US2438367A - Transmitter-receiver switching system - Google Patents

Description

' March 23, 1948. J. E. KEISTER 2,438,367

TRANSMITTER-RECEIVER SWITCHING SYSIEM I Filed 0ct. 24, 1942 Fig.1.

K I i s TRANSMITTING RECE/V/NG APPA RATUS APPARATUS Inv'e ntor:

James E. Keistev His Attorneg.

Patented Mar. 23, 1948 TRANSMITTER-RECEIVER SWVITCHING SYSTEM James E. Keister, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application Gctober 24, 1942, Serial No. 463,173

3 Claims. 1

My invention relates to transmission lines of the type employed in transmitting high frequency energy and in particular to the use of such lines as impedance transformers.

It is an object of my invention to provide an improved means for transformin the value of an impedance connected across such a high frequency transmission line to a difierent desired value.

My invention relates further to high frequency transmission systems and it has for one of its -b- J'ects to provide improved means for protecting the receiver of such systems from high voltages to which the line, over which the received oscillations are transmitted, may be subjected.

More particularly, my invention relates to transmission systems in which both a transmitter and a receiver are connected to a common line which may lead, for example, to an antenna by which desired oscillations are radiated and received, and it has for one of its objects to provide an improved means to protect such a receiver from the intense oscillations produced by the associated transmitter.

Another object of my invention is to provide such means particularly adapted for use at short wave lengths.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, 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 shows diagrammatically my transmission line impedance transformer; Fig. 2 is a modification of the transformer of Fig. 1; and Fig. 3 shows diagrammatically a transmission system embodying the impedance transformer of Fig. 1.

The impedance transformer shown in Fig. 1 comprises a specially constructed transmission line In shown as a concentric line having a tubular outer conductor l i and an inner conductor !2 coaxial therewith. A load impedance 13, which impedance it is desired either to transform or invert to a desired value, may be connected. at the end of the line between conductors M and I2. The inner conductor 52 consists of a series of sections of different diameters, alternate sections being of the same diameter. Thus, the sections it are of a particular diameter, while the sections l5 have a smaller diameter. Each of the sections M and 55 has a length equal to a quarter of a wave length at the desired operating frequency of the line. In this manner, the transmission line IB consists of a series of transformer sections connected in tandem and each having a length equal to a quarter of a wave length at the operating frequency and of alternating low and high characteristic impedances.

If the odd numbered sections M, counting from the load I3, are made identical and the even numbered sections l5 are likewise made identical, each odd numbered section has the same characteristic impedance between its outer and inner conductors, which may be represented by ZA, and each even numbered section has an impedance ZB. ZL, the input impedance at the point .I, according to well-known transmission line theory, is

Z ZL and the impedance at the point 2 is i Z nZ The input impedance for an n number of sections is, therefore, where n is an even integer This represents a degree of transformation of the impedance Z1. which is dependent upon the characteristic impedance of the individual sections and the number of such sections.

When n is an odd integer, the input impedance is Zia-H) This represents an impedance inversion so that the input impedance, when ZB is greater than ZA, is considerably less than the load impedance.

As an example of the manner in which the input impedance transformer of Fig. 1 is able to effect a substantial increase or decrease of load impedance ZL, the following practical values are given. If the inside diameter of the outer conductor II is 1 inch and the outer diameter of the section [4 is .715 inch, the value of ZA is approximately 20 ohms. Also, if the outer diameter of the section I5 is .189 inch, the value of Z3 is approximately 100 ohms. A four section line, constructed in accordance with these measurements, gives an impedance transformation" of 625. Also, if the load impedance 13 represents a transmission line having a characteristic impedance of ohms, three sections of the line In invert this load impedance to give an input impedance Z3 which is only .229 ohm.

If the load impedance it is represented by The operation of the impedance transformer shown in Fig. 1 depends upon comparatively accurate lengths for each section l4 and I5, small deviations from the exact length being effective to introduce undesired reactive components of impedance. However, it is found that it is possible to compensate for the reactive components introduced by small errors in the lengths of the quarter wave sections by making one section adjustable in length. A transformer embodying this feature is illustrated diagrammatically in Fig. 2. In Figs. 1 and 2, corresponding reference numerals have been placed on corresponding elements to facilitate their comparison. In the transformer of Fig. 2, corresponding sections 2| and 22 of the outer and inner conductors ll and I2 are adjustable in length. While such adjustment may be achieved in any suitable manner, in the method shown in Fig. 2, the section 2! has a flared portion 23 which surrounds the end of the outer conductor H and the section 22 has a sleeve portion 24 which telescopes the conductor 25 connected to the section Id of inner conductor l2. The adjustable section, of course, may be inserted at any suitable point in the transformer and may be an intermediate section, rather than the end section as shown. Conductor 25 is provided for connecting section 22 to a source of input voltage and with conductor 21 forms a connecting line whose impedance is difierent from that of the transformer section 2|, 22 and preferably matches the impedance of apparatus to which it is connected.

A transformer, such as is shown in Fig. 1 and in the right-hand portion of Fig. 2, may be easily constructed by inserting the inner conductor It in a lathe and reducing the diameter of the sections l by cutting the desired amount of material from these sections. The adjustable sec tions 2i and 22 make it possible to compensate easily for any errors which may arise from slight differences in the lengths of the sections in conductor l2. Simply by sliding the portions 23 and 24 on the conductors II and 25, accurate adjustment may be made to compensate for any reactive component of transformation.

In Fig. 3, I have shown a transmission system embodying the impedance transformer of Fig. 1. The transmitting apparatus 3%) and the receiving apparatus 3| are connected to a main transmission line 32. Transmission'line 32, in turn, is connected by means of transmission line 33 to a single directive antenna 34. The transmitter 39 may be of the type which, at predetermined intervals, supplies a pulse of high intensity oscillation over the transmission lines 32 and 33 to the antenna and then is inoperative while this signal travels through'space to a refleeting object and returns to the system to be picked up by the antenna 34 and detected in the receiving apparatus 3!. As may be readily understood, the reflected signal ismuch weaker than that supplied by the transmitting apparatus to the antenna. Since both the transmitting and the receiving apparatus are connected to the transmission line 32, the receiving apparatus may be adversely affected by the strong signal of the transmitter unless means is provided for preventing the transmitted signal from reaching the receiving apparatus directly over transmission line 32 during the transmitting period.

In accordance with my invention, 'a branch transmission line 35 is connected to'the transmission line 32 at a point '35 lying between'the point of connection 31 between lines 32 and 33 ohm.

and the receiving apparatus 31. Moreover, the distance between the points 36 and 31 is made equal to an odd multiple of an electrical quarter wave length of the radiated oscillations of the pulses transmitted over the transmitter 30. In using the term odd multiple, I mean to include the multiple one. Because, of the spacing of the junctions 36 and 3?, a short-circuit across line 32 at point 36, by reason of the impedance inversion effect of the line 32, well known in transmission line theory, appears as a very high impedance across the line at point 31 and prevents transmission of signals to the receiver.

As one means for short-circulating the transmission line 32 at point 36 during a transmitting period, the branch transmission line 35 comprising an impedance transformer of the type shown in Fig. 1 is provided and is terminated at its end remote from the, point 36 by means of a non-linear impedance device 38. The inner conductor of the branch line 35 consists of the alternate sections 39 and 40 connected in tandem and having'high and low characteristic impedances respectively. For purposes of illustration the device 38 has been shown asv an electron discharge device of the diode type having its anode ll connected directly to the inner conductor of line 35 and its cathode 42 connected through any suitable biasing means, such as the battery 43, to the end of the outer conductor of a line 35, and thence through apparatus connected to the line 32,150 the inner conductor and to the anode, capacitor 54 being provided to bypass alternating currents from battery 43.

The biasis selected so that in the presence of received signals on the transmission line 32, device 38 is non-conducting and has a very high impedance. In the presence of the intense signals from the transmitter on the lines 32 and 35, however, the device 38 becomes conductin and its resistance is reduced to a very smallvalue. The line 35, moreover, is constructed to reduce this small resistance so that it appears as a virtual short-circuit across the line 32 at the point 35. For example, the line 35 may act as an impedance transformer and in accordance with the principles explained above may easily be constructed so that the resistance across the device 38 is decreased a thousandfold. Under such conditions the impedanceof the devic '38 at conduction, which, though of a small value, is of a definite size, is decreased by the use of the impedance transforming line 35. When the device 38 is non-conducting, such as duringa period when received signals are being supplied to the apparatus 3!, 1e impedance of the device 38 is very high, for example about 1' meg- If this is reduced a thousand times by the line 35, the impedance of the line 35 appears to be of the order of approximately 1000-ohms at its point of connection with the line 32. Transmission lines 32 and 33 are preferablyof the type customarily used with this type of apparatus and have a surge. impedance of approximately 70 ohms. ance value of 1000 ohms, line 35, during a receiving period, has a negligible effect on the signals Hence, having an apparent impedreceiving apparatus during the receiving period. The device moreover has a low cost, is easily constructed from available materials, and is simple and reliable in operation.

In the construction of the protective system of Fig. 3, an insulator 45 is provided at the end of the outer conductor of line 35 in order that connection may be made between the anode of device 38 and section 39 of the inner conductor and the connections between the anode and cathode of device 33 and the line 35 are made as short as possible. Also, the adjustable impedance transformer shown in Fig. 2 may be employed in the protective device of Fig. 3.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

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

1. In combination, a transmission line, receiving apparatus connected thereto to receive oscillations over said line, a transmitter connected to said line to supply oscillation-s thereover, an impedance means connected across said line at a point between said transmitter and receiver, an output terminal connected to said line between said point and said transmitter, the impedance of said means being low during the high voltages produced by said transmitter and high during the absence of said high voltages, said means comprising a branch transmission line having a length equal to a multiple of a quarter wave lengthiof said oscillations and having a plurality of quarter wave length sections of alternately high and low impedance, and a non-linear impedance element connected across said branch line at its end remote from said point, the distance between said end and said terminal being equal to an odd number of quarter wave lengths at the frequency of said oscillations.

2. In combination, a transmission line, receiving apparatus connected thereto to receive oscillations over said line, a transmitter connected to said line to supply oscillations thereover, an impedance means connected across said line at a point between said transmitter and receiver, an antenna connected to said line at a point between said first point and said transmitter, the impedance of said means being low during the high voltages produced by said transmitter and high during the absence of said high voltages,

said means comprising a branch transmission line having a length equal to a multiple of a quarter wave length of said ocsillations and having a plurality of quarter wave length sections of alternately high and low impedance, a diode connected across said branch line at its end remote from said first point, and means biasing said diode to cutoff during the absence of said high voltages, said end being spaced from said first point by a distance equal to an even number of quarter wave lengths at the frequency of said oscillations and said second point being spaced from said first point by a distance equal to an odd number of quarter wave lengths at the frequency of said oscillations.

3. In combination, a transmission line, a source of oscillations and receiving apparatus connected to said line at spaced points thereof, said apparatus being adversely afiected by high voltages, an output circuit connected to said line at a point between said source and said apparatus, and means for protecting said apparatus from high voltages comprising, a branch transmission line connected to said first line intermediate said receiving apparatus and the point of connection with said output circuit and having a length equal to a multiple of a quarter wave length of said oscillations and having a plurality of quarter wave sections of alternately high and low impedance, and a non-linear impedance element connected across said branch line at its end remote from said transmission line, and means rendering the impedance of said element low in the presence of said high voltages and high during the absence of said high voltages, said branch line having a length equal to an even number of quarter wave lengths at the frequency of said oscillations and the distance between points of connection of said output circuit and said branch line with said first line being equal to an odd number of quarter wave lengths at the frequency of said oscillations.

JAMES E. KEISTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,898,180 Hansell Feb. 21, 1933 2,178,299 Dallenbach Oct. 31, 1939 2,189,549 Hershberger Feb. 6, 1940 2,205,874 Buschbeck June 25, 1940 2,219,922 Gossel Oct. 29, 1940 2,235,010 Chaffee Mar. 18, 1941

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