US2570579A - Transmission line system - Google Patents

Description

Oct. 9, 1951 w, MASTERS I 2,570,579 V TRANSMISSION LINE SYSTEM Filed Dec. 6, 1946 Z Z I 3} I 17 v I I Ja /V0 mw/wrmz INVENTOR ATI'ORNEY Patented Oct. 9, 1951 TRANSMISSION LINE SYSTEM Robert W. Masters, Erlton, N. J., assignor to Radio Corporationof America, a corporation of Delaware Application December 6, 1946, Serial No. 714,620

11 Claims.

The present invention relates to transmission 7 line systems and more particularly to diplexing arrangements whereby a plurality of 'radio frequency carriers may be fed to a single load circuit or antenna.

An object of the present invention is the ;re moval of a source of ghost images in television transmitters.

. Another object of the present invention is the provision, in a diplexing system, of means for absorbing power reflected back from the antenna into the transmission line system. 7

A further object of the present invention isthe improvement of diplexing arrangements.

Another object of the present invention is to provide alow cost simplified television antenna sy tem- ,A further objectof the present invention is to provide a feed system whereby a narrow band antenna may be emciently operated as a television antenna.

A further object is to provide a network which can act to balance or equalize the power absorption of two unequal loads bearing the relation ship, Z1Z2=Zo Where Z is the surge impedance of the transmission lines feeding the two unequal loads. In particular, the balancing action re stores to a turnstile antenna the nominal circularity which is destroyed when the elements mismatch the lines which feed them.

A still further object of the present invention is to materially reduce or eliminate completely all intermodulation effects between a number of radio frequency signal generators connected to a single load.

The foregoing objects and others which may appear from the following detailed description are attained in accordance. with principles of the present invention by providing, in a diplexing transmission line arrangement, one or more absorber networks between the diplexing network derstood by reference to the following detailed description which is accompanied by a drawing in which:

Figure 1 illustrates in schematic form an embodiment of the present invention, while Figure 2 illustrates a modification of a portion of the embodiment shown'in Figure 1.

Referring now to Figure 1, the general operation of the system will first be described. There is shown in Figure l picture transmitter P from which a picture modulated carrier wave is carried by coaxial transmission line I to the diplexing arrangement. The coaxial transmission line has an inner conductor 2 and an outer conductor 3. At the end of transmission line I remote from the picture transmitter P, the outer conductor 3 of the transmission line is surrounded by an outer sleeve 1. In an end-to-end coaxial relationship with the outer conductor 3 of line I is a section of a conductive tubing 5, coaxially' surrounding an extension 4 of inner conductor 2 of transmission line I. The outer sleeve I is of such length as to enclose the end of outer conductor 3 andtubular conductor 5 each for a distance approximating a quarter wavelength. At its ends sleeve 1 is connected to conductors 3 and 5 respectively.

The action of coaxial conductors 2, 3 and l, 5 is such as to transform the electrical system from an unbalanced to a balanced-to-ground method of transmission. At points 8 and 9 at the adjacent ends of conductors 3 and 5 the voltages are balanced to ground; that is, at a given instance the voltage on one will be positive while'the other is negative but of equal amplitude.

To points 8 and 9 is connected a half wave loop of coaxial transmission line I 0. The coaxial line II] has an outer casing I2 connected at each end to outer sleeve 1 while the ends of the inner conductor I l are connected to points 8 and 9 of conductors 3 and 5. Since loop I0 is ahalf wave the diplexing unit. This is true since sound and picture frequencies are close together. The reactance look ng into loop It) at points 8 and 9 is nearly infinite. However, since the'point I3 at the center of the loop is effectively at ground potential with regard to the picture carrier frequency but is not physically connected to ground, it is possible to feed in a sound modulated carrier wave at this point from the sound transmitter S through coaxial transmission line I5 having an inner conductor I6 and an outer conductor I1. Since both sides of the loop II] are of equal length the energy from sound signal transmitter S arrives at points 8 and 9 in an in-phase relationship. That is, at any given instant, when point 8 is positive, point 9 is likewise positive. Hence,

feeding the opposite halves of each radiator in,

phasing opposition. In the present arrangement the loads are represented by a' turnstile televisionantenna A. While I have shown antenna A as being a broad band radiator, by utilizing the present arrangement, a simple crossed, rod turnstile arrangement may be made to operate effectively over the band widths required for television broadcasts. Line 23 contains a quarter wave phasing loop 24 along its length whereby line 23' is electrically a quarter wave. longer than transmission line 22. Thus the phase rotation of the radiated picture signal is in the opposite direction to. that of the sound signal. The signals are therefore inde endent y radiated and do not interact with one another.

Now, if the antenna A is not a perfect match for transmission lines 22 and 23, some reflection may occur of, for example the picture signal, from the. antenna back into lines 22 and 23. The reflected waves arriving at points 8' and B: of the diplexing system, are now in phase instead of out of phase since line 23 has caused a 180' degree phase lag from the two trips through the phasing loop- 24. The returning reflected picture modulated carrier wave cannot enter the picture signal line I because the energy in lines 22 and 23 is in an in phase relationship but it can proceed toward the sound transmitter S along transmission line l5. This reflected wave may act to cause unwanted modulation in the sound transmitter. If the returning wave is allowed to be reflected from the sound transmitter S and again proceed to the antenna it would, if sufficiently large, produce a ghost image on the viewing screen at the receiver. Such reflection may easily take place since sound transmitter S is a very good reflector for picture signal frequencies. Thus, at certain points along the transmission line l the reflected wave sees'a very high impedance; that is, large standing waves exist.

Now, in order to prevent such reflection and reradiation, a selective network, such as that indicated by reference numeral 25, which is inactive and acts as if it were absent at the sound modulatedcarrier frequency but which acts to absorb the reflected picture modulated carrier wave may be placed at a high impedance point such as that indicated at 3| in the sound signal line l5. That is, it is placed at a location along line [5 where there are large standing wave loops of voltage at the picture modulated carrier frequency. The network 25 operates as follows:

A coaxial branch line 26 has its outer conductor '21 connected to outer conductor I1 and its inner conductor 28 connected to the inner conductor l6 of. coaxial line I5. The branch has a length equal to one quarter of the operating wavelength, A S, at the sound modulated carrier frequency. At the end of line 26 is connected a second coaxial line section 34 having an inner conductor 33 and an outer conductor 29. Line 34 is arranged so that with line 26 a generally T shaped figure is formed. The inner conductor The respective 33 of second section of coaxial transmission line 34 is connected to conductor 28 at a distance from one end equal to one quarter of the sound modulated carrier wavelength. It is to be understood that where the specification specifies a quarterwavelength at some particular frequency, an odd multiple of such quarterwavelengths may be employed if desired with equally efiicient results as is well known to those engaged in the art. The junction is identified by reference character 32'. The portion of the second coaxial line 34 to the right of point 32 in shunt with line 28 produces a short circuit at point 32 at the sound frequency. Beingshort circuited at junction 32 the effect at 31 is as though an open circuit at the; soundfrequency is placed in shunt with the line l5 and therefore resistor 40 at the junction 32 cannot absorb any radio frequency power at the wavelength of the sound modulated carrier. The: operation of transmission line with respect to the sound modulated carrier is therefore unaltered. However, to the reflected wave of picture modulated carrier travelling toward the sound transmitter S the absorber network does not look like an open circuit. Since the sound line at point 31 is at a high impedance point the reflected wave enters the absorber network 25 through coaxial line 25;. The portion of the second coaxial line section 34' tothe left of the junction point of conductors 2B and 33 is so adjusted that the reflected waveentering" branch transmission line 26' at picturefrequency is dissipated in resistor 4!]. Theenergy therefore cannot again return to the antenna and the-ghost image'is effectively eliminated as is also the intermodulation effect. The order of magnitude of the reflected power to be absorbed by resistor 40 will be of the order of only 11% of the total" picture signal power for an antenna mismatch of 2 to l on the feeder lines, and rapidly decreasing power losses for better and better degrees of 7 match.

The embodiment of the presentinvention so far described depends for its successful operation upon the connection of the branch line 26 to. the main sound transmission line 15 at a point which has a high, impedance to the picture modulated carrier. In order, to avoid this careful placing of the absorber network the modification shown in Figure 2 may be utilized instead. The absorber network 25 shown in Figure 2 is the same as 25 of Figure 1. At a distance equal to one quarter of the picture modulated carrier wavelength from the junction of branch transmission line 26 and sound transmission line 15 is placed a second branch transmission line. 45. This line has the same length as branchline 26 and is terminated in a further Tzconnected c0- axial line section 49 similar to the second branch line 34 of absorber network 25. However, no resistor is placed across the junction of inner conductor 43 of the T connected line. 49 and inner conductor 48.. The second shown network 45 acts as, a short circuit to the picture modulated carrier wave in line [5 due. to the impedancev inverting characteristics of quarter wave line section 46,. It therefore causes av high impedance to. the picture modulated carrier to exist at thejunction of branch line 26 and the main sound transmission line I5. Thus, all of the reflected picture modulated carrier wave is forced to be dissipated in the dissipati r sist r 48-. The, a dition of this second T branch acts tofurther decrease any'intermodulation effects in a'mqre positive manner. Absorbers similar to absorbers 25 and 45 may, if desired, be placed in the picture modulated carrier transmission line land for the same reason they are placed in transmission line I5. The sound carrier can have an echo as well as does the picture carrier and the diplexing system described above is reciprocal in its action to the extent that the reflected sound modulated carrier wave willgo back to the picture transmitter and from the picture transmitter be reflected back to the antenna in the same way as described with reference to the picture signal wave. The bilateral operation of the diplexing system also presents the opportunity for serious cross modulation of the picture transmitter.

The present invention permits operation when as high as a 2 to 1 voltage standing wave ratio exists on the lines due to mismatch between the antenna A and transmission lines 22, 23. Even with a mismatch of this magnitude, only about 11 percent of the transmitter power is wasted in dissipating resistor 40.

If the antennas are actually disconnected all of the power from the picture signal'transmitter P is dissipated in resistor 40. Then connecting an antenna to the free ends of transmission lines 22, 23 will reduce the power lost in resistor 40 only by the amount it absorbs from lines 22, 23. A perfect match over the entire band will of course abstract all of the power from the lines leaving none to be dissipated in resistor 40.

Furthermore, the present arrangement reduces or entirely eliminates inequalities of current distribution antenna itself. Thus a turnstile radiator composed of thin rods might, for example, have a two to one unbalance in the currents drawn by the radiators. Such unbalance would cause the normally circular radiation pattern to become somewhat oblong, to the extent that a two to one difference in signal strength is obtained about the antenna. The absorbing network 25 restores the circularity. The sound transmitters, in the present example, could be entirely disconnected without disturbing, in any way, the balancing action of the invention, both networks 25 and 45 being used, of course.

While I have illustrated a particular embodi ment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in their arrangement without departing from the spirit and scope of the invention.

What is claimed is:

1. A conductor "arrangement for a plurality of high frequency radiant energy waves including individual sources of said waves, individual transmission lines from said sources to a common junction point, and means in at least one of said transmission lines effective to absorb energ appearing therein from the other of said sources but ineffective as to energy from the source to which said transmission line is connected, said means including a T-shaped branch transmission line having its foot connected to said one transmission line, the height of said T and one arm thereof each having a length equal to one quarter of the operating wavelength of the source to which said line is connected, and an absorber of high frequency energy at the junction of the leg and top of said T.

2. An arrangement for radiating a plurality of high frequency radiant energy waves including individual sources of said waves, individual transmission lines from said sources to a common junction, means coupling said junction to a radiating means, and means in at least one of said transmission lines effective to absorb energy appearing therein from the other of said sources but ineffective as to energy from the source to which said transmission line is connected, said means including a T-shaped branch transmission line having its foot connected to said one transmission line, the height of said T and one arm thereof each having a length equalto one. quarter of the operating wavelength of the source to which said one line is connected, and an absorber of high frequency energy at the junction of the leg and top of said T.

3. An arrangement for radiating a plurality of high frequency radiant energy waves including individual transmission lines adapted to couple sources of said waves to a junction, means connecting said junction to a common antenna, and means in at least one of said transmission lines effective to absorb energy appearing therein from the other of said sources but ineffective as to energ from the source to which said transmission line is connected, said means including a T- shaped branch transmission line having its foot connected to said one transmission line, the height of said T and one arm thereof each having a length equal to one quarter of the operating wavelength of the source to which said one line is connected, and an absorber of high frequency energy at the junction of the leg and top of said T.

4. A conductor arrangement for a plurality of high frequency radiant energy waves including individual transmission lines adapted to be connected to individual sources of said waves to couple said sources to a common junction point, and means in at least one of said transmission lines effective to absorb energy appearing therein from the other of said sources but ineffective as to energy from the source to which said transmission line is connected, said means including a T- shaped branch transmission line having its foot connected to said one transmission line, the height of said T and one arm thereof each having a length equal to one quarter of the operating wavelength of the source to which said one line is connected, and an absorber of high frequency energy at the junction of the leg and top of said T, said T being located at a point along said line where a voltage loop exists at the frequency of the other of said sources.

5. An arrangement for radiating a plurality of high frequency radiant energy waves including individual sources of said waves, individual transmission lines from said source to a junction, means coupling said junction to a common antenna, and means in at least one of said transmission lines effective to absorb energy appearing therein from the other of said sources but ineffective as to energy from the source to which said transmission line is connected, said means including a T-shaped branch transmission line having its foot connected to said one transmission line, the height of said T and one arm thereof each having a length equal to one quarter of the operating wavelength of the source to which said one line is connected, and an absorber of high frequency energ at the junction of the leg and top of said T, said T being located at a point along said one line where a voltage loop exists at the frequency of the other of said sources.

6. A conductor arrangement for a plurality of high frequency radiant energy waves, including individual transmission lines from sources of said waves to a common junction point, and means in mea ows atleast one. of. said transmission lines. effective to absorb energy appearing therein from. the other ofsaid sources but ineffective as to energy from the source to which said transmission line is connected, said. means including a T-shaped branch transmissio line having its foot connected to said one transmission line, the height. of said: T and one arm thereof each having a length equal to one quarter of the operating wavelength. of the source to which said one line is connected, and an absorber of. high frequency energy at. the junction of the leg and top of said T, and means connected to said one transmission line for causing, a voltage loop at the frequency of the other of said sources to occur at the point of connection of said T to said one line.

7. An' arrangement for radiating a plurality of high frequency radiant energy Waves including individual sources of said waves, individual transmission lines from said sources to a junction, means. coupling said junction to a common antenna, and means in at least one of said. transmission lines effective to absorb energy appearing therein from the other of said sources but ineffective as to energy from the source to which said transmission line is connected, said means including a T-shaped branch transmission line having its foot connected to said one transmission line, the height of said T and one arm thereof each having a length equal to one quarter of the operating wavelength of the source to which said one line is connected, and an absorber of highfrequency energy at the junction of the leg andtop of said T, and means connected to said one transmission line for causing a voltage loop at the frequency of the other of said source to occur at the point of connection of said T to said one line.

3. A, conductor arrangement for discriminatingly transmitting radiant energy of one wavelength to the exclusion of radiant energy of another wavelength, including a length of transmission line and means coupled thereto effective to absorb energy appearing therein at said other wavelength but ineffective as to energy at said one wavelength, said means comprising a T- shaped branch transmission line having the foot thereof connected to said length of transmission line, the height of said T and one arm thereof each having a length equal to one quarter of said one wavelength, and an absorber of high frequency energy connected at the junction of the leg and the top of said T.

9-. An arrangement for translating energy of a given frequency appearing at one end of a transmission line substantially without attenuation and absorbing energy of a predetermined frequency appearing at the other end of said transmission line without absorbing any energy at said given frequency, including a branch transmission line having a length equal toan odd multiple including unity of a quarter wavelength at said given frequency connected at one endto said transmission line, a section of transmission line having a length equal to an odd multiple including unity of a quarter wavelength at I said given frequency connected at one end to the other end of said branch transmission line, a

portiorr of: transmission: line having a length: substantially equal to an& odd multiple: including unity of the difference: between a half wavelength at said predetermined frequency and a quarter wavelength at said given frequency connected at one end to thejunctionbf said transmission. line section and, said branch transmission line, and anabsorber of high frequency energy connected: to said junction.

10. A transmission; line arrangement for discriminatingly transmitting wave energy obtained from a pluralityof radiant energy sources, including: a: transmission: line, means effective to absorb energy appearing in said transmission line from one of said sources but ineffective as to energy fromztheothert of. saidzsources, said means comprising a branch transmission line having a length equal to an odd, multiple including unity of one. quarter; of the. operating wavelength. of the energy from said'one source, a transmission line section having alength' equal to;an'odd multiple including unity of one half. of the operating wavelength of the energy from;said other source, said branch transmission. line being connected at one end to the first said. transmission line and connected at the other end to said transmission line section at a point an odd multiple including unity of one quarter of. the. operating wavelength of the energy from said. one source from onelen'd, and an. absorber ofihigh' frequency energy connected tothe junction of said branch transmission line and; said transmission line section. 1

11. A transmission line arrangement for ab.- sorbing wave energy" of a predetermined frequency introduced in a length of transmission line provided for the'translat'i'on' of wave ener y at a given frequency, including a pair of transrnission line networks connected to said length of transmission line at points spaced substantially a quarter wavelength at saidpredetermined frequency, each of said networks comprising a section of transmission line having a length substantially equal to a halfwavelength at said predetermined frequency and a transmission. line branch substantially a quarter wavelength long at said given frequency connected at one end to 'said length of transmission line and connected at the other end to said section of transmission line" at a point substantiallya quarter wavelength at said given frequency from one end thereof, and an. absorber of higlrr'frequency energy connected to the junction of said transmissionline branch and said transmissionline section of" one of said transmission line: networks.

ROBERT W". MASTERSz' REFERENGES CITED.

The following references areof record in the file of this patent:

UNITED STATES PATENTS" 2,474,277 Pickles; June 28, 1949

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