US2794958A - Transmission line directional coupler - Google Patents

Description

June 4, 1957 l Rfw. PETER 2,794,958

TRANSMISSION LINE DIRECTIONAL coUPLER Filed oct. s1, '1951 '2 sheets-sheet 1 finger/0N T 1 L IND/Mraz f IIIIII' ATTORNEY June 4, 1957 R. w. PETER 2,794,958

` TRANsmssIoN LINE DIRECTIONAL coUPLER Filed Oct. 31, 1951 2 Sheets-Sheet 2 V'Il/IA ilrlllz infill/lll; IIIIIIIIA rlllli/ FEO/V SOI/ECE INVENTOR Rolf Wren-r BY im ATTORNEY United States Patent Mice 2,794,958 TRANSMISSION LINE DIRECTIGNAL COUPLER Rolf Walter Peter, Princeton, N. J., assgnor to Radio Corporation of America, a corporation of Delaware Application October 31, 1951, Serial No. 254,048

17 Claims. (ci. ssa- 10) The present invention relates to signal wave transmission systems and particularly to directional coupler arrangements especially well suited for coaxial transmission lines and for measuring signal reections on the line from a load.

A compact directional coupler arrangement is frequently desirable in which the proportion of power transfer is large compared to the space or to the length of line utilized for coupling. Other desirable qualities of directional couplers are that the coupling be matched, in which event reflections of incident power from the coupling structure are lsubstantially absent; and broad banding, that is, the coupling characteristics including power transfer, do not vary over a broad band of frequencies about an operating frequency. Previous coaxial transmission line directional coupler networks have involved ring or rat race arrangements, and the like.

Generally a directional coupler arrangement includes a linear passive network coupling between four transmission line arms. Energy incident on the network from one arm is.divided between vtwo of the other arms but is notcoupled directly. to the fourth. Similarly, energy incident from the fourth arm is divided between the second and third arms, and is not coupled to the rst arm. v Ideally there are no reections and the arms look into matched impedances. Consider the first and second arms .as part of one transmission line, and the third and fourth as part of another. first arm in a direction toward the network (thus travelling in one direction in one transmission line) is partially coupled to the third arm, and travels in this third arm in a direction away from the network. Similarly, energy vcoupled into the fourth .arm incident from the second a reverse direction in the one trans- (thus travelling in mission line) travels away from the network in the fourth arm. Therefore, the direction of energy flow in the ysecon-d transmission line is determined selectively by the'direction of energy flow in the one transmission line. The converse is also true.

It is an object of the invention to provide a novel directional coupler arrangement for coaxial transmission lines.

It is another object of the invention to provide a coaxial transmission line directional coupler arrangement that is compact, easy to construct, and broadbanded.

It i-s another object of the invention to provide a novel compact power monitoring device for coaxial transmission lines.

In accordance with a preferred embodiment of the invention, a pair of coaxial transmission line sections are coupled in a directional coupler network over a lengthA in which an intermediate conductor serves both as the outer conductor of the inner line section and as the inner conductor of the other line outer section. The coupling is provided by a helical slot in the said intermediate conductor through which the electromagnetic fields of one line .are coupled to those of the other. A coaxial line arm is connected at one end to supply energy. At the Then energy incidentin the i Patented June 4, 1 957 teristic impedance of the inner line section and' its ex-v tensions may be maintained constant throughout to aid in securing broad-banded matching.

In other embodiments of the invention, preferred for power monitoring, the output or energy withdrawal end of one line is terminated with an absorbent termination and the other end has a detector crystal termination forl monitoring power output or for monitoring for the amount of reflected power, to test for matched conditions of a load.v

The foregoing objects and other objects, advantages, and novel features of the invention will be more apparent from the following description, in which like reference numerals refer to like parts, and in which:

Fig. 1 is .a longitudinal cross-sectional view of one embodiment according to the invention which is broadbanded in operation because of connections to the outer of two coaxial transmission lines coaxial with each other;

Fig. 2 is a longitudinal cross-sectional fragmentary view of another embodiment of the invention in which the characteristic impedance of both the inner and outer of two coaxial transmission line sections coaxial with each other is maintained the same over the coupling region;

Figs. 3 and 4 are longitudinal cross-sectional views of power monitoring arrangements according to the inventionl mission lines couples coaxial transmission line arms 10,y

12, 14, and 16. The network 8 includes an inner continuous conductor 18, an intermediate conductor 20, and an outer conductor 22. The intermediate conductor serves as the inner line outer conductor of the section 8 and also as the outer line inner conductor of the section 8. The inner conductor 18 of the inner line is extended at each end directly into the inner conductors respectively of coaxial line arms 14 and 16. The section intermediate conductor 20 is extended at each of its ends respectively ldirectly into the outer conductors of arms 14 and 16. The arms 10, 12 are coupled to the section V8 respectively by broad band stub supports 24 and 26. Thev stub support 24 is connected between intermediate conductor 20 near one end of section 8 and the inner conductor of arm 10. The stub support 26 is connected between intermediate conductor 20 near the other end of section. 8 and the inner conductor of arm 12. At the end-s of section 8, a quarter wavelength respectively from the points of connection of stub supports 24 and 26, the section outer conductor 22'is short circuited to the section intermediate conductor 20. Each of the stub supports 24 and 26 is of the type illustrated on page 58 (Fig. III-42) and described in the associated text of Microwave Transmission Design Data, Publication No. 23-80 of Sperry Gyroscope Co., Inc. This type of 3. right angle lstub support is hereinafter termed a lright angle broadband stub support.`

A helical slot 28 is cut in the section intermediate conductor 20 between the pointsof connection thereto of the stub supports 24 and 26. The-pitch, width, and number of turns of the slot 28 are selected to provide theldesired division of power-or coupling 'from energy incident :at the operating frequency band in one of the Apairof arms 10, 12.through the section to one of the arms '16, 14 respectively, the remaining energy being transferred to the other arm 10,12 of the pair;

In the preferred arrangement, tthepitch of the slot is preferably infinite at each end and gradually mergesinto a helical slot of constant pitch, as illustrated. This preferred arrangement provides a broader band of operating frequencies over which `the desired division of power is secured than if the longitudinally slotted infinite pitch portion of the slot were not employed.

In operation, let energy be supplied from a source, as indicated on the drawing, to waveguide 10. This energy at the junction of lines and section 8 sees an open circuit to the left beyond the end of section 8 as viewed in Fig. 1, due to the quarter wavelength short-circuited portion of conductors and 22. Therefore, the energy ows to the right. Some of the energy is supplied through the helical slot 28 which allows communication between the inner coaxial line section 18, 20 and the outer coaxial line section 20, 22.` The energy thus supplied to the inner coaxial line section 18, 20 ilows to the right to a load A (notshown). The remaining energy in the outer coaxial line section 20, 22 `flows to the stub supported right-angle bend 26. t The energy here faces an apparent open circuit to the right, due again to the short circuit of conductor 22 to 20 at this end. The only remaining pathtis out coaxial line 12 to a load B,(not'sliown). The actionqof the right angle stub supports gives broad banding energy transfer as described in the said volume `on Microwave Transmission Design Data.

The amount of energy coupled from the outer coaxial linesection 20, 22` to the inner coaxial line section 18,` 20 through the'helical slot 28 depends on the axial length, pitch, and width of the slot 28. These may be varied empirically to secure a desired proportion of energy transfer.

In Fig. 1 it may be assumed that it is desired to divide the energy applied from the sourceequally between two loads, A and B. Then any energy detected at line 14 mustbe reflected energy, returned from the loads. Here it is shown how to couple the loads A and B to the source, preferably equally if the section `8 is arranged to couple 50% of the energy from line 10 to line 12 and 50% to line l16. If `desired a detector crystal with ammeter 32 is used as a reflection indicator.`

It is apparent that the ammeter will indicate current only when at least one of loads A andB is` not matched, and will vshow zero current only when both loads A and B are matched to their respective lines 16 and 12. Notice that lines 10 and 14 are decoupled, and that `lines 12 and 16 are decoupled. The stubs 24 `and 26 of diameter enlarged respectively over that of the inner conductors of lines 10 and 12 give a broad banding effect for the stub supported right angle bend. This effect is conjoined with that due to the tapering of the` sections.

In order to enhance the matching of the transmission line arms 10, 12, 14, and'16 to the section 8, section 8 may have a central portion `as shown in Fig. 2. The section intermediate conductor -20 is slottedwitha helical slot 28 as in Fig. l. However the section 8-outer condoctor-is replaced by an outerconductor 34 which has a portion'34a of reduced internal diameter about the slotted portion of the section intermediate conductor 20. Also, the section inner conductor 18 of Fig. 1 is replacedfby a. section inner conductor `36 having aportion 36a of enlarged diameter inside the slotted portion of the section intermediate conductor.

By proper selectiontof the diameters of the enlarged diameter section inner conductor portion 36a, the characteristic impedance for the section inner line may be maintained constant throughout the length of section 8. By proper selection of the diameters of the reduced diameter section outer conductor portion34a, the characteristic impedance of the section outer line `may be maintained constant throughout the length of section 8. It is desirable that the characteristic impedance Zo of the portion with slotted conductor remain equal to the characteristic impedance Z1 of the unslotted conductor. Let 1r=S/L, where S is total length of the slot, and L is the axial length of the slot. It can be shown that if the decrease in spacing between conductors is such that the capacitance C0 per unit of line length along the portion of line where one of the conductors is formed with a helical slot is increased by a factor 17, then Za is made equal to Z1. The impedances Zo' and Z1 of the inner line can be made. equal in a similar manner.

If this method of equalizing impedances is employed, in the section shown the device is thereby made even more broad banded as hereinbefore mentioned.

Referring to Fig. 3, a section 8 is shown having inner and outer coaxial transmission lines 20, 36 and 20, 34 respectively, coaxial with each other and similar to that shown in Fig. 2. At one end, the section 8 has the section outer transmission line 20, 34, extended and joined to a coaxial transmission line 38 leading from a source (not shown). Att the other end, the section outer line 20, 34 i's joined to and terminated by a resistive or iabsorptive termination 40. At this other end, the section 8 inner coaxial transmission line has an extension 42 leading to a load. At the one end, the section inner line 20, 36 is terminated. by a crystal 30 in a suitable holder (not shown) inserted between the inner and outer conductor. At the other end, a pair of D. C. leads 44 is taken from the conductor ofthe load transmissionline 36, 42. AniR. F. choke, R. F. C., `keeps A. C; `currents out of meter 32. The crystal by-pass capacity is not-shown in the various figures.

In operation, suppose the slot affords a'50% coupling between the inner and outer coaxial lines 20, 36 and 20, 34. Then half the energy from the sourceis coupled to the termination 40; half is coupled to the load by line 42.,

It" the load is matched, there is no reflected energy to I. be rectified by crystal 30 and no D. C. current ow in meter 32. If the load is not matched, however, `one-half the returned radio frequency energy is returned `to the source, and one-half is returned to crystal 30,` which recties the R. F. energy incident on it, `and the rectified D. C.`currents are read in the meter 32. Therefore, the

i axial with` each other.

meter 32 indicates whether or not the load is matched to the load line 42.`

, Referring to Fig. 4,a section8 has inner `and outer coaxial line sections 18, 20 and 20, 22 respectively co- Transmission line 14 from a source acts as one arm of the directional coupler section and feeds the section outer coaxial line 20, 22 end-to-end. Transmission line l16 leads toa load and acts as another arm of the directional coupler and is connected end-toend to the other end of section outer line 20, :22 `from the end connected to source line 14. The section inner line 13, 20 is terminated at one end in a matched crystal arrangement including crystal 30 and at the other end in matched resistive load 46. The end terminations of the section inner line 18, 20 `act as the third and fourth arms. It will be apparent from whathas been heretofore said, that crystal v30 is coupled to reliections from the load and decoupled from energy from the source. The load 46 is coupled to energy from the source and not to energyfrom theload. Preferably in this arrangement,l the coupling coefficient is small. Reflections from the load are detected yand measured by current ow in the crystal 30 and meter 32 circuit.

aree-95e Referring to Fig. 5, the source line 14 is connected to the section inner line 18, Z and the load line 16 is connected to the Section inner line 18, 20. The section outer line 20, 22 is terminated at the end coupled tothe source' in a resistive matched termination 40. The end of section outer line 20, 22 coupled to the loadretlections is terrninated in a crystal arrangement.

In Fig. 5, the directional coupledrcoupling through theV slot 28 is preferably small, say 2 or 3% as in Fig. 4. The crystal 30 and meter 32 circuit is such'that the crystal leads to the meter do not interfere with energy passage from source to load, `asthey need .not cross the section line through which energy passes from sourceto'load.

In Fig. 6, energy from source` line 14 passes to the section inner line 18, 20. The 'number of turns `and length of helical slot 28 is such that the percentage of coupling is preferably substantially greater than 50%. Line 16 is coupled to receive energy from line 14 through the sec-` tion outer line 20, 22. The crystal termination arrangement is similar to that in Fig. 5. It is clear that the crystal is de-coupled from energy incident from the source, and is responsive only to energy returned from the load.

It will be apparent that there is disclosed herein a particularly desirable, compactarrangement for detecting 'and measuring reilections from a load, and a particularly useful directional coupler arrangement, especially for applications where large coupling is desired.

What is claimed is:

1. A directional coupler comprising a section of inner and outer coaxial transmission lines coaxial with each other, said outersection having a central portion and portions next adjacent tosaid central portion, the inner of said section lines having an inner conductor, the outer of said section lines having an outer conductor, and said lines having in commonan intermediate conductor serving as the inner line outer conductor and the' outer line inner conductor, said intermediate conductor having a helical slot therein throughout said section central portion aording energy communication between said section inner and outer lines, the said outer line outer conductor having a diameter throughout said central portion reduced under its said diameter in said adjacent portions.

2. A directional coupler comprising a section'of inner and outer coaxial transmission lines coaxial with each other, said inner section having a central portion and portions next adjacent to said central portion, the inner of said section lines having an inner conductor, the outer of said section lines having an outer conductor, and said lines having in common an intermediate conductor serving as the inner line outer conductor and the outer line inner conductor, said intermediate conductor having a helical slot therein throughout said section central portion affording energy communication between said section inner and outer lines, the said inner line inner conductor having a diameter throughout said central portion enlarged over its said diameter in said adjacent portions.

3. A directional coupler arrangement comprising a section of inner and outer coaxial transmission lines coaxial with each other, said section having a central portion vand portions next adjacent to said central portion, the inner of said section lines having an inner conductor, the outer of said section lines having an outer conductor, and said lines having in common an intermediate conductor serving as the inner line outer conductor and the outer line' inner conductor, said intermediate conductor having a helical slot therein throughout said section central portion aiording energy communication between said section inner and outer lines, the said outer line outer conductor having a diameter throughout said central portion reduced under its said diameter in said adjacent portions, and the said inner conductor having a diameter throughout said central portion enlarged over its said diameter in said adjacent portions.

4. A directional coupler arrangement comprising a section of inner and outer coaxial transmission lines coaxial with each other, the inner of said section lines having ari inner conductor, the outer of said section lines having an outer conductor, Iand said section lines having in corn-l mon for a portion an intermediate conductor serving as the inner line outer conductor and the outer line inner conductor, said intermediate, conductor having a helical slot therein throughout said portion thereof to afford energy communication between said section inner and outer lines, one of said lines having `an extension beyond one end of said portion, said `one line having a ratio of inner diameter of outer conductor to outer diameter of inner conductor in said portion that is less than the like ratio for said extension.

5. The coupler arrangement claimed in claim 4, the other of said lines having an extension beyond an end of said portion, said other line having-a ratio of inner diameter of outer conductor to outer diameter of inner conductor in said portion that is less than the like ratio for said other line extension.

6. The coupler arrangement claimed in claim 4, said one line having 'another extension beyond the other end of said portion, said ratio for said one line being also less than the like ratio for said other extension.

7 A directional coupler arrangement comprising a sectionrof inner and outer coaxial transmission lines coaxial withl each other, the inner of said section lines having an inner conductor, the outer of said section lines having an outer conductor, and said section lines having in common for a portion thereof an intermediate conductor serving as the inner line outer conductor and the outer line inner conductor, said intermediate conductor having a helical slot therein throughout said portion to atord energy communication between said section inner and outer lines, each of said lines having extensions beyond each end of said portion, said outer line having a ratio of inner diameter of outer conductor to outer diameter of inner conductor in said portion that is less than the like ratio for each of said outer line extensions, and said inner line having a like ratio in said portion that is less than the like ratio for said inner line extensions.

8. A coaxial line including an outer conductor and an inner conductor, one of said conductors being formed as a helix along one portion of its extent; and the spacing between said two conductors being reduced to a suicient extent along the length of said helix to improve the impedance match between the portion of the coaxial line which includes the helix and the remainder of the line.

9. A coaxial line including an outer conductor and an inner conductor, one of said conductors being formed as a helix along at least one portion of its extent; and the distributed capacitance between said two conductors along the portion of the coaxial line which includes the helix being increased an amount suflicient to render its characteristic impedance substantially equal to that of the remainder of said line.

l0. A transmission line including a pair of substantially parallel conductors, one of said conductors being formed as a helix along one portion of the extent of said one conductor, and the spacing between said pair of conductors being reduced to a sufficient extent along the portion of the line in Which the helix is located to render the characteristic impedance of said line substantially uniform throughout its entire length.

ll. A coaxial line including an outer conductor and an inner conductor, said outer conductor being formed with a helical slot along at least one portion of its extent, land the inner conductor of said line being increased in diameter along the portion of the line in which the helical slot is located an amount suflicient to make the characteristic impedance of said portion of said line substantially equal to that of the remainder of said line.

l2. A coaxial line including an outer conductor, and an inner conductor, said inner conductor being formed as a helix along at least one portion of its extent, and the inner diameter of said outer conductor being reduced` the `distributed capacitance per unit of length f the sec.

tion of coaxial line in which the helix is located being increased `by a factorn, where S "rr and S is the extended length of the helix,` and L the axial length of the helix.

14. In combination, two coaxial lines including a rst conductor serving as the inner conductor of one line, a hollow `third conductor coaxial with `said rst conductor serving as the outer conductor of the other line, and a hollow second conductor coaxial with said first conductor and intermediate the first and third conductors serving as the inner conductor of said other line and the outer conductor of said one line, said hollow second conductor being formed with a helical slot along one portion of` its extent, and the diameter of the trst conductor being in.

creased along the portion thereof adjacent the helical slot au amount suicient to make the characteristic impedance of said one line substantially uniform throughout its length, and the inner diameter of the outer conductor along the portion thereof adjacent said helical slot being reduced an amount suicient to make the characteristic impedance of said other line substantially uniform throughout its length.

15. A coaxial line including an outer conductor and an inner conductor, one of said conductors being formed as a helix coextensive `with the remainder of said one conductor along at leastlone portion `of its extent, the pitch of said helix being substantially greater. at Ian end thereof coextensive with the remainder of said one conductor than the pitch of the helix in the center portion thereof; and the impedance of the section of line in which the conductor is a helix being substantially equal to that of the remainder. of the line.

16. A coaxial line including an outer` conductor and an inner conductor, at least a part of said inner conductor being formed as a helix; and the impedance of the section of line in which said inner. conductor is a helix being substantially equal to that of the remainder of the line.

17. A coaxial line including an outer conductor and an inner conductor, at least a part of said outer conductor being formed as a helix; and the impedance of the section of line in which said outer conductor is a helix being substantially equal to that of the remainder of the line.

References Cited inthe file of this patent UNITED STATES PATENTS 2,524,857 Seeker Oct. 10, 1950 2,580,678 Hansen et al. Ian. l, 1952 2,588,832 Hansell Mar. 1l, 1952 2,705,305 Bailey Mar. 29, 1955 i FOREIGN PATENTS `621,167 Great Britain Apr. 5, 1949 OTHER REFERENCES Proceedings of the I. R. E, vol. 34, No. 10, October 1946,. page 792. (Copy in Division 51.)

Moreno: Abstract of S; N. 605,417.0. G. vol. 640, nage 343, published Nov. 7, 1950.

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