US2584600A - Radio-frequency phase shift network - Google Patents

Description

Feb. 5, 1952 aOKlMMlE 2,584,600

RADIO-FREQUENCY PHASE SHIFT NETWORK Filed June 29, 1948 Patented Feb. 5, 1952 cunning-stares PATENT OFFICE RADIO-FREQUENCY PHASE SHIFT V j NETWORK 'GeorgeB. MacKimmie, Montreal, Quebec, Canada, assignor to .Radio Corporation of America, :a corporation of Delaware Application June 29, 1948, Serial No. 35,895

This invention relates transmission line networkspand more particularly to improvements in phase shifters and the like for radio irequencyenergy.

The principal object of the inventionis to provide phase shifters in which the amount of phase shift can be varied without producing any amplitude variations. 1 r I Another object is to provide. phase shifters whose input and output impedances are constant and therefore independent of the amount of phase'shif-t introduced. I i

- A further -object-is to provide phase shifters oi the described type including reactance elements in the form of short circuited transmission lines, wherein the positions of the short circuiting' elements in said line sections bear a simple linear relationship to the amount of phase shift produced.

11; is a' further object of this invention to provide phase shifters which fulfill the foregoing objects notwithstanding electrical or mechanical irregularities in the said .reactance elements, and therefore are ,simpler and more economical to design and construct than prior art devices having the same purposes.

to radio frequency I germs; (01. 178-44) V The invention will be describedlwith reference to the accompanying drawing, wherein; I Figure 1 is a schematic circuit diagram of a variable phase s'hifter'network embodying the instant invention and illustrating the principles thereof,.and

Figure 2 is a schematic diagram of a preferred embodiment of the invention including coaxial transmission line elements.

Reference is made first to Figure 1, which an alternative, all of the lines except one may include suchimeans.-

A furthertransmission line section I9, .provided with longitudinally adjustable short circuitlng means such as a shorting bar 2| "is connected to the junction I15 between the lines 5, and

1. Another line section .23, similarv to the.'.:sec-' tion.;I-9 but one uuanter. wavelengthlongengm and .3.

The line section 23. is provided with shorting means .25 like that on the line I9. The junctions 9 and I3 are connected respectively to a source and aload (not shown). m

In the operation of the device of Figure 1., the

shorting means 2| and 25 are set at positions on the lines I9 and 23, such that the, effective lengths of said lines differ by a quarter wave.- length. Thus, if the distance from the junction I5 to the bar 2| .is x, the distance from the Junetion .I I to the bar 25 is :c-i-A/4.

Assuming that the line sections I., 3, 5. and 1. are all of the same characteristic impedance, incident energy applied to the junction ,9 will divide equally between the lines I. and I, arriving'in the .same phase at the junctions 'II and lfil 'xiAt these points the energy divides again, part going out the respective lines .23 :and .-2.I, and part going along the lines 3 and 5. the characteristic .impedances of. the lines 23 and I9 are equal, the currents flowing into th lines 3 and 5 will be equal.

. Since the line 3 includes the transposition II, the current and voltage at the end .of the linex3 which is connected to the junction I3 will be equal and opposite to those at the corresponding end of the line 5 which is connected thereto; Thus, that part of the current which travels directly past the junctions II and I5 balances out at the junction I3, simply circulating around the loop and producing no output to the load. The current flowing into the line 23 is re-, flected at the short circuit 25. and returns, to the junction II, arriving at that point Witha phase radians. These two currents are equal, and each divides equally at the respective junctions I i ,and

I5. 'The currents flowing from the junctions H and I5 into the lines I and I arrive at the point 9 with a phase difierence of 41:10

Or radians"; i 80 degrees ,outof enn c edtotheiunct qulibetween thez-iinesiiph sea d theteiorecancel...

Assuming The reflected currents from the lines 23 and I9 which fiow into the lines 3 and 5 arrive at the point I3 in phase with each other, owing to the reversal at the transposition II. These currents combine to flow from the junction I3 to the load. Since no energy (except for losses in the line elements) is dissipated in the network, substantially'all bf the energy applied to the junction 9 flows out of the junction I3 to the load. The phase at the point I3 lags that at the point 9 by a constant 11' introduced by the lines I and 5, plus an amount a introduced by the shorted line I3. The phase lag introduced by the path including the lines I,

3 and 23 is 360 degrees greater and thus is the same.

It will be apparent that the phase difference between the input and the output to the described network may be varied at will by moving the shorting. bars 2| and 25 together, and that the change in phase will-be directly proportional to the distance the bars are. moved. The amplitude of the output is determined only by that of the input as long as no substantial dissipation occurs in the lines I9 and 23, so that no variationin amplitude is introduced by changing the positions of the shorts 2i and 25.-

Since a line terminated in an open circuit will reflect as well as one terminated in a short circuit, variable length-open ended lines could be substitutedifor the shor-ted'lines I9 and 23. At present, however, it appears preferable for mechanical reasons to use short-circuited lines.

A shorted line of length a: in general exhibits, at a definite frequency, the characteristics of a pure reactance such as .a capacitor or an inductor. Consequently, the line section l9 may be replaced wholly or in part by a lumped reactance element, providing the terminal portion of length x of the line 23 is similarly replaced. Thus, a variable capacitor can'be connected to the junction IS, with a similar capacitor connected to the junction I I through a quarter wave line. If the two capacitors are varied simultaneously in the same manner, the phase at the junction I3 will shift accordingly, without variation in amplitude.

The phase shift'network is designed to match the impedance of the source and load devices as follows: Suppose the input and output'impedances are to be Z1. The characteristic im-' pedances of the lines I, 3, 5 and I are made equal and their impedance is denoted Z2. The characteristic impedances of the line sections I3 and 23 are also equal and the impedance is denoted Z3. In order to match impedances to prevent reflections at the junctions 9 and I3, the following relationship must be satisfied:

Thus, assuming Z1 and Z3 are each 50 ohms, Z2

must be /5 times 50 ohms, or about '70 ohms.

The system'of Figure 1 may be constructed to the similarly designated elements-in Figure ZP The transposition I! of Figure 1 is replaced by a section ll of line one half wavelength long. Since the delay in a half wavelength line section is 1r radians or degrees, the effect of this section is simply to reverse the phase, exactly as a transposition would do. It should be noted incidentally thata transformer or other known phase reversingdevicemay be substituted for the transposition I! in Fig. 1 or the half wavelength line section II of Figure 2. In the illustrated embodiment, the total length of the line 3 and the additional section I1 is three quarters wavelength. The lines I, 5, and 'I are each one quarter wavelength,.as in Figure 1.

The ends of the lines 23 and I9 remote from the junctions II and I5 respectively are provided with slottedextensions 23 and II! respectively which contain movable shorting plugs 25 and 2I. The slots are provided for access to the plugs, which are mechanically ganged together as by a rod 29. Also connected to the shortin plugs is an indicator pointer 3I, which moves over a scale 33 calibrated in degrees or other units of phase, to show the amount of phase shift being introduced by the network. V

The operation of the system of Figure 2'is substantially identical with that of Figure 1. It will be apparent that the lines 23 and I9 can be made of any convenient length, as long as they differ by a quarter wavelength or anodd number of quarter wavelengths. ;A point of considerable practical importance which should be noted is that the extensions23 and I9 need not have the same characteristic impedance as the lines 23 and I9, as long as they are identical with each other. Thus, whileat first glance the described structure appears to be more. come. plicated than a simple variable length lineor trombone for phase shifting, it is in fact much simpler to build because no impedance matching is required between the variable elements and the remainder of the system, and any irregularities in the variable-length line stubs will not. change the input and output impedances, nor

said wavelength, one of said sections differing in effective length from the others of said sections' by an amount equal to an odd integral number of half wavelengths, means applying said energy to two of said line sections at their junction, means providing output energy from the othertwo of said line sections at their Junotion, two reactance elements each coupled to one of said first two line sections at its junction.

with the corresponding one of said second two line sections and the coupling between one of said reactance elements and the respective line 5 section junction including a furtherline section 6 having an effective length greater than the coupling of the other of said elements by an odd integral number of said quarter wavelengths.

2. The invention as set forth in claim 1, wherein all of said transmission line sections are of equal effective lengths, and one of said sections includes means reversing thephase of energy travelling therethrough. 3. The invention as set forth in claim 1, where'- in three of said transmission linesections are" each one quarter wavelength long and the fourth of said line sections is three quarters Wavelengths long.

4. The invention as set forth in claim 1, wherein said reactance elements are substantially identical.

5. The invention as set forth in claim 4, wherein said reactance. elements comprise a pair of transmission line sections, each including movable short-circuiting means, and means mechanicallv coupling said short circuiting means together for gang operation.

6. The invention as set forth in claim 5, wherein the characteristic impedance Z2 of said line sections connected to form said closed loop, the characteristic impedance Z3 of said line sections comprising said reactance elements, and the impedances Z1 of said means applying energy REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS Number Name Date 2,416,790 Barrow Mar. 4, 1947 2,438,912 Hansen Apr. 6. 1948 2,438,914 Hansen Apr. 6, 1948 Tyrrell July 2'7, 1948

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