US2150246A - Transmission line system - Google Patents

Description

March 14, 1939. w,.vAN B, ROBERT 2,150,24

TRANSMISSION LINE SYSTEM Filed Sept. s, 1937 72 b fib Z'RANSM/ TTEI? R m E O M 0 m v v v 05 9 8 w/ J 3 J 1 0 a 7 lloo L mm E n M A my 2/ T B N R o 0 M m A Of 0 0 0 0 0 w ,0, u m m 3% 3 wuzfiuwwk Y WEE "AN 5'. ROBERTS B A TTORNEY.

Patented Mar. 14, 1939 UNITED STATES 2,150,246 TRANSDIISSION LINE SYSTEM Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 3, 1937, Serial No. 162,302

9 Claims.

This invention relates to transmission line systems, particularly to radio frequency transmission systems, and has for its primary object to transform in a simple and inexpensive manner the value of a load or terminating impedance, such as an antenna, to a pure resistance of a value equal to the characteristic impedance of the associated line.

In order to avoid reflection losses in high frequency systems, a transmission line should be terminated by a pure resistance equal to the characteristic or surge impedance of the line. In practice, however, it is found that the load to be fed by the line is often not a pure resistance. It is well known, however, that the load may be made effectively a pure resistance by connecting suitable reactance elements in series or shunt to it. It is also known that a suitable length of transmission line, either open or shorted at its far end, may be connected across the load to function as a suitable reactance for the purpose of neutralizing the reactance component of the load.

Fig. 1 is given for the purpose of exposition, and

illustrates avknown transmission line system;

Fig. 2 illustrates a simple embodiment of the present invention; and

Fig. 3 illustrates a graphical method for determining the constants of the structure employed in thepresent invention.

In Fig. 1 there is shown a load Z, such as an antenna, fed by a transmission line TL. An extra section of transmission line T1L1 is provided across the load to transform the load Z to a pure 5 resistance.

The efiect of the extra section of line T1L1 is to make the load an effective resistance which, however, in general differs in value from the characteristic or surge impedance of the main transmission line TL feeding it. There is therefore'inserted, in accordance with known principles, a' matching section I comprising two spaced conductors whose length is one-quarter the length of the communication wave and whose characteristic impedance is the geometric mean between the eifective load resistance and the characteristic impedance of the main line. The

. impedance of this quarter Wave matching section I is determined by the geometry of the section,

in well known manner.

One disadvantage of the arrangement of Fig. 1 is that there must be provided an extra length of line, suchas T1L1. whose sole purpose is to make the load efiectively a pureresistance.

In accordance with the present invention, the

foregoing disadvantage is overcome and the extra section of line dispensed with by inserting the quarter wave matching. section I not between the main line TL and the load, as shown in Fig.

1, but at a particular point in the main line itself, 5 note Fig. 2. In this way no greater total length of transmission line is required than the minimum necessary to reach from the source to the load.

Fig. 2 illustrates the present invention as ap- 1Q plied to a simple transmission line system comprising a transmitter T feeding high frequency energy to a load Z overa main transmission line TL. In accordance with the present invention a quarter wave-length section of line I having 15 terminals 0., a and b, b is inserted in the main line at a particular location between the load Z and the transmitter T, and at an electrical distance 0 which is preferably less than one-quarter the length of the communication wave. 20

The length of the section 0 between the matching section I and the load Z may be determined by a simple graphical method which is illustrated in a particular case in Fig. 3.

In Fig. 3 it is assumed that the impedance of 25 the load Z and the characteristic impedance p or" the line 2 are known, and that Z has a value of 320-14240, while p is 300. When the points Z and Z+5i in the complex plane may be plotted and connected by a straight line, in the manner 30 shown in Fig. 3 by the vertical line V, the lower end of said line'V being Z and the upper end being Z+i Let us now subdivide this vertical V line into equal portions, for example ten portions, and number the division points on the line 35 uniformly so that the bottom of the line bears the number zero and the top of the line bears the number 1, as illustrated.

Let us next draw a line joining the points p and +7Z and similarly subdivide into equal por- 40 tions and number in the manner shown in Fig.

3 by the slanted line S. Let us furtherlay off a straight line M from the origin 0 of the complex plane, such that it cuts the two scales of numbers V and S at the same Value. This value 45 is tan 0, so the electrical length of the line 0 is thus determined which will make the impedance measured at terminals a, a looking toward necessary whenever the required electrical length exceeds 45.

The impedance measured at terminals a, a when 0 is given the value determined, as explained above, is a pure resistance equal to the characteristic impedance of the line 2 (between a, a and Z) multiplied by the ratio of the distance from the origin 0 to the intersection of lines V and M, to the distance from the origin 0 to the mtersection of lines S and M. In Fig. 3 this is 300 is a real quantity, and for determining the value of said expression when 0 is so chosen. Z is the complex load impedance and p the characteristic impedance of the line 2.

It should be noted that it is not necessary to make the line section 2, which is adjacent the load Z, of the same characteristic impedance as the main line TL. In practice, the impedance of the section 2 adjacent the load and the impedance of the load itself, if possible, will be adjusted by mathematical cut and try, in accordance with the method explained above, so as to obtain the ost convenient spacings between conductors for the quarter wave section I and the section 2 adjacent the load.

It should be understood that the matching line section I can be a quarter wavelength or any odd multiple thereof, and the section 2, while preferably less than oneequarter wavelength, may be increased by any multiple of a half wavelength without affecting the principles involved.

What is claimed is:

1. The method of preventing wave reflection in a transmission line connecting a source of high frequency energy to a complex load impedance which comprises changing the spacing of said line for a distance equal to an odd multiple including unity of a quarter wavelength at a posi tion in said line which is removed from said load by a length so chosen as to make the effective impedance of said load and said length of line a pure resistance, and further so choosing the constants of said changed section of line as to make its characteristic impedance equal to the geometric mean between said resistance and the characteristic impedance of the line located between said source and changed section whereby said pure resistance is transformed to a value equal to the line impedance.

2. Themethod of preventing wave reflection in a transmission line connecting a source of high frequency energy to a complex load impedance which comprises changing the spacing of said line for a distance equal to an odd multiple including unity of a quarter wavelength at a position in said line which is removed from said load by a length equal to an integral number of half wave lengths plus an additional length greater than zero and less than a quarter wave length, said additional length being so chosen as to make the effective impedance of said load and said length of line a pure resistance, and further so choosing the constants of said changed section of line as to make its characteristic impedance equal to the geometric mean between said resistance and the characteristic impedance of the line located between said source and changed section, whereby said pure resistance is transformed to a value equal to the line impedance.

3. In combination in a high frequency system, a complex load impedance Z, a section of transmission line terminated thereby and having an electrical length 0 so chosen as to make the input impedance of said section of line expressed by the term Z+j tan 0 p+jZ tan 6 a real quantity, where p is the characteristic impedance of said line, whereby the input impedance of said section of line is a pure resistance.

4. In combination in a high frequency system, a complex load impedance Z, a section of transmission line terminated thereby and having an electrical length 0 so chosen as to make the input impedance of said sction of line expressed by the term Z+jp tan 6 p+jZ t 0 a real quantity, where p is the characteristic impedance of said line, whereby the input impedance of said section of line is a pure resistance, a source of high frequency energy including a main transmission line coupled to said section of line, and means between said main transmission line and said section of line for matching the resistance of said source to the input impedance of said section of line.

5. In combination in a high frequency system, a complex load impedance Z, a section of transmission line terminated thereby and having an electrical length 0 so chosen as to make the input impedance of said section of line expressed by the term Z+j tan 8 +jZ tan 0 a real quantity, where p is the characteristic impedance of said line, whereby the input impedance of said section of line is a pure resistance, said electrical length 0 being less than one-quarter wavelength long.

6. In combination in a high frequency system, a complex load impedance Z, a section of transmission line terminated thereby and having an electrical length 6 so chosen as to make the input impedance of said section of line expressed by the term a real quantity, where p is the characteristic impedance of said line, whereby the input impedance of said section of line is a pureresistance, said electrical length being a quantity less than one-quarter wavelength long plus a multiple of one-half wavelength, 7

'7. In a high frequency system,a complex load impedance, a 'first section of' line having one terminal coupled to said lead, a second section of line a'quarter wavelength long or an odd multiple thereof, a source of energy coupled to one terminal of said second section of line, the other terminal of said second section of line being coupled to the other terminal of said first section of line, said first'se ction of line having such an electrical length asto make the effective impedance of said load and said first section of line a pure resistance, said second section of line having a characteristic impedance which is the geometric mean between said pure resistance and the impedance of said source, whereby said pure resistance is transformed to a value equal to the line impedance.

8. A system in accordance with claim 7,inc1uding a third section of line connected between said source and said second section, said third section of line having the same geometrical structure as said first section.

9. A system in accordance with claim '7, including a third section of line connected between said source and said second section, said third section of line having a different geometrical structure than said first section.

WALTER VAN B. ROBERTS.

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