US2127408A - Transmission line termination - Google Patents

Description

Aug. 16, 1938. l. J. KA'A'R TRANSMISSION LINE TERMINATION FilcLSept. 13, 1935 Patented Aug. 16, 1938 UNITED STATES TRANSMISSION LINE TERMINATION Ira J. Kaar, Stratford,

Conn, assignor to General Electric Company, a corporation of New York Application September 13, 1935, Serial No. 40,435

5 Claims.

My invention relates to high frequency transmission lines and more particularly to transmission lines of the concentric type.

It has for one of its objects to provide an im- 5 proved impedance network for use in connection with. such lines.

A concentric tube transmission line is commonly used for conveying power from an ultra high frequency transmitter to an antenna. In order to reduce the losses on such a line to a minimum, the impedance looking from the end of the line into the load must equal the surge impedance of the transmission line. One method of matching the surge impedance of the transmission line with the impedance of the load is to place an inductance in series with the line and a capacitance in shunt across the line, on one side or the other of the inductance, depending upon which of the two impedances to be matched is the greater, these elements being proportioned, in the Well known Way, to effect the desired impedance match. Where ultra high frequencies are used the value of the capacitive reactance and the inductive reactance required are relatively small. For example, where the transmission frequency is 44 megacycles, the transmission line impedance is '72 ohms, and the antenna load is 5000 ohms, then the value of capacitive reactance necessary is approximately 5.9 micro- 30 microfarads and the value of inductive reactance necessary is approximately 2.16 microhenries. By suitably proportioning the relative diameters of the conductors of a concentric transmission line at the load terminal of the transmission line 35 a sufficient shunt capacity and series inductance may be introduced at the end of the transmission line to permit proper surge impedance termination to be achieved.

Another object of my invention is to provide 40 an improved means for terminating a concentric transmission line.

It is a further object of'my invention to provide a shunt capacity and a series inductance in a concentric tube transmission line by suitably proportioning the transmission line conductors themselves.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 of the drawing is a cutaway view of the antenna end of a transmission line and Fig. 2 is the equivalent circuit diagram of the transmission line termination.

Referring to Fig. 1 of the drawing, I have illustrated a transmission lineconstructed in accordance with my invention, and. which comprises two concentric conductors I and 2. An extension 3 of conductor 2 beyond the end of conductor 1 forms the antenna to which the transmission line is connected. In the particular embodiment of the invention shown, the inner conductor 2 is provided with a constricted section 4 and an en.- larged section 5 at the antenna end of the transmission line. By thus constricting a portion of the inner conductor 2 a small amount of series inductance is added to the transmission line. By enlarging the inner conductor 2 as at 5 a small amount of shunt capacity is placed across the line. The degree of constriction and the length of the constricted section 4 determines the value of the series inductance which is introduced. The relative enlargement and the length of the enlarged section 5 determines the value of the shunt capacity which is introduced across the transmission line. By properly selecting the dimensions of the constricted section 4 and the enlarged section 5 of the inner conductor 2, the impedance looking toward the end of the transmission line and into the load may be made equal to the surge impedance of the transmission line.

The equivalent circuit of a transmission line termination of this type is shown in Fig. 2. Resistance R represents the load placed on the transmission lines I and 2 by an antenna 3. R indicates the ohmic impedance of the transmission line itself. X1 represents the series inductance placed in the line by the constricted section 4 of the inner conductor 2 while X2 represents the capacity placed across the line by the enlarged section of the inner conductor 2.

Dimensions for the proper termination of any given concentric tube transmission line may be determined from the following well known formulae which express the inductance and capacitance of two concentric tubes having an air dielectric.

log a where,

L=henries per centimeter, C=farads per centimeter, D=inside diameter of the outer tube, d=outside diameter of the inner tube.

As an example of an application of the above formulae, assume that a concentric tube transmission line having a surge impedance of '78 ohms is to be terminated in a vertical antenna. Assume further that the outside diameter of the inner tube is 2.54 centimeters and that the inside diameter of the outer tube is 9.327 centimeters; that the vertical antenna has a length approximately 1.75 meters and an outside diameter of 2.54 centimeters; and that the operating frequency is 853x10 cycles. Under these conditions, the antenna will present a resistive load of 550 ohms to the transmission line. The problem, therefore, is to match the '78 ohm transmission line to a 550 ohm load.

In order to match correctly the surge impedance of the transmission line to the antenna it can readily be shown that if a network such as that illustrated in Fig. 2 be connected between a source having a resistance R and a load having a resistance R and if the values X1 and X2 be adjusted as follows:

and

and

It will thus be seen that the constricted section 4 of Fig. 1 must be so proportioned as to have an excess of inductance of .356 10- over that which would be obtained if the inner tube were continued at uniform size. In order that transmission line eifects will be minimized in the constricted section, the length of the constricted section should be made as short as practicable. Assume that a length of 100 centimeters is chosen as the length of constricted section 4. The inductance of the normal transmission line per centimeter (from Equation (1) above) is 2.6l 10" The inductance of 100 centimeters of a normal section of the transmission line is .261 10 Therefore, the total inductance required for 100 centimeters of constricted section 4 is .26l 10- +.356 10 :.617 10- The inductance per centimeter of this section is .0061'7 10 Applying Equation (1), it will be found that the outside diameter of the inner tube over the constricted portion 4 is .425 centimeters when the length chosen for section 4 is 100 centimeters.

The required excess capacity (as determined from Equation (3)) is 8.28 10- Assume that this excess is to be obtained over a length of 25 centimeters. From Equation (2) the normal capacity of the transmission line per centimeter is xi25 10- and for 25 centimeters is 10.625 10*. Since the excess capacity to be achieved over this length is 8.28 x l0 the total capacity required is 18.905 10- and the required capacity per centimeter is .756 10- Applying Equation (2) it will be found that the outside diameter of the inner tube for the enlarged section is 4.49 centimeters.

As previously stated, the amount of shunt capacity which is placed across the line and the amount of inductance added in series to the line depends upon the relative diameters of outer conductor I and inner conductor 2. It will, therefore, be apparent that if upon applying the above equations for a given application the diameter of the inner conductor becomes unduly small from the standpoint of commercial application that the diameter of the outer conductor may be enlarged at the point where it is desired to add series inductance rather than decreasing the diameter of the inner conductor, or both the outer conductor may be enlarged and the inner conductor constricted.

It is frequently convenient to support the antenna at the. point where it leaves the transmission line by a suitable insulator 6. The effect of insulator 6 is to introduce a certain amount of lumped capacity across the transmission line. This lumped capacity may be used as a part of the capacity provided by the enlarged portion 5 of the inner conductor 2 to provide the proper shunt capacity.

Although I have described an embodiment of my invention wherein the impedance of the load is greater than the surge impedance of the line, it is apparent that where the surge impedance is greater than the load impedance I may match the two impedances by placing the capacitance X2 on the input side of inductance X1. By reversing the position of the constricted section 4 and the enlarged section 5 in Fig. 1 such a result may be attained.

It will be further understood that my inven tion is not limited to impedance matching networks but that the conductors of the concentric transmission line may be suitably formed, in accordance with my invention, inherently to form the elements of impedance networks of other types as well, as for example, those used to determine the frequency characteristic of the transmission line.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since different modifications may be made, and I therefore contemplate by the appended claims to cover all such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:-

1. In a system for matching the impedance of a concentric transmission line to the impedance of a load connected thereto, said load impedance being different from the impedance of said line, the combination with said line of a portion thereof adjacent to said load comprising a plurality of sections, one of said sections being proportioned to offer series inductance per unit of length greater than the series inductance per unit of length of the line and another of said sections being proportioned to offer shunt capacity per unit of length greater than the shunt capacity per unit of length of said line, said different sections being so proportioned relative to each other that the impedance looking into said portion from either end thereof matches the impedance looking in the opposite direction from the same end.

2. In a concentric tube transmission line extending between a high frequency apparatus and an antenna, said transmission line comprising an outer conductor and an inner conductor, means including a section of said inner conductor constricted with respect to normal for adding series inductance to said transmission line, and means including a section of said inner conductor enlarged with respect to normal for adding shunt capacitance to said transmission line, said sections being adjacent and positioned at the end of said line.

3. A concentric tube transmission line extending between a source of oscillations and an antenna, said line comprising an outer conductor and an inner conductor, said inner conductor being connected to said antenna through an impedance matching portion of said inner conductor, said portion having a constricted section and an enlarged section adjacent thereto and in proximity to said antenna.

4. A concentric tube transmission line extending between a high frequency apparatus and an antenna, said transmission line comprising an outer conductor and an inner conductor, said transmission line having a section in proximity to said antenna Where the difference in the diameter of said outer conductor and said inner conductor is increased with respect to normal and a second section where the difference in the diameter of said outer conductor and said inner conductor is decreased with respect to normal.

5. In a system for matching the impedance of a concentric transmission line to the impedance of a load connected thereto, said load impedance being different from the impedance of said line, the combination with said line of a portion thereof adjacent said load comprising a plurality of sections, said sections being proportioned relative to each other to constitute the electrical equivalent of an impedance network matching the impedance of said load to that of said line, said network comprising a series impedance element and a shunt impedance element, each section of said portion corresponding respectively to one of said elements.

IRA J. KAAR.

Images