US2128400A

US2128400A - Transmission line system

Info

Publication number: US2128400A
Application number: US88073A
Authority: US
Inventors: Philip S Carter
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-06-30
Filing date: 1936-06-30
Publication date: 1938-08-30
Anticipated expiration: 1955-08-30
Also published as: GB487282A; BE422353A

Description

A. 30, 193. P. s. CARTER 2,128,400

TRANSMISSION LINE SYSTEM Filed June 30, 1936 5 Sheets-Sheet 1 INVENTOR PHILIPS. CARTER ATTORNEY Aug. 30, 1938. P. s. CARTER TRANSMISSION- LINE SYSTEM Filed June 50, 1936 3 Sheets-Sheet 2 IMPEDA NCE MA TCH/NG INVENTOR PHIL/IF BY CARTER n ATTORNEY Patented Aug. 30,1938

PATENT OFFICE 2,128,400 TRANSMISSION mm SYSTEM Philip 8. Carter, Port Jeflerson, N. Y., asslgnor to Radio Corporation of America, a corpora- "tion of Delaware Application June 30,

9 Claims.

This invention relates broadly to high frequency signalling systems, and more particularly to arrangements for antenna transmission lines over which it is desired to operate a plurality of transmitters otdifierent frequencies for simultaneously feeding the same antenna.

One of the objects of the invention is to enable two or more transmitters of different frequencies to feed the same transmission line over different branch circuits in such manner that each branch has-negligible effect on the flow of energy from the other branch or branches toward the common load circuit. This object is achieved first by shunting each branch at a predetermined location with a circuit of predetermined electrical constants, whereby the branch enables the unrestricted flow of energy thereover from its associated transmitter toward the common antenna system, and the shunt circuit provides an eifective short circuit for energy from any other branch, and secondly, by making each branch appear at the point of junction to the common transmission line as a circuit of extremely high impedance to the flow of energy from any other branch.

Another object is to enable matching the characteristic impedance of the main transmission line for all transmitter frequencies used to simultaneously excite the load.

The invention has been found to be especially useful in the broadcasting of television signals wherein both voice and video transmitters of diflerent frequencies are used to feed simultaneously the same antenna.

A better understanding of the invention may be had by referring to the following description and accompanying drawings, in which Figs. 1 to 5 illustrate difierent embodiments of the invention, and Fig. 6 illustrates impedance curves showing the manner in which the shunt elements of Fig. 5 function.

. Fig. 1 schematically illustrates a simple case embodying the principles of the invention wherein two transmittersvTi and T2, respectively generating waves whose lengths are i1 and A2, the wavelength of M being chosen exactly twice that of A1, feed the same antenna A over a common transmission line TL. Transmitter T1 feeds line TL over a branch circuit comprising lines In andci. and transmitter T: feeds line TL over another branch circuit comprisinglines b: and 02, both lines 01 and or joining the common line T1. at the junction points J. Antenna A comprises two coaxially arranged arms, each oi'which' has a-length 1936, Serial No. 88,073

equal to half the length of the communication wave A1, i. e.,

or one-quarter the length of the longer communication wave A2.

In other words, antenna A is a double half-wave dipole at the higher frequency corresponding to the shorter wavelength A1, and a single halfwave dipole at the lower frequency corresponding to the longer wavelength in.

For obtaining the desired restrictions in the branch circuits, there is employed at the junction at of lines in and c1 of one branch circuit, an impedance element at; which acts as an open circuit for the flow of energy of the frequency of the wave M and as a short circuit for the flow of energy of the frequency oi the wave in, but which acts at the junction J as a high impedance to the flow of energy from transmitter T2. At the junction b of lines hr and c2 of the other branch circuit, there is employed an impedance element d2 which acts as an open circuit for the flow of energy of the frequency of the wave is and as a short circuit for the how of energy of the frequency of the wave M, but which acts as a high impedance at J to the flow of energy from transmitter T1. For this purpose element d1 is here shown as a straight line which is open at the far end and has a length equal to which is also Element do is here also shown as a straight line and closed or short-circuited at the far end and has a length equal to which is equal to Section 01 is designed to have a length equal to and section 0: a length equal to In order to more fully understand the reason for the particular lengths chosen for the straight line sections d1, (1: and c1, 0:, it should be remembered that a straight section of line a halfwavelength long or a multiple of a half wavelength acts as a line of zero length, whether it is open or short circuited at the far end; and that a quarter wavelength line, or an odd multiple thereof, closed at the far end acts as an open circuit, while a quarter wavelength line or an odd multiple thereof open at the far end acts as a short circuited line. Putting it still another way, a half-wave section of line, short circuited at the far end, acts, for a particular frequency, as a series tuned circuit of lumped inductance and capacity, whereas an open half-wave section of line acts, for a particular frequency, like a parallel tuned circuit of lumped inductance and capacity.

In operation, the energy from. transmitter T1 flows without impediment over

lines

221 and 01 to the line TL, since element d1 acts as a very high impedance shunt on the branch line b1, 01, i. e., infinite impedance or zero loss, for energy from T1. However, the closed line or element d2 acts as a short circuit at the Junction b for any energy from T1. At junction J, however, which is a quarter of the wavelength of A1 fromb, the entire branch circuit for T2 has a very high impedance to the flow of energy from T1 so that the effect of the branch circuit of T2 is negligible on the flow of energy from T1 to the line TL and antenna A.

As for the energy from transmitter T1, the element (in acts as an open circuit and has negligible effect on the flow of energy from. this transmitter to the line TL and antenna A. Because of the length of 01, however, the element d1 will act at a as a short circuit to any energy from transmitter T2. At junction J, however, the entire branch circuit of T1 acts as a high impedance to the flow of energy from T: so that the effect of the branch circuit of T1 is negligible on the flow of energy from T2 to the antenna.

Fig. 2 shows a more general arrangement wherein the two frequencies of the transmitters T1 and T: have no simple relationship as in the special case of Fig. l. The circuit of Fig. 2 is similar to Fig. 1 except that instead of the line elements I11 and d2 of Fig. 1 there are provided lumped reactance circuits (1'1 and d'z, and also provision is made for matching the antenna circuit to the characteristic impedance of the main line TL by means of circuits m and n. It should be noted that circuit dz comprises a series branch of inductance U1 and capacity C1 shunted by a capacity C2, whereas circuit d1 comprises a series branch of inductance L1 and capacity C1 shunted by an inductance In. The circuit d1 acts as a parallel tuned or anti-resonant circuit to energy of the frequency of the wave A1, and as a series tuned or effective short circuit to energy of the frequency of wave A2. This is accomplished by making the reactance values of L1 and C1 equal at the frequency of A2. At the same time L1 and C1 are so chosen that at the frequency of A1, the series circuit of L1 and C1 becomes in effect a capacity reactance equal to the inductive reactance of L2.

The following mathematical relations must exist to obtain this result:

11 and h, respectively corresponding to the transmitter wavelengths A1 and A2, it being understood that A1 is shorter than A: from which it follows that W1 is greater than W2.

Similarly the circuit d: is designed to act as a parallel tuned or anti-resonant circuit for frequency f2 and as a series tuned or short circuit for frequency f1. To obtain this result the following relations must exist:

It should be observed in passing, that the circuit d'1 across the branch through whidh the higher frequency f1 flows must provide a shunt inductance to its series tuned circuit, whereas the circuit d: across the lower frequency branch must provide a shunt capacity to its series tuned circuit.

For matching the impedance of the antenna system to the characteristic impedance of line TL, for the different frequencies of transmitters T1 and T2, there may be employed circuits m and n which may take the form of lumped reactances, or straight line sections, as shown in Figs. 3 and 4. The broad principles underlying the manner of matching the impedance of the antenna system is described in my copending application Serial No. 31,756, filed July 17, 1935, to which reference is made for a detailed description thereof.

Fig. 3 illustrates schematically a system like that of Fig. 2, wherein any relationship of frequencies for the transmitters can be used. Here again it is assumed that the wavelength of transmitter T1 is shorter than that of T2. The impedance elements d: and (1411]. this figure are sections of transmission lines rather than lumped reactances and are connected to the branch circuits at points intermediate their ends. It should be noted that the distance from one open end of d; to its point of connection to the branch circuit of T1 is one-quarter of the wavelength of A2, while the distance from the free end of d4 to the point of connection to the branch circuit of T2 is one-quarter of the wavelength of A1. The length of 01 and 02 from J to the junction points of the impedance elements are here again one-quarter of the length of the communication wave emanating from the other branch circuit. An analysis of Fig. 3 will show that each branch acts as a high impedance at J for the flow of energy from the other branch toward the antenna and consequently has negligible effect thereon, and the elements d: and d4 act like open and short circuits to the different frequencies in precisely the same manner hereinabove mentioned in connection with the elements d'1 and d'2 of Fig. 2.

With certain frequency relationships of f1 and is, it may be desirable to use odd multiples of the quarter wavelengths shown in Fig. 3, rather than a. simple quarter wavelength. Similarly, in certain instances we may wish to use any multiple of a half wave for the half wavelengths appearing in the drawings. In a practical application of the invention where losses must be considered, such as may be the case where the

frequencies

11 and 12 are close to one another, it may be necessary to use the above mentioned multiples of the lengths shown in the drawings in order to obtain the desired frequency accepting and rejecting actions of the elements.

The system of Fig. 3 is not limited to the case where the line sections d: and d4 are either open at both ends or at one end only, since it is feasible to use in certain cases, instead of (1: and/or d4, a line section closed at both ends. In other words, any type of line section'inw be used which satisfies the requirement of being in eifect an open circuit at one frequency and a short circuit at the other frequency. The following formulas show the reactances for the three diiferent types of line sections which may be used:

1. Line section closed at both ends:

where Sis the distance angle from either end.

2. Line section open at both ends:

where S is the distance angle from either end.

3. Line section open at one end and closed at the other end (U section).

where S =the distance angle from'the closed end, where Z is the surge impedance of the line section and L is the total length of the line section expressed as an angle.

The distance angle .5 in the above formulas equals 360 X distnnce wavelength The above formulas for the reactance of the line sections take no account of losses, and where it is desired to take account of losses, hyperbolic functions should be substituted for the circular functions after substituting thecomplex line angle consisting of the attenuation and phase constants in place 01' the simple angular lengths above, accordingto well known transmission line y.

Impedance matching of the antenna system and line TL for the two frequencies of T1 and T: is obtained by elements m and n in the manner described in my copending application serial No. 31,756, supra.

Fig. 4 shows a system wherein three transmitters T1, T2 and T: of different frequencies are employed, according to the invention, to excite simultaneously the same transmission line TL and the antenna. The principles involved in the construction of Fig. 4 are the same as those of Fig. 3 with the exception that each of the shunt circuits d5, ds and d1 are so designed as to result in very high impedance for the frequency of the transmitter associated with the same branch and an effective short circuit for the other two frequencies oi the other transmitters. It is assumed in this case that the wavelength is generated by T1 is shorter than the wave is generated by T2, in turn, shorter than the wave-As generated by Ta. Each of the three shunt circuits d5, do and d1 has three branches. Circuit d: is here shown consisting of two branches. whose lengths l1 and l: are each closed at one end, and a third branch whose length I: is open. Circuit do has only one branch 11 closed and the other two branches 1: and la open. Circuit d1 has all three of its branches 11, l: and I: open. The lengths of the branches for each of the shunt circuits is, do and d1 shown in the drawings are determined by the following mathematical relations: For circuit "dc" eot m l +eot nub-tan nub-=0 cot m l -l-cot nub-tan m l :l; m=2rlh For circuit "do" cot nub-tan nub-tan m ls= :t: w

cot nul -tan zmI -tan smb=0 cot nul -tan nub-tan m h= :l: so

For circuit "d-l tan nul -Han m ld-tan m l :l: w

tan m l +tan nub-Han m;l;=0

The invention, it is-to' be understood, is not limited to the particular circuits d5, do and (17 shown with their types of open and closed branches. since other combinations of line sections, or lumped reactance circuits, may be used, provided the essential requirement of having a high impedance at one frequency and a very low impedance at the other two frequencies be satisfled.

Where impedance matching of the line TL is desired, we may employ circuits m'i, 12% and o. The manner in which the valuesand locations of m; and n'r is determined is described in my copending application, Serial No. 31,756, supra. Circuit 0 also employs the same principles shown in my copending application and shows another embodiment wherein three branches are used in order to obtain a certain finite value of reactance at the third frequency and at the same time a very high reactance at the other two irequencies. The lengths of the branches of the matching circuits m'1, n'1 and o are determined from the following relations, it being understood that the locations of these'circuits and the values of the reactances x1. .17: and as may be obtained from the procedure outlined in my copending case.

For circuit W1 20 tan mili=X1 For circuit "ma" cot nub-tan a1,1,=o cot nub-tan m l,==%

' For circuit 0" cot nul -tan rmb-tan m I =0 cot m l -tan nub-tan rml =0 In Fig. the voice transmitter 01) has a car-.

rier frequency of 52 megacycles and a very small hand between 51.98 and 52.02 megacycles. whereas the videotransmitter (M) has a carrier frequency of 49.75 megacycles and a very wide band between 48.25 and 51.25 megacycles. The shunt element 9 across-the voice transmitter branch is here designed to give a high impedance to the small band of frequencies emanating from the voice transmitter, and to provide a very low impedance at the junction at over the very wide band of video frequencies. Similarly, the shunt circuit k, r is designed to give a high impedance to the wide band of video frequencies and to provide a very low impedance at the junction b for the small band of voice frequencies. At the junction J each branch has negligible effect on the other branch since each branch acts as a high impedance at this junction to the frequencies coming from the other branch.

A circuit g, in this particular instance where it is essential that losses be taken into account, gives a very high impedance over a relatively narrow band of frequencies and a quite low impedance over a band of considerable width. This type of shunt line section circuit is very satisfactory for the small voice frequency range of the voice transmitter. For the video branch we require just the reverse, that is, a shunt circuit which will give a very high impedance over a very wide frequency band and a quite low impedance over a relatively narrow frequency band. This is obtained in Fig. 5 by providing circuit 1" which has the same dimensions as circuit g and connecting through a quarter wave section K. In this case we may say that K acts as an impedance inverter.

The reason why the shunt line section open at both ends, of the type shown in Fig. 3, was not used at the junction b in Fig. 5 is because such an open line section provides a high impedance over a small range of frequencies and low impedance over a wide band of frequencies, whereas the opposite effect is required in the case of Pi 5.

ig. 6 shows the general shape of the impedance curve for the shunt line sections g and k, 1'. It will be seen that the curve for g shows a relatively low impedance over the video frequency band and a high impedance over the voice frequency band, whereas the curve for k, 1' shows a relatively high impedance over the video frequency band and a low impedance over the voice frequency band.

In practice it will be found that some current from one branch will find its way into the other branch but the degree of such undesired feedback depends principally upon the efiiciency of the shunt sections across the branches. A very high degree of freedom from undesired feedback may be obtained by using concentric branch lines of relatively large diameter.

The invention, it will be understood, is not limited to the precise arrangements of parts shown since various modifications may be made without departing from the spirit and scope thereof. For example, although two conductor lines have been shown in all the figures of the drawings, the principles and formulas of the invention apply equally well to single conductor lines with ground return.

What is claimed is:

1. In combination, a first source of high frequency energy of wavelength x, a second source of high frequency energy'of wavelength 2x, a main line and a load connected to said main line, a first two-conductor line connecting said first source to said main line, a second two-conductor line connecting said second source to said main line, a first shunt circuit connected across said first two-conductor line at a distance from the junction of said first line with the main line equal to one-quarter of the length of the wave 2x, said first shunt circuit comprising a two-conductor line section whose length is equal to one-quarter of the length of the wave 2A, the ends of said line section being open, a second shunt circuit connected across said second two-conductor line at a distance from the junction of said second line with the main line equal to one-quarter of the length of the wave i, said second shunt circuit comprising a two-conductor line section whose length is equal to one-quarter of the length of the wave 27x, said last shunt circuit being closed at its end farthest removed from its point of connection, whereby each shunt circuit presents, at its point of connection across its two-conductor line, high impedance to the fiow of energy of the frequency of the source directly associated with said line, and a low impedance to the flow of energy of the other source, and each two-conductor line presents a high impedance at the point of junction to said main line to the flow of energy coming from the other two-conductor line.

2. In combination, a first source of high frequency energy of wavelength A1, a second source of high frequency energy of longer wavelength M,

said shunt circuit being open-ended and having an overall length equal to the distance from one open end of said first shunt circuit to its point of connection across said first two-conductor line being a second shunt circuit also comprising a line section connected across said second two-conductor line at a point located from the junction of said second line with the main line a distance equal to said last line section being closed at only one end and having an overall length equal to the distance from the open end of said last line section to its point of connection across said second two-conductor line being and a pair of line sections connected to different points on said main line for matching the load to the characteristic impedance of the main line for the different frequencies of said two sources. 3. In combination, a first transmitter of one frequency, a second transmitter of another frequency, a main transmission line, an antenna connected to said main line, a two-conductor line connecting each transmitter to said transmission line, and an impedance circuit in shunt across each of said two-conductor lines, each of said impedance circuits being constructed and arranged to provide at its point of connection to its line equal to line with the main line i I 2,128,400 two-conductor line, a high impedance to' the fiow of energy in its associated two-conductor line 01' the frequency of its associated transmitter and a low impedance to the fiow of energy in its associated line ofthe frequency of the other transmitter, the shunt impedance circuit across one two-conductor line being located from the junction of said one two-conductor line with the main transmission line a distance equal to an odd multiple of one-quarter of the wave generated by the transmitter-connected to the other two-wire line, saidother shunt impedance circuit being located from the junction of said other two-conductor line with the main transmission line a distance equal to an odd multiple of one-quarter of the wave generated by the transmitter connected to said one two-conductor line, and means connected across said main transmission line between said junction and said antenna for matching the antenna to the characteristic impedance of the main transmission line for the different frequencies of said two transmitters.

4 In combination, a first source of high frequency energy of wavelength in, a second source of high frequency energy of longer wavelength M, a main line and a load connected to said main line, a first two-conductor line connecting said first source to said main line, a second two-conductor line connecting said second source to said main line, a first shunt circuit connected across said first two-conductor line at a distance located from the junction of said first line with said main said first shunt circuit comprising an inductance in parallel with a series circuit of inductance and capacitance, the values of said inductance and capacitance elements being such as to provide at the terminals of the shunt circuit connected across saidfirst tWOrCOHdUCtOI line a high impedance to the flow of energy from said first source and a low impedance to the fiow of energy from said second source, a second shunt circuit connected across said second two-conductor line at a distance located from the junction of said second equal to tance in parallel with a series circuit of inductance and capacitance, the values of the capaci tance and inductance elements of said second shunt circuit being such as to provide at the terminals thereof a high impedance to the fiow of energy fromsaid second source and a low impedance to the fiowof energy fromsaid first source.

5. Apparatus in accordance with claim 4, characterized in this that the reactance values of the series connected inductance and capacitance elements of the first shunt circuit are made to be equal at the frequency of the energy of the second source, and the reactance connected inductance and capacitance elements of the second shunt circuit are made to be equal at the frequency of the energy of the first source.

6. In combination, a first source of high ;frequency energy having a narrow band of frequencies and a mean wavelength M, a second source of high frequency energy having a wider band of slightly lower frequencies and a mean wavelength A2, a main line and a load connected tosaid main line, a first two-conductor'line connecting said first source to 'said main line, a second two-convalues of the series ductor line connecting said second source to said main line, a first shunt circuit connected across said first two-conductor line at a distance from the junction of said first line with the main line equal approximately to 4 and a second shunt circuit second two-conductor line at a distance from the junction of said second line with the main line equal approximately to 4 said first shunt circuit comprising a two-conductor line section open at one end and closed at the other and connected intermediate its ends to said first two-conductor line, the overall length of said first shunt circuit being approximately M, and the length of the shunt line section between the closed end and the intermediate point 01 connection to the first two-conductor line being approximately 2 said second shunt circuit comprising a first twoconductor line section and a second two-conductor line section, said last first line section having a length equal approximately to and being connected at one end to said second two-conductor line and at its other end to said second line section at a position intermediate the ends of said second line section, said second line section being open at one end and closed at the other end and having an overall length approximate equal to M, the distance between the closed end of said second line section and the intermediate point of connection being approximately a 7. A system in accordance with claim 6, characterized in this that said first and second twoconductor lines, said main line, and said shunt circuits comprise concentric inner and outer conductors.

8. A system in accordance with claim 6, characterized in this that said first source has a frequency range between 51.98 and 52.02 megacycles and said second source has a frequency range between 48.25 and 51.25 megacycles, the mean or carrier frequencies of said sources being respectively 52 and 49.75 megacycles.

9. In combination, a first source of high frequency, a second source of high frequency energy of another and higher frequency, a common load lines.

PHILIP S. CARTER.

connected across said DISCLAIMER 2,128,400.Phz'lip 8'. Carter, Port Jefferson, N. Y. TRANSMISSION LINE SYSTEM.

Patent dated August 30, 1938. Disclaimer filed October 19, 1939, by the patentee; the assignee, Radio Oorporation of America, consenting.

Hereb disclaims claim 3 of said Letters Patent.

[ Gazette November 7, 1.989.]