US2899650A

US2899650A - carlin

Info

Publication number: US2899650A
Authority: US
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11

Description

Aug. 11, 1959 u H. J. CARLIN I I MULTIPLEX SIGNALLING SYSTEM Filed June 22, 1955 I t 2 Sheets-Sheet 1 Tunable Detector Resonontcovitytuninq |b In accomplished by this sliding section H "In" 3 4\\\\\\\ \2\ fill/III d II/I/l/A b c c A (next coupler) 0 i r. Antenna Resonant fl y 90 H 9 T V 2 S Anti-resonant (transmitter 7c" cavity Impedance or receiver) q Mel comm 7d 4 Equivalent circuit 6b Mfla.

(T) Trunsmitter' 6c Recelyer INVENTOR HERBERT J. CARLIN' ATTORNEY -11,1959 H. J. CARLIN 2,899,650

v MULTIPLEX SIGNALLING SYSTEM Filed June 22, 1955 S 2 Sheets-Sheet 2 Z Zi,35. 4.

M l8 TURNS IN DECOUPLING LOSS D) 3 A COUPLING LOSS (C) u |6 J 20 g D I2 10 a v C [c 1 INSERTION LOSS (1) FREQUENQY (MC) INVENTOR HERBERT J- CARLIN DECOUPLING (db) ORNEY United States Patent Q MULTIPLEX SIGNALLING SYSTEM Herbert J. Carlin, Hicksville, N.Y., assignor to Polytechnic Institute of Brooklyn, Brooklyn, N.Y., a corporation of New York Application June 22, 1955, Serial No. 517,252

7 Claims. (Cl. 333-9) This invention relates to a multiplex signalling system for operating several transmitters and/or receivers on a single antenna.

While the invention is especially useful in the ultra high-frequency band, it is not limited to this frequency band.

A broad object of the invention is to devise a multiplexing system for the simultaneous operation of several transmitters and receivers on a single antenna with low frequency separation of the channels, low insertion loss, and good isolation between the different channels.

A more specific object of the invention is to devise a multiplexing coupler formed of coaxial line elements, and being suitable for operating several transmitters and receivers on a single antenna in the ultra high-frequency band.

With the coupler described herein, for operation in the band of 225 to 400 mc./s., it is possible to obtain satisfactory multiplexing with a frequency separation between adjacent channels of only 3 megacycles per second, an average insertion loss in a four-coupler system of approximately 1 db per coupler, and an isolation between adjacent channels of approximately 60 db.

The invention is illustrated in the accompanying drawing in which:

Figure l is a circuit diagram, partly in block fonn, showing a multiplex signalling system according to the present invention and embodying three couplers connected in cascade for operating two transmitters and two receivers from a single antenna;

Figure 2 is a circuit diagram illustrating the bridge arrangcn'ient of the elements embodied in each coupler;

Figure 3 shows a coaxial embodiment of the coupler acccrmng to the present invention, the coupler being shown in section along a longitudinal cutting plane; Figure 3a shows the equivalent circuit of Figure 3; and

Figure 4 shows typical performance curves of the coupler.

Referring to Figure 1, three multiplexing couplers C1, C2 and C3 are connected in cascade to a single antenna A connected to coupler C1. A transmitter T1 operating at a frequency 7, is coupled to the antenna A through coupler C1 and a second transmitter T2 operating at a frequency f is coupled to the antenna through another channel completed by couplers C1 and C2. All three couplers complete another channel to receiver R3 operating at a frequency f and the three couplers also complete another channel to a tuned detector or. second receiver R4. L1, L2 and L3 represent balancing impedances connected to the three couplers for purposes described later.

The circuit arrangement for each of the three couplers of Figure 1 is illustrated in Figure 2. As will be seen, the coupler is formed of a six-arm impedance bridge connected between the terminals a, b, c and d. The arm a-b comprises a series resonant circuit formed of an inductance L and a condenser C having an impedance Z The arm ad is formed of a balancing load or variable impedance L. The arm cd is formed of a parallel tuned circuit (an anti-resonant circuit) consisting of an inductance L and a parallel condenser C forming an impedance Z,,. The arm bc comprises an antenna or another coupler connected between the terminals b and c. The arm ac comprises transmitting or receiving equipment connected across terminals T, and the arm b--d comprises another coupler or an output device connected in the position N. Letters A, L, N and T in Figure 2 represent ports or terminals in the different arms of the bridge. The bridge is balanced when the product of the impedances in opposite arms are equal. That is Z Z -:Z =1

The impedance Z, is inverse to Z, at all frequencies over the working band. Under these conditions power at port T is entirely decoupled from arm N. At the same time it is easily seen that port A is also decoupled from port L. This decoupling is theoretically independent of frequenc though parasitic effects in the coaxial coupler prevent the full realization of this frequency-invariant performance. The decoupling is related to the Directivity of directional couplers and, in fact, the actual device is a coaxial directional coupler.

In addition it may be shown that the on resonance insertion loss from a transmitter to an antenna (T to- A) or the 01f resonance loss of a coupler (A to N) can be made small providing a high enough value of Q for the resonant circuits of Fig. 2 is used, and provided that the resonant impedances of these tank circuits are suitably chosen. Ideally, Z should be low at resonance and Z high at that point. This permits coupling of (T) to (A) via path abc. The oif-resonance value of Z must increase rapidly and Z decrease so that power from (A) bypasses (T) and goes to (N) via path b-dc. With a finite Q, the resonance level of Z should not be set too low, for the impedance level would then never reach high enough off resonance values to produce sufficiently low insertion loss. Similarly, the resonant level of Z must not be permitted to be too high at resonance. A further problem is the loss of effective directivity when the coupler is operated with a mismatched antenna; This is taken into account by the use of a variable impedance as a balancing network L in arm ad of Fig. 2. The antenna balancing network not only balances out the mismatch due to the antenna but also cancels out parasitic effects in the coupler at whatever frequency the device is tuned to.

Figure 3 shows a coaxial embodiment of the coupler bridge of Figures 1 and 2. The series resonant circuit forming the arm a-b of the bridge is embodied in a coaxial structure extending horizontally in the upper part of Figure 3, and the parallel resonant circuit forming the arm c-d of the bridge is embodied in a coaxial structure extending vertically downward from the first coaxial structure. The left end of the series. resonant unit is connected at port L to an adjustable balancing impedance or load by a coaxial line having conductors marked ad, while the right end of this coaxial unit is connected at port N to the next coupler by a coaxial line with conductors marked bd. The two coaxial structures are supported from a common base formed of block l which is bored horizontally for the series resonant cavity and is bored vertically for the parallel resonant cavity. The

- outer conductor of the series resonant structure is formed unit is formed of three sections, a short fixed section 2a connected with the center conductor (a) of the line at port L, a longer fixed section 2b spaced from section 2a, and a movable section 20 sliding within section 2b and connected to the inner conductor (b) of the line at port N.

Mounted in the center of tubular section 2b is a center conductor 3, the left end of which is supported by insulating plug 4 having threaded engagement with the interior of section 2a, the plug 4 being rotatable to control the extent to which the center conductor 3 extends out of the tubular section 2b, across the gap between tubular sections 2a2b, and into the section 2a. A short-circuiting slider 5 is carried on the inner end-of tubular section 2c and serves to vary the length of the coaxial line formed of conductors 2b-3. Sliding of the outer conductor section 1c also varies the length of the coaxial line formed by the concentric conductors 1alc and 2b2c. The length of the un-shorted portion of conductor 3 should be about one-quarter of the operating wavelength. As seen across the gap between conductor sections 2a and 2b, the inner coaxial structure presents a series-resonant unit, the approximate equivalent circuit of which is shown in Fig. 3a between terminals (a) and (b).

The parallel resonant coaxial structure is formed of

sleeve extensions

6a and 6b mounted upon the block '1 in alignment with the vertical bore therein. The inner conductor 7 is supported from the outer conductor 6 by two oppositely extending arms 7a and 7b passing through the walls of block 1 and forming the outer conductors respectively of coaxial lines cb and c-a connected to the coupler at ports A and T and leading to the antenna and transmitter respectively. The upper end of center conductor 7 is suitably bored to permit the inner conductor (a) of line T to be connected to the inner end of tubular section 2a, and the inner conductor b of coaxial line A to be connected to the inner end of tubular section 2b. The outer end of inner conductor 7 is formed of a tubular section 7c, into which is slid-ably mounted another tubular section 7d closed by a plug 7e. The outer end of sleeve 6b is shorted by a plug 6c, and adjustment of the telescoping sleeve 7d is accomplished by means of an insulating rod 8 secured to the plug 7e and passing through a central hole in the plug 60. As seen from the upper end of center conductor 7, the vertical coaxial structure acts as a parallel-resonant unit, the approximate equivalent circuit of which is shown in Fig. 3a connected between terminals (0) and (d). Varying the length of the inner conductor 7c7d varies the tuning of the parallel resonant unit. The resonant frequency of the unit depends on the length of the adjustable section '7c'7d,and also upon the length of the arms 7a and 7b within the block 1, and the. lengths of these arms may be varied by suitable means, if desired, such as the level control sleeve 9 shown on arm 7a, which may be duplicated on arm 7b if necessary, as shown at 9a.

The approximate equivalent circuit of Figure 3 is shown in Figure 3a. The parallel combination L C in Figure 3a represents the near resonant line formed of conductors 2b3, while the condenser C represents the open-circuited line section formed of conductors 241-3. By varying the axial position of the conductor 3 with respect to the coupling slot a-b in Figure 3 (by adjusting plug 4), the position of the coupling slot along the resonant line is thereby varied, giving the elfect of an impedance transformation by which the resonant impedance level may be controlled.

The anti-resonant unit of Figure 3 consists of an opencircuited length of line in shunt witha short length of short-circuited line formed of inductive stubs 7a and 7b which produce an effect of animpedance transformer. By adjusting the lengths of the inductive stubs 7a and 7b 'by the

sleeves

9 and 9a, the impedance level of. the anti-resonant unit is varied. Theinductance-addedby 4 arms 7a and 7b is shown at L, in Figure 3a. The third reactive element added to each of the cavity equivalent circuits causes a second resonant point which is located close to the first.

The arrangement shown in Figure 3 provides for various junction points to be brought out and made accessible for external coaxial line connections, such as those pro vided by coaxial stubs 7a and 7b which provide space for an inner coaxial conductor as explained above. Also, the various coaxial lines are arranged within the main junction block '1 insuch manner that a minimum of space is required for the inner connection of the-lines, and so that the points shown to'be directly connected together in Figure 3a are actually so connected in the coaxial embodiment. The dimensions of the junction block should be as small as possible with respect to the operating Wavelength.

Simply by way of illustration, the approximate dimensions will be given for a coupler designed for operation over a freqency band from 225 to 400 megacycles per second.

Referring to Figure 3, the two coaxial lines at ports'L and N connecting the coupler to the adjustable load or impedance and to the next coupler are of the same size, the effective diameters of the inner and outer conductors of these lines being 0.356 and 0.811 inch, respectively. The coaxial lines at the ports A and T connecting the coupler to the antenna and to the transmitter respectively are of the same size, and the effective diameters of the inner and outer conductors of these lines are 0.175 and 0.285 inch, respectively. These lines enter the block 1 through bores of a diameter of 0.625 inch. The horizontal and vertical bores through block 1 for the two coaxial resonant units are both of a diameter of 2 inches. The inside diameter of sleeve 1b is approximately 1.3 inches, and the inside diameter of sleeve in is 3 inches. Tubular conductor section 2a has an inside diameter of 0.811 and an outside diameter of 0.875 inch, respectively, and the end of tubular conductor section 2b located within the block 1 is of the same dimension, while the section of this tubular conductor located within the sleeve M has an inside diameter of 1.265 and an outside diameter of 1.315 inches, respectively. The section of the inner conductor 3 located within the sleeve 1a has an outside di ameter of 0.55 inch, while the section of this conductor located within theb lock 1 has an outside diameter of 0.356 inch. The length of the gap between tubular conductor sections 2a and 2b is 0.25 inch. Tubular section 1b has a length of approximately 2 inches. The horizon tal bore in blockl has a length of approximately 5 inches, and the length of tubular section 1a is approximately 13 inches. Tubular section 10 sliding within section 1a has a length of approximately 10 inches. Section 20, including the short-circuiting slider 5, has a length about the same as section 10.

Tubular sections

10 and 20 have a range of anal movement of about 9 inches. The center conductor 3 has an overall length of about 15 inches, and its effective lengthis varied by slider 5 from about 4 inches to about 13 inches. The spacing between the center of the horizontal bore in block 1 and the axis of the bores receiving the lines 7a and 7b is approximately 2.25 inches, and the distance between the axis of the lines 70 and 7b to the end of the coaxial line6a-6b is approximately 15 /2 inches. The sleeve 70 has an outside diameter of 1.097 inches and the sleeve 6a has an inside diameter of 2.5 inches. Themovable sleeve 70! has an outside/diameter of 1.02 inches, and the sleeve 6b has an inside diameter of 2.37 inches. This sleeve has a range of axial movement of about 4 inches.

A typical three-coupler system is shown diagrammatically in Figure 1. To operate this system the first transmitting coupler C1 has its tuning cavities set at the desired transmitter frequency f There is a precalibrated frequency scale on the coupler, and the cavity plungers are Set m dssfieifi auens m r n Th t n g piungers of the other couplers are set at the assigned frequencies of f and f respectively. There is also a tuning screw which adjusts the resonant impedance level of the resonant cavities when required.

:The final adjustment of each coupler is made with the antenna balancing network associated with the coupler. A tunable detector R4 at the end of the coupler chain or array is employed as a null indicator in securing proper adjustment of the couplers. This is set for the transmitter frequency assigned to the coupler being adjusted and the balancing network is then tuned for a null indication on the detector. Thus, indicator R4 would first be tuned for f and then the balancing net work L1 of the transmitting coupler C1 set for a null indication by the detector. Next the detector R4 would be set for E, and the balancing network L2 of the coupler C2 (presuming its cavities have already been set at the precalibrated scale indication for f adjusted for a null indication at R4. Note that since the next coupler port of coupler f has been decoupled from the transmitter port T1, the adjustment of coupler C2 has no elfect whatever on the previously tuned setting of coupler C1, and, in fact, the adjustment of each coupler in the chain is entirely independent of all the others.

a :Typical performance curves of the coupler described above, when tuned at 300 mc./ sec. and operating with a balancing network, are shown in Fig. 4. In this figure, the insertion loss curve I is the coupling from A to N (values of I at other than the resonant frequency are the olf resonance insertion losses). The coupling loss curve C shows the loss measured from A to T (the resonant value of C is the on resonance loss), and the curve D shows decoupling loss (synonymous with isolation) as measured from T to N. The letters refer to the port designations of antenna (A), transmitter or receiver (R), or next coupler (N) of Figs. 2 to 3a. Performance data of a 4 coupler system (a total of 4 equipments-transmitters and/or receivers with a matched load on port N of the last coupler) based on curves of this type, are given In the above table the average loss per coupler is the total loss to the last coupler set for the frequency shown divided by 4.

'It should be emphasized that the freedom from interaction eifects between units of the coupler system is almost entirely independent of the closeness of frequency spacing of adjacent equipments; If a UHF receiver of kc. bandwidth is used, it can operate even closer than 1 me. to an adjacent high power transmitter and the residual transmitter signal will still be low enough in level to be adequately discriminated against by the receiver selectively. The main limitation on closeness of frequency spacing is almost entirely a matter of tolerable insertion losses, and if these are to be of the order of ldb average, then a minimum spacing of about 3 mc./sec. is required. If higher losses are permissible (of the order of 3 db per coupler), 1 mc./ sec. separations are entirely feasible.

One advantage of the coupler bridge of the invention is that the adjustable impedances L1, L2, etc. provide means for compensating for a mismatched antenna. This is easily understood by reference to Figure 2. If the load in that figure is an adjustable impedance then it may be set to balance the bridge, i.e. produce a null in arm N, when the antenna arm A presents an impedance other than a purely resistive impedance. The adjustable balancing impedance should be capable of producing either capacitive or inductive reactance of adjustable value.

I claim:

1. A multiplexing signalling system comprising a transmission line, a plurality of parallel-resonant circuits connected in cascade in said transmission line at .a plurality of station locations, said circuits being tuned to different frequencies, a signalling device and a balancing impedance connected in series relation in a path shunting each parallel-resonant circuit, and a series-resonant circuit connected across said transmission line at each station and being tuned to the same frequency as the parallel-resonant circuit at the same station, one terminal of each series-resonant circuit being connected to the mid-point of the path shunting the parallel-resonant circuit at the same station, the parallel resonant circuit at each station location having an impedance which is the inverse of the impedance of the series-resonant circuit at the same station.

2. A multiplex signalling system comprising .a plurality of coupler networks connected in a cascade array, each coupler comprising four terminals a, b, c, and d, a signalling device for each coupler connected across terminals a and c and operating at an assigned frequency different from frequencies assigned to other devices, a balancing impedance for each coupler connected between terminals a and d, a parallel-resonant circuit connected between terminals 0 and d and being tuned to said assigned frequency, a series-resonant circuit connected between terminals a and b and being tuned to said assigned frequency, and connections from terminals b and d of one coupler to terminals b and c of the adjacent coupler in said cascade array, the parallel resonant circuit in each network having an impedance which is the inverse of the impedance of the series-resonant circuit in the same network.

3. A multiplex signalling system comprising a plurality of coupler networks connected in a cascade array, each coupler comprising four terminals a, b, c, and d, a signalling device for each coupler connected across terminals a and c and operating at an assigned frequency different from frequencies assigned to other devices, a balancing impedance for each coupler connected between terminals a and d, a parallel-resonant circuit connected between terminals 0 and d and being tuned to said given frequency, a series-resonant circuit connected between terminals a and b and being tuned to said given frequency, connections from terminals b and d of one coupler to terminals b and c of the adjacent coupler in said cascade arrangement in said cascade array, a multi-channel circuit for the transmission of a plurality of said frequencies connected between terminals b and c of the coupler at one end of said array, and a signal device operating at one of said assigned frequencies connected to terminals b and c of the coupler at the other end of said cascade array, the parallel resonant circuit in each network having an impedance which is the inverse of the impedance of the series-resonant circuit in the same network.

4. A multiplexing coupler comprising two sections of coaxial cable arranged in spaced axial alignment and having load terminals and coupler terminals respectively, a first tubular conductor connecting the outer conductors of said aligned cable sections and being of [larger diameter than the outer conductors of said cable sections, a second tubular conductor mounted concentrically within said first tubular conductor and being formed of two sections separated by a gap, one section being relatively short and being connected to the inner conductor of one cable section and the other section being relatively long and connected to the inner conductor of the other cable section, a center conductor mounted within said long section of said second tubular conductor and being slid-- 7' able axially thereof to extend out of said [long section across said; gap and into said short section, means for varying the length of the coaxial line formed between said first and second tubular conductors, means for varying the length of the coaxial line formed between said long section of tubular conductor and said inner conductor, a tunable coaxial line coupledto said first tubular conductor through an opening formed in the wall thereof adjacent said gap and embodying a parallel-resonant circuit tunable over a band of frequencies, a coaxial line for connection to signalling apparatus and having an inner conductor connected to the end of said short section adjacent said gap, and another coaxial line for connection to a signal channel and having an inner conductor connected to the end of said long section adjacent said gap.

5. A multiplexing coupler comprising two sections of coaxial cable arranged in spaced axial alignment and having load terminals and coupler terminals respectively, a first tubular conductor connecting the outer conductors of said aligned cable sections and being of larger diameter than the outer conductors of said cable sections, a second tubular conductor mounted concentrically'within said first tubular conductor and being formed of two sections separated by a gap, one section being relatively short and being connected to theinner conductor of one cable section and the other section being relatively long and connected to the inner conductor of the other cable section, a center conductor mounted within said second tubular conductor and being slidable axially thereof to extend out of said long section across said gap and into said short section, means for varying the'length of the coaxial line formed between said first and second tubular conductors, means for varying the length of the coaxial line formed between said long section of tubular conductor and said inner conductor, a third.tubular.conductor coupled to said first tubular conductor through an opening formed in the wall thereof adjacent said gap and being closed at its outer end, a center conductor of adjustable length mounted within said third tubular conductor and having a front ,end portion terminating in said opening in said first tubular conductor, said last mentioned center conductor being supported by a pair of oppositely extending arms spaced from the front end thereof and passing through openings in said third tubular conductor, a coaxial line for connection to signalling apparatus and embodied in one of said arms and having an inner conductor extending through aborein said front end section of said center conductor and being connected to the end of said short section adjacent said gap, and another coaxial line for connection to a signal channel and embodied in the other arm of said center conductor and having an inner conductor passing through a bore in said front end section of said center conductor and connected to the end of said long section adjacent said 6. A multiplexing coupler comprising two sections of coaxial cable arranged in spaced axial alignment and having load terminals and coupler terminals respectively, a first tubular conductor connecting the outer conductors of said aligned cable sections, a second tubular conductor mounted concentrically within said first tubular conductor and being formed of two sections separated by a gap, one section being relatively short and being connected to the inner conductor of one cable section g i and the other section being relatively long andc'o'nnected to the inner conductor of the'other cable section, a center conductor.mounted within said long section of said second tubular conductor and having oneaend shorted to .said second tubular conductor and the other end terminating adjacent said .gap-,..said secon'd tubular conductor and said center conductor forming a seriesresonant circuit across saidgap; atunable coaxial line coupled to said first tubular conductor through an opening formed in the wall thereof adjacent saidgap and embodying a parallel resonant circuit tuned to the same frequency as said seriesfresonant circuit; a coaxial line for connection to signalling apparatus and having an inner conductor connected to the end of said short sec tion adjacent said gap; and another coaxial line for connection to a signal channel and .having an 'inner conductor connected to the end of said ;long section adjacent said gap.

7. A multiplexing c 'lupler comprising two sections of coaxial .cable arranged in spacedaxial alignment and having load terminals and coupler terminals respectively, a first tu bular conductor connecting the outer'conductors of said aligned cable sections, a second tubular conductor mounted concentrically Within said first tubular conductor and being formed of two sections separated by agap, one sectionbeing relatively short and being connected to the inner conductor of one cable section and the other section being relatively long and connected to the inner conductor of .the other cable section, a center conductor mounted within said second tubular conductor and having one end shorted to said second tubular conductor and the other end terminating ad jacent said gap, said second tubular conductor and said center conductorfonning a series-resonant circuit across said lgap; a tunable coaxial line coupled to said first tubular conductor and forming a parallel-resonant circuit tuned to the same frequency as said series-resonant circuit, said coaxial line'comprising a-third tubularconductor coupled to said first tubular conductor through an opening in the wall thereof adjacent said gap, asceond center conductor mounted within said third tubular conductor and having a front end portion terminating in said opening in said first tubular conductor, said second center conductor being supported by a pair of oppositely extending arms spaced from the front end thereof and passing through openings in said third tubular conductor; a coaxial line for connection to signalling apparatus and embodied in one of said arms and having an inner conductor extending-through a bore -in said front end section of said center conductor, and bein connected to the end of saidshortsection adjacent said gap, and another coaxial line for connection to a signal channel and embodied in the other arm of'said center conductor and having an inner conductor passingthrough a bore in said front .end section of said center conductor and connected to the end of said long section'adjacent said gap.

References Cited in the file of this patent UNITED STATES PATENTS 2,495,589 Masters Jan. '24, 1950 2,713,152 Brown July '12, 1955 2,779,000 Sosin Jan.-22, 1957