undue reflections and other deleterious'etlects, these spurious modes must .be eliminated in some manner prior to'enter- ,ing load '12;

In accordance with the present invention; the spurious modesintroduced by path 13 are eliminated from the output thereof by'a filter 14icomprising the helically wound conductors 15 and 16 surrounded and supported by a lossy jacket 17.' If the distance between source 11 and load 12 is great, more thanone filter is used. In practice a filter is placed at regular intervals of from to 1 00 feet. 'For the purposes of illustration only one filter is shown between the source and 1am; 1

Fig. 2 shows in detail the construction of the filter embodying the principles of the'invention. The inner member 15 comprises a conductor wound as a' helix having a uniform pitch and an internal diameter d equal to theinside diameter of cylindrical guide 13. Conductor 15 may be solid or stranded and may comprise a base metal such as iron or steel plated by a highly conductive material such as copper or silver. Adjacent turns 19 and 20 of the helix are electrically insulated from each other by either leaving a small air gap 21between adjacent turns when bare wire is used, or by using enameled or plastic insulated wire. Conductor 15 may, for example, be a number 37 size, enameled or plasticinsulated solid copper wire'(0.005 inch overall diameter). By close winding the helix, a uniform and appropriate spacing be tween successive turns is provided by the insulation. The pitch angle of the helix, should be as small as possible consistent with the above-mentioned insulating requirement. This distance in any event must be less than onequarter wavelength and is preferably such that'the gap 21 between adjacent turns is less than the diameter of conductor 15.

'space'between successive turns increasing with each successive turn until the space therebetween is greaterthan one-half wavelength of the lowest frequency to be transmitted. The longitudinal extent of helix 16 is represented in Fig. 2 as e, and is between one-half to one heat wavelength, a beat wavelength being defined as MM la-M where A is the wavelength of the exciting mode andk is the wavelength'of an induced spurious'mode; For example, k could be the Wavelength in the filter of the TM mode and A could be the wavelength in'the filter of the 'TE mode. Calculations must be made ofjall possible combinations of exciting and induced modes to assurethatthe proper helix length is obtained. However, experience will generally indicate to ones'killed in the art which of the spurious modes are of importance and which are not, thus the number of calculations are-generally very small. In a typical 2-inch diameter transmission system, the length of helix 16 will be about one foot.

Conductor 16 maybe in all physical respects similar to conductor 15 except that it has a larger diameter. A typical wire. diameter for 16 would beapproximately four times the diameter of 15. a a

If the system is limited to transmissions in only one direction, it is only necessary to have a tapered helical member at the input end ofthe filter. If, however, wave energy is to be transmitted in both directions, a second tapered helix 18 is placed on the other end of the filter. Unless considerations dictate otherwise, .helix 18 identical to helix 16. V V The space between adjacent turns of helix 15, 16; and 18, are exposed toelectrically dissipative or lossymaterial. This may be done by enclosing the helices Tina cylinder-like casing 17 of material having'a high electrical loss. Casing 17 may be made of any suitable plastic ,or dielectric material, such'as polyethylene, in

which small particles 23 of resistive material, such as iron dust or carbon black, are suspended. It is not desirable that'the materialof casing 17 extend into the space between adjacent helix turns and therefore casing '17 preferably has a smooth internal surface of diameter substantially equal to the outside diameter ofhelices 16 and 18 where they extend over helix 15 and the outside diameter of helix 15 over theregion between 16 and 18.

that the cross-section of helix 15 be maintained as nearly circular as possible. This condition may be maintained by the resilience of casings 17 and 22. It may, however, be maintained by employing for the conductor of helix 15 a spiral of spring steel plated by a highly conductive material. Thus, the efiectof the spring itself willrnaintain the desiredcircularity and shield 22 may then be 'wound of overlapping, thin strips or may be made of a woven braid. Other means of constructing the helical filter are disclosed in a copending applicationbyKohman et.al., Serial No. 679,835, filed August 23, 1957.

The inside diameter d of helix 15 is equal to the diameter of the solid wall circular guide 13 transmitting waves of the same frequency. This diameter d must be greater than the critical or cut-ofi diameter for the TE mode in the circular guide. This cut-ofi diameter is equal to 1.22%, where A is the wavelength-in free space of the longest wave in the transmission band. In practice, d might be in the range 1.25 to times the cut-off diameter, depending onthe transmission loss desired.

In operation, the circular electric TE wave being transmitted along guide 13 will be excited within helix 15 asit enters the filter section. A major component of the circular current associated with this mode is conducted along the helical path by each turn. Since the pitch of the'helix is small, this component constitutes substantially the entire current of the wave. A very small component of the wave is presented with a small reactance caused by the discontinuity between adjacent turns. This reactance will have the effect of changing the phase velocity of the total wave very-slightly. Very little of the total TE current will pass through the resistive material of casing 17 and therefore the attenuation constant of the TE mode is changed very little and the filter will not afiect the latter to any substantial degree.

With respect to the TM mode however, the picture is quite different. The TM mode has a predominantly longitudinal current fiow along the walls of the waveguiding path and will, as a consequence, be seriously affected by any discontinuities between adjacent turns of helix 15. By forcing the longitudinally flowing currents to pass through the dissipative material of casing 17 exposed between successive turns of the helices, the eifective attenuation constant of the wave-guiding path to the TM mode energy will be greatly increased. While this is the purpose of the filter section, unless this increase is brought about gradually, the 'IM mode will degenerate into other spurious modes having smaller longitudinal current components. Since the attenuation per unit length is directly related to the strength of the longitudinal component of current, these spurious modes would not be as effectively attenuated in a helical filter and consequently a longer filter section would be required.

The extent of exposure of the longitudinal currents to the dissipative material, and hence the attenuation, is a function of the ratio of the spacing between turns to the diameter of the wire, and for a fixed spacing such ratio is inversely related to the wire diameter. Larger wires present greater turn-to-turn capacitance and efiectively shield the inner region from the outer region. It is for this reason that helix 16, used to couple the non-circular modes to the mode filter is made of larger siZe wire. By close winding the larger diameter wire over the end of the filter, the exposure of the casing 17 to the longitudinal currents is initially very small. As the pitch of helix 16 opens up, more and more of the lossy jacket is exposed until the second helix is discontinued, at which time, the dissipative efiect of the lossy jacket is a maximum. The eifect of helix 16 might be likened to an aperture which is slowly opened, allowing more and more of the dissipating effect of the resistive casing to become effective. By extending the aperture opening over a length equal to from one-half to one beat wavelength, moding is reduced to a tolerable level within a reasonably short distance.

An alternative embodiment of the present invention uses a resistance film in between the lossy jacket and the helical waveguide instead of a tapered helix. The resistance film acts as a shunt impedance to the surface impedance of the lossy jacket. By varying the resistivity of the film, any prescribed distribution of surface resistance may be realized. Thus, the transition from the high isotropic conductivity of the copper wall to the low and anisotropic conductivity of the helix and lossy jacket structure may be made either gradually or in steps to reduce spurious mode conversion in the filter.

In allcases it is understood that the above-described arrangements are illustrative of a small number of tire many possible --specific embodiments which can represent applications of the principles of the invention. Numerous and varied *other arrangements can readily-be devised in accordance with these principles by thoseskilled in the art withoutdeparting from the spirit and scope of the invention.

Whatis claimed is:

' l. 'A selective mode filter for the preferential transmission of circular electric wave energy comprising -'a first means for propagating circular electric and noncircular electric modes of wave energy, said first means having afirst and a "second attenuation constant for said modes respectively, second means coupled tosaid first means for propagating said circular electric and said nonacircularelectric modewave energy, said secondm'eans havi'n'g a third and -a fourth attenuation constant for said modes I respectively, :and third means coupled to said first; and saidisecond :means 10 vary gradually theeattenuation constant for said non-circular electric mode wave energy between said second and said fourth attenuation constants, said first and said third attenuation constants being substantially equal.

2. Means for selectively attenuating high frequency non-circular electric mode wave energy comprising a first elongated member of conducting material wound in a substantially helical form having equally spaced turns, a second member of conducting material wound over a longitudinal region of said first member in a substantially helical form having progressively increasing spaces between successive turns, and a casing of dissipative material surrounding said helices.

3. The combination according to claim 2 wherein the diameter of said second conducting material is approximately four times the diameter of said first conducting material.

4. The combination according to claim 2 wherein said second helix extends longitudinally along said first helix firom between one-half to one beat wavelength where a beat wavelength A is defined as A being the wavelength in the attenuator of an incident non-circular electric mode and x being the wavelength in the attenuator of a spuriously generated mode.

5. The combination according to claim 2 wherein the maximum distance between successive turns of said second helix is approximately one-half wavelength of the lowest non-circular mode'to be applied to said attenuator.

6. A high frequency electromagnetic wave mode filter comprising a first helix of insulated closely spaced turns for the propagation therethrough of circular electric wave energy, second and third helical members conductively insulated from each other and from said first helix having non-uniform spacing between turns thereof wound directly over said first helix each of said members extending longitudinally from an end to a region towards the center of said first helix being close wound at said ends and having a progressively increasing pitch, said members terminating when the space between successive turns has increased to approximately one-half wavelength of the lowest frequency non-circular electric mode to be filtered therein, and a high loss outer jacket surrounding said helices.

7. The combination according to claim 1 wherein said third means comprises an elongated member of conducting material wound over a longitudinal region of said second propagating means in a substantially helical form having progressively increasing spaces between successive turns, and a casing of dissipative material surrounding said helix.

8. In an electromagnetic wave transmission system propagating circular electric and non-circular electric j mode wave energy, a selective mode filter for the preferential transmission of said circular electric wave energy comprising an elongated member of' conducting material Mound-in a substantially helical form having-.equally spaced turns, saidhelix having a first and a second at- 7 :tenuation constant for said circular and said non-circular electric mode wave energy, respectively, a second transmission path having third and fourth attenuation constants, respectively, for said circular and said non-circular electric mode wave energy comprising a conductively bounded circular cylindrical pipe, and means coupled to said helix and to said pipe to vary gradually the attenuation constant for said non-circular electric mode wave energy between said second and said fourth attenuation beingsubst'antially equal. a I, 9. An electromagnetic wave transmission system adapted for propagating circular electric and non-circular .electn'c mode wave energy comprising a section of solid conductive pipe connected at one point to a transmission onstants, said first and said third attenuation constants medium comprising an elongated member of "conducting material wound in the form of a helix, 'said piperand said helix having low attenuationconstants for both of said modes, means "for introducing'aregion of'high atsurrounding said helix adjacent said one point for gradually exposing said non-circular, mode to said dissipative material thereby gradually increasing the attenuation constant presented to said non-circular modes from said 3 low attenuation constant to said high attenuation constant. 7

References Cited in the file of this patent V UNITED STATES PATENTS 2,343,790 Cutler j July 15, 1958 7 FOREIGN PATENTS France V Mar. 19, 1956

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