US2779006A - Spurious mode suppressing wave guides - Google Patents

Description

Jan. 22, 1957 w. .1. ALBERSHEIM 2,779,006

SPURIOUS MODE SUPPRESSING WAVE GUIDES Original Filed Dec. 2. 1949 /A/ VEN TOR L4. J. ALBERSHE/M- A 7'v TURA/EV ,fous losses.

United States Patent F"ice sPUaIoUs MODE sUPPREssING WAVE GUIDES Walter J. Albersheim, Interlaken, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application December 2, 1949, Serial No. 130,670. Divided and this application October 10, 1951, Serial No. 250,752

13 Claims. (Cl. 333--98)A This invention relates to the guided transmission of ultra-high frequency electromagnetic waves and, more particularly, to the propagation of waves of the circular electric or TEol mode through Curved bends and elbows in circular wave-guide structures. As used throughout the specification, the term bend will be taken to refer to gradually curved sections of wave guide having large bending radii while the term elbow will be used to refer to relatively sharp curved sections of wave guide having short bending radii.

This application is a division of my copending application Serial No. 130,670, filed December 2, 1949, now Patent 2,649,578 granted August 18, 1953, and entitled Wave-Guide Elbows. l

The propagation of microwave energy in the form of TE01 waves in circular wave guides is ideally suited for the long distance transmission of wide band signals since the attenuation characteristic of this transmission mode, unlike that of all other modes, decreases with increasing frequency. However, one difficulty with this method of transmission is that the TEo1 mode is not the dominant mode supported in a circular wave guide, and consequently energy may be lost (by transformation) to other modes also capable of transmission therein. In an ideal wave guide which is perfectly straight, uniform and conducting, the propagation of TEo1 waves therethrough is undisturbed, but slight deformations and imperfections ,in the guide and especially curvature of the wave-guide `axis may excite waves of other modes and produce seri- These losses are attributed mainly to the ,fact that the bending of the guide produces a coupling ,between the desired TEoi and other transmission modes, mainly the TMn mode.

In his article entitled Propagation of TE01 Waves in :Curved Wave Guides appearing in the January 1949 'issue of the Bell System Technical Journal, vol. 28, No. l, applicant describes the nature of this mode coupling and 'likens it to that between traveling alternating-current .waves in coupled transmission lines. Each mode capable of transmission in the wave guide is analogous to a separate transmission line. Since the predominant losses in the bends and/or elbows considered herein are due to interaction between the TEor and TM11 modes, it will be sufficient to consider only two coupled lines; a primary line representing the desired or TEoi mode and a secondary line representing the undesired or TMn mode.

In two such lines there exist for each frequency and direction of travel two distinct traveling-wave configurations, from superposition of which all possible current distributions may be built up. In the limiting case when the lines are uncoupled, these configurations consist of a wave in the primary line alone and a wave in the secondary line alone. When the lines are coupled it is not possible to impress a current on one line alone without generating a secondary current in the other line. The two possible wave configurations in the second case are (a) one in which the electromagnetic elds generated by the 2,779,006 Patented Jan. 22, 1957 currents in the two lines tend to be in phase and reinforce eaeh other, and (b) one in which the fields tend to be in opposite phases and weaken each other. Due to the greater energy storage, configuration (a) has a slower phase velocity than configuration (b), so that if both configurations coexist, there will be beats between them as they travel along the two lines. Due to the different phase relations between primary and secondary currents in the two configurations, these beats alternately increase and decrease the current in each line in a sinusoidal manner. If at the point of origin, current is made to flow in the primary line only, configurations (a) and (b) will coexist in such amplitude and phase relations that their components cancel each other out in the secondary line at that point; at other points along the lines energy will be transferred in increasing or decreasing amounts into the secondary line so that a sinusoidal current flow in that line will be observed.

The amount of energy transfer between lines per unit length has been shown in applicants above mentioned Bell System Technical Journal paper to depend upon the coupling discriminant 17 which'is defined as the coupling coefiicient k divided by the relative difference in propagation constants, and may be expressed as follows:

The coupling coefficient k may be expressed broadly in terms ofthe energy stored in the individual lines 1 and 2 and the energy transferred from one line to the other. Applicants paper shows that if the coupling discriminant is much smaller than one, only a small fraction of the energy originally flowing in the primary line will be transferred to the secondary line before the energy flow is reversed; if the coupling discriminant is much larger than unity, nearly the entire energy flows back and forth between primary and secondary lines.

For the purpose of further exposition ofthe present invention the term effective interaction length is introduced. This term may be defined mathematically as the integral of the coupling discriminant 17 over the entire length of the coupled line section and will therefore indicate the total energy transfer between lines. It should be noted that the coupling coefficient k may vary from positive to negative values (for instance by reversing the polarity of a coupling inductance) and it is therefore possible to make the effective interaction length zero even though the coupling coefficient is finite over nearly the entire coupling length. v

Applying the above transmission line analogy to the coupling between the desired TEoi mode and the TMu mode (or in exceptional cases, other undesired transmission modes) in a Wave guide containing intentional or unintentional bends or elbows, it has been shown in applicants above-mentioned Bell System Technical Journal paper that the coupling coefficient k is proportional to the curvature of the bend or elbow and to the diameter of the wave guide. The relative difference in propagation constants (Tij) (w/rlrz) between the TEo1 and TM11 modes is very small in a smooth, highly conductive wave guide, and approaches zero in a wave guide of zero resistivity and substantially zero curvature.

From this, it follows that in an ordinary smooth waveguide bend or elbow, the coupling discriminant n as expressed in Equation 1 supra, tends to be large so that nearly the entire energy of the TEni mode impressed upon the beginning of the bend may be transferred to the TMu mode by interference between the two con- C iigurations consisting of combinations of primary and secondary currents, that is, of TEOi and TMn Cmponents.

Accordingly, itis a primary object of the present invention to provide waveguide bend and'elbow designs .for the transmission of T E01 circular waves, wherein losses dueto curvature of the guide are substantially reduced or eliminated.

Another object of the invention is to provide broad band, low loss bend and elbow, designs for the transmission of TEO1 circular waves in wave guides.

Y A specific object is to provide circular wave-guide bends and elbows wherein degeneration of the TEO1 wave into TM11 wave power is substantially eliminated.

In furtherance of the objectives of the -present invention means are provided for .minimizing Aor canceling the effect of interaction between TEOi 'and other mode waves in the deformations, bends or' elbows of circular wave guides. In the designs'presented in' the' present application, this is accomplished by increasing the relative'differencein propagation constants of the l'desired and unwanted mode waves and by attenuating 'the unwanted modes.

The nature of the present invention' and other objects, features and advantages thereof will be apparent from na consideration ofthe following detailed'description, from the appended claims, and from the drawings in which:

Figs. 1 and 2 illustrate'the transverse electromagnetic eld patterns of the TEO; andyTMu waves, respectively;

Fig. 3 diagrammatieally illustrates a microwave oscillator supplying energy inthe form of TEOr waves to a 'load'through a circular wave-guide passage including a smoothly` curved elbow Yor bend; and

' Figsl 4, 4A and 4BV are embodiments of the invention utilizing" transverse slots inwave-guide elbows or bends.

Referring to the figures, Figs. 1 and 2 illustrate the distribution of the electricand magnetic fields in transverse lsci :tions of a pair of circular wave guides supporting the TEOi and TMm transmission modes, respectively. The `transverse electric TEOr wave illustrated lin Fig. 1 is :designated as the circular electric typey inasmuch as the 'electric field, shown by the solid lines, consists of circular 'lines coaxialv with `the guide and lying transversely thereto without any longitudinal components. The transverse component of theA magnetic iield, indicated by the dotted flines, forms at various points along the guide' a radial ern. The intensity of the electric held attains a maximumy approximately 'half way between the axis and the surface ofthe Vguide'and drops to zero at the surface. 'The current flow associated with the VTEOi wave is predminantly circular around the "periphery of the guide as Yillustrated in Fig. 1. p l y l The configuration ,of thev transverse magnetic TMm inode'shown'in `ig.,2` is similartothat of a shielded conductorfpair.Qv Therriagnetic field pattern is entirelyV transversal ,with'ou :any longitudinal'v components and is indicated bythe dotted lines encircling the'respective poles A17,1?" whichfinthecase of a plane bend, exhibit lanorientation in afplane normal to theplane of the bend. Since lthlefr'r'ignetic lines must form closed paths', they tend to spreadut near the `center'vof the guide and to crowd close together at the inner surface mostly near thevertical axis of the guide thus inducing a considerable longitudinal conduction current ow irizthe wall of the guide as shown conventionally in Fig. 2. p .y

In va microwave system for the transmission of '[501 jwavestheinside radius a of the circular pipe guide selected foryfthe propagation of these waves must be greater than the critical or'cut-off radius ctc for the TEO; mode. The cut-off radius a@ for the lTEOi mode is equal to 0.61m, Awhere O is the' wavelength in free space' ofthe longest wave, (i. A .e. lowest frequency), in the transmission bandof frequencies. In practice a is made greater than'ac'and ymayvary indifferent systems', from about 1.5)(Oto 15M, for' example.v For'illustrativepurposes, a' suitable inner lradius for the Wave-guide structures described herein can be about 2a@ or 1.21m. Thus, if a hollow-pipe guideb five inches in diameter (internal dimension) were selected for transmission of TEO1 waves, AO, in accordance with the above, would be two inches.

In Fig. 3, there isvshown a simple wave-guide installation wherein a variable frequency source 1G of any suitable wellk'nown type supplies microwave energy inthe form of TEOrwaves to a load l2, such as, for example, a microwaverepeater or antenna, through a circular 'waveguiding passage having a hollow interior. Vl`h`rough out Athe greater part of its length, the passage comprises a pair of angularly disposed straight uniform sections of wave guide .14, 15, which'are joined' by `a relatively shortI curved elbow or bend 17, the design of which may assume any of the forms describedhereinafter.

The characteristic transformation of TEOr into undesirable TM11 wave power is ascribed to the fact that these waves have substantially the same phase constants, i. e., phase velocity and wavelength and, therefore, interact strongly in a manner analogous to coupled transmission lines as `set forth hereinabove. In the following embodiments of the invention, the elbows or bends are so treated as to change'the phase velocity of the TMt; wave relative to the TEOr wave to increase the relative difference in propagation constants. ln addition, attenuation of the unwanted Inode is'in certain instances also introduced. This reduces the effective interaction length and coupling discriminant as expressed in Equation l and `avoids conversion of TEOi into TMm wave power.

Pig. 4 illustrates in plan the appiication of one method 'of affecting the phase constants of the TMm mode which consists of inserting predominantly reactive effects in the wave-guide elbow or bend in such a manner that the phase constants of the TE and TM modes of the same operating frequency will be substantially different. lf the .wave-guide bend is cut apart by transversal gaps or slots -20 into ring-shaped sections 22, the slots do not a'p- Vpr'eciably interfere with the transversal current flow of the TEO1 wave and, therefore, do not substantially change the'congu'ratiom wavelength, orl velocity of this mode.

` However, the TMm mode has a predominantly longi; vtudinal current flow in the wave-guide walls, and it is seriously affected by the division of the guide intoshort "cylindricalrings, At low frequencies, each slot would vbe a complete open circuit and suppress all current flow, but at`the` microwave frequencies, involved in wave-guide propagation, 4each gap constitutes a large capacitive series d"e'a'ctancewhich serves to increase the phase velocity of the TMirmode. If the slot width were increased sufcientlyto cause the inserted capacitive reactance to exceed the'nductive reactance of the remtainingtmetal wall,`the TEO; 'mode would not be altered' appreciably' while spurious transmission modes like the TMu would be'supv"tfrsfsfaflgatgerhelr ii a` p rajctical4 structure,'the rings VVshown in Fig. 4 may "beenclosed or embedded in a protective casingl 25, which frnayfbe a rubberihose, a metal or plastic braid or any f'other suitable, protective covering known to the art. The 'casingmayfeither be rigid, forming a permanent struc- "turejor'pliable,forming a ilexible one, as shown in Fig. ,4.1 .If lthe V,covering is made of a non-'conductor ora'nonv`"c :l'ntluc':tingf dielectric' material', a small, fraction of the 'TEOi energy and a large part of vtheV spurious T M11Y energy maybe `dissipated laterally by radiation through the ring fslots`20fand cause the cross-talk between adjacent wave guides. `Radiation losses and cross-talk can be prevented fbycovering'the outer and/ or'inner surfaces of the dielec- V'tricfc'asing 25 with thin metal shieldinglayers '27, ,27' `which' may be plated, sprayed, dusted, or painted on. Due to skineifect, the currents flowing in the walls of a wave guide are confined near its inner surface, I and accordingly thethickness of the shielding layers needrfnot be greaterthanapproximately 0.1 mil. In the presence the shielding layers, the longitudinal 'current flo'wof the TMii nide follows an irregularly'raised path which' alters the. waveguidel reactancef to the` undesired'mnde zand affects its, phase'velocity.

Some of; the stray, energy reaching theA layers .27; 12-7! is reected' back into the1 waveguide further ajfectingthe waveguide.reactanceftothciundesred.transmission modes. These.. reflecticn. effects; vars'. with. frequency; and; .may exhibit-Sharp: cavity Yresonance eiects4 at some. frequencies-e A. m9l'ez-unf0rm;modification of; theWave-guide reactance may" beaobtained. by, .using a lossy material; inA Athe casing 2 5, such,; for; example, as a plastic material in which carbonzdust; has Abeen dispersed.; In. this case, only the metallic shielding. layen27, located on the1outside ofthe casing. should' be used.l

A.' self-'supporting and'. semirigid. structure similar in performanceto the ring-.type'structureof Fig. 4 may be obtained by slotting the. waveeguide bend only partially around its circumference, as.- shown in Figs. 4A and 4B. Even the partial slots 21 interferesuiciently withrthe T-Mlrmode to increase its phase velocity. considerably andl thus reduce the mode coupling and the resulting transfer-'of energy to the TM mode.

Theguide may be slotted'beforebeing bent toformvas illustrated in Fig.'4A. In Figs. 4 and 4B, only the curved section of the guide need be slotted. Itmay not be necessary' to employ anyform Yof impedanceV matching transition into the unslotted straight section ofthe guide". Fig. 4 is suited Vfor sharp bends or elbows which may be further defined arbitrarily as a bendwhose inner radius of curvature Ri is less than about three-fourths of the outer radius of curvature R0. In Fig. 4, the distance h between slots should' preferably be ofthe order of 'magnitude of or less. The slot width s maybe on the order of or less Wide. Where M, the wavelength in free space of 'the longest wave in the transmission band, is two inches, by way of a specific example, the distance h can be approximately 0.5 inch and s, approximately 0.2 inch. In very gradual bends (where the inner radius of curvature is at least three-fourths of the outer radius of curvature), such as those for which Fig. 4B is intended, it is per! missibfle and economical to make the interval h greater than it may be suicient to make the slots at intervals corresponding to about one degree of bending, regardless of their separation. For sharper bends or to provide increased iiexibility the distance h can be made appreciably less than without substantially impairing the transmission properties o f the structure.

In order to prevent contamination, radiation loss, and cross-talk, and to introduce substantial dissipation of unwanted mode waves, the structure of Fig. 4B may, as indicated, be supplied with the same type of coverings discussed above in connection with Fig. 4.

In Figs. 4, 4A, and 4B, a modification of undesired waves, principally of the TM11 transmission mode, is accomplished by the removal of metal from the Wall of the wave-guide bend, thus changing the phase constants of lche bend only slightly for TE and very appreciably for TM waves of the same operating frequency.

Viewed from the standpoint of coupled transmission lines as discussed in detail above and in my above-menlili rinsed.; raser;- in, the-f Bellv System. Technical, Journal; the nlurality. .ci rings (Fie 4). er the. slotted. wave guide (Bres-1 4A and, 4B) may be. Saidtc cQnStiurte-e practically unbroken cendlmorY fr the weak. circular currenis ci. the transmission line for the'TEoi mode. waves, whereas the conductor f orfthe strong longitudinalI currents: .Off-the transmission line for the TM11 andsimilar modesjincludes thetcapacitive gaps, thefcasing membertZS anltheouter conductive shielding ilayer 2;'7 of Fig. 45er asimilar casing andv shielding memberk applied .-to,` the structurlefof;- Figs... 4A and 4B as suggested-above. The twolinesarercou.- pled-mainly through. the non-circular energycomponents in the dielectricinsideof'the-main wave guidecaused by thev Y,bending of thexguide.V

In the above-suggestedstructure, where the-casing: 2S is made ofY lossy material, such as a plastic material in which carbon dust has been, dispersed, the energy converted. to the TM11 mode'` can be largelydissipatedsin the casing 25'.I As is well known. tov those skilled in` the art, if` one oftwoloosely intercoupledv transmission lines is highly, dissipative .while the other is substantially non@ dissipative, the highlyv dissipative line will' not-absorb large amountsv of power from the relatively non-dissipative line. In'vi'ew of ,this fact,-thestructures of the inventionappear yto offer outstandingadvantageslas round wave-guide structures for transmitting TEoi mode waves in'that they substantiallyeliminate the most troublesome of the spurious-or-unwanted modes without :the sacrifice of a s'ubstantial amountlof thepower ofthe TEbr wave being transmitted. f

The above-described embodiments areillustrative of the applicationof the principles ofthe invention;` Numerousl andevaried other ,applications vcan readily beY devised by those skilled in the art, without departing from the spirit and scope of said principles.

What is claimed is:

l. A- hollow'p'ipe wave guide ofcircular cross-section for thegtransmission of high frequency electromagnetic TE'oi Waves comprising a plurality of conductive ring members eoaxially arranged with respect to a common longitudinal axis and spaced along said axis, and" a continuous casing or sheath of lossy dielectric material surrounding said members.

2. The wave guide of claim l, in which said casing or sheath of lossy dielectric material is chosen and arranged to provide iiexibility to said wave guide.

3. A high frequency electromagnetic wave guide transmission line comprising a conductive means defining a low-loss dielectric transmission ypath of circular cross section, an annular dielectric path surrounding said conductive means throughout the entire length of said transission line, said dielectric path comprising lossy material uniformly distributed along the length or" said transmission line, said low-loss and said lossy dielectric paths being electrically coupled by a plurality of substantially transverse openings in said conductive means, said openings being regulariy spaced throughout the length of said transmission line. 'A

4. The transmission line of claim 3, in which said transverse openings extend completely around said first conductive means.

5. A waveguide for the transmission of high frequency electromagnetic TE-oi waves comprising a cur-ved hollow conductive pipe of circular cross-section, said pipeY including transverse slots spaced regularly along said pipe, said slots extending at least half way aroundsaid pipe, alternate slots being positioned on opposite sides of said pipe, and a continuous casing of lossy dielectric material surrounding said pipe.

ln an electromagnetic wave transmission system, means for producing electromagnetic wave energy in the circular electric mode, means for utilizing said wave energy, and means connecting said utilizing means to said producing means, saidconnecting means comprising a plurality of conductive ring members coaxially arranged withfrespect to a commonlongitudinal axis and spaced along said axis, a continuous highly dissipative casing surrounding said members and a layer of conductive material surrounding said highly dissipative casing.

7. The transmission system of claim 6, in which said highly dissipative casing is flexible. 8. A high frequency, electromagnetic wave, waveguide transmission line comprising a first conductive means defining a circularly cylindrical low-loss dielectric transmission path of substantially uniform diameter throughout the length of said transmission line, an annular lossy dielectric path having substantially uniformly distributed loss along said path surrounding said condctive means, said low-loss dielectric path being electrically coupled by a plurality of substantially transverse openings in said first conductive means to said lossy dielectric path at a plurality of spaced points thereon, said `openings being regularly spaced along the entire length of said transmission line, and a second conductive means comprising a shielding member surrounding said annular high-loss dielectric transmission path.

9. The transmission line of claim 8, in which said lossy dielectric path and said second conductive means are chosen and arranged to provide flexibility Ato said transmission line.

-. 10. The transmission line of claim' 3 in which said low-loss and said lossy dielectric paths are surrounded by a continuous casing of a protective material.

11. In an electromagnetic wave transmission system, a source of electromagnetic Wave energy in the ycircular electric mode, means for utilizing said VWave'energy, and transmission means comprising a longitudinal shield connecting said source to said utilizing means, said shield comprising a conductive casing of lcircular cross-section interrupted by a plurality of substantially transverse discontinuities, each of said discontinuities having a longitudinal dimension substantially less than one half of a Wavelength in free space of the longest Wave in the transmission band, said discontinuities comprising lossy dielectric material.

- 12. In an electromagnetic wave transmission system, a ksource of electromagnetic Wave. energy in the circular electricvmode, meansl for utilizing said Wave energy, and transmission means comprising a longitudinal shield connecting said utilizing means to said source, said shield comprising a conductive casingvof a circular cross-section interrupted by a plurality of discontinuities in the path of longitudinal currents in said shield, each of said discontinuities having a longitudinal dimension substantially less than one half of a Wavelength in free space of the longest wave in the transmission band, said discontinuities comprising lossy dielectric material.

13. In an electromagnetic wave transmission system, a source of high frequency electromagnetic TEoi wave energy, means for utilizing said wave energy, and a transmission medium comprising a substantially cylindrical shield connecting said utilizing means to said source, said shield of said transmission medium having a plurality of lossy dielectric regions for longitudinal current components in every wavelength of high frequency wave energy conducted along said medium, said regions being uniformly distributed along the length of said shield, said shield being electrically conductive around the major portion of its circumference for circumferential current components of high frequency Wave energy conducted along said medium.

vReferences Cited in the le of this patent UNITED STATES PATENTS 869,734 l France Nov.l 17, 1941.

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