US2935705A

US2935705A - Constant impedance balance line phase shifter

Info

Publication number: US2935705A
Application number: US612183A
Authority: US
Inventors: Herbert J Reis
Original assignee: Martin Co
Current assignee: Martin Co
Priority date: 1956-09-26
Filing date: 1956-09-26
Publication date: 1960-05-03
Anticipated expiration: 1977-05-03

Description

y 3, 1960 H. J. REIS 2,935,705

CONSTANT IMPEDANCE BALANCE LINE PHASE SHIFTER Filed Sept. 26, 1956 lNVENTOR HERBERT J. REIS BY G i 2 g I g; ATTORNEY 'utilize vacuum tubes, of one kind or another. balanced type networks and transmission lines are iniline as its physical length is changed. Another commonmeans involves the use United States Patent 7 CONSTANT IMPEDANCE BALANCE LINE PHASE SHIFTER Herbert J. Reis, Middle River, Md., assignor to The Martin Company, a corporation of Maryland This invention relates to phase shifting devices and,

in particular, to a phase shifting device for balanced-line type transmission lines which maintains a substantially constant impedance throughout the range of phase shift.

Many different varieties of apparatus are known for changing the electro-magnetic wave phase characteristics in transmission linm and networks. Probably the best of these depend upon active networks, i.e., those which Where volved, one'of the most common means for introducing phase shift is merely to change the electrical length of the line by altering the physical length of the line. This is simply accomplished by inserting different lengths of two wire conductor at the appropriate location in the circuit. Obviously, this method is very inconvenient where frequent changes of the phase shift characteristic are necessary.

So-called line stretchers have been devised which permit the introduction of a variable phase shift characteristic between two points in a transmission line or network. Generally, these comprise telescopic joints in the conductors, but they are not wholly satisfactory because it is diflicult to construct the joints with good noise-free contacts between the telescoping parts. Moreover, such devices change the impedance between the nominal input and output terminals of the transmission of so called stub lines which are actually short lengths of trans- -mission line shunted across the main pair of conductors.

By varying the lengths of the stub lines the amount of phase shift in the main line may be changed. Telescoping type stub lines are used to obtain a variable phase shift, or sliding shorting bars in contact with the stub lines can, be made to serve the same purpose. In

most cases two such stub lines are required, though some lines or networks necessitate the use of three stub lines to obtain the desired degree of phase shift. At best such arrangements are bulky and they are awkward to use. A paramount disadvantage of the stub line system is that the range of load impedances that can be ,matched to the transmission line is limited.

I have invented constant impedance phase shifting apparatus for variably shifting the phase of electric currents transmitted over two wire, balanced-type trans- .mission lines. The apparatus has utility generally vthroughoutthe very high frequency (VHF) and ultra high frequency (UHF) frequency ranges which nominally [include frequencies from 30 to 3,000 megacycles per second.

Recently there has been introduced to the art a type of constant impedance phase shifting apparatus which is similar in some theoretical respects to the present invention, but is practically applicable to coaxial type transmission lines which operate generally at frequencies above 3,000 megacycles. That apparatus is described in an application for United States Letters Patent of Herbert 2,935,705 Patented May 3, 1960 Constant Impedance Coaxial Phase Shifter.

According to the present invention I provide two elongated conductive elements mounted in spaced relation to each other. At least one of these elements has disposed along its length at least one conductive fin or plate, the plane of which extends outwardly of the conductor surface such that the distance along the surface of the conductor and the plate or plates fixed thereto generally in the direction of the length of the conductor is greater than the length of the conductor itself between the extremes of the plate bearing portion. I also provide means, for varying the lateral spacing between the surfaces of the conductors and plates. By changing the spacing the capacitance between the conductors is varied and simultaneously the mutual inductance of the current elements along the adjacent surfaces of the conductors and plates is changed. Through proper proportioning and positioning of the plate or plates the capacity and the inductance of the device may be made to vary proportionally so that the impedance of the device is maintained constant while the degree of phase shift of a current of particular frequency is variable.

The phase shifting device I have invented is markedly more compact, more easily adjustable, and less susceptible to the electro-mechanical difficulties which are encountered in the use of telescoping joints.

A paramount feature of this new phase shifting apparatus is its substantially constant impedance throughout its adjustable range. The striking fact is that this result is achieved with apparatus which does not rely upon complicated mechanical structure nor upon vacuum tube circuits or other active devices and networks such as are found in balanced-type transmission line phase'shifting devices presently known in the art.

In the following part of the specification I describe in detail particular embodiments of my invention which I have found to function especially well at the frequencies for which they were designed. These embodiments are illustrated in the accompanying drawing in which:

Fig. 1 is a plan view of a balanced-type transmission line phase shifting apparatus;

Fig. 2 is a sectional view taken along the line 2--2 of the device shown in Fig. 1;

Fig. 3 is a view similar to Fig. 2 in which the movable parts of the device are shown in different positions;

Fig. 4 is a perspective view of another embodiment of my new phase shifting device.

As shown in Figs. 1, 2, and 3 of the drawing, the principal embodiment has a suitable base 1 on which there are mounted at the opposite ends of the base two pairs of conductor supports 2, Sand 4, 5, respectively. In this particular embodiment each pair of supporting posts is slidably mounted in a transverse slot in the base. For this purpose, the posts have their lower ends reduced in the cross-section to fit into the slots. The posts of each pair may be spaced apart or moved together. As will appear in due course this spacing of the posts of each pair of supports provides one mode of adjustment of the apparatus. e

As is well known, a balanced-type transmission line comprises two substantially parallel, spaced conductors. The basic conducting elements of the instant apparatus are the elongated crank-like elements indicated generally at 8 and 9. Inasmuch as these elements are substantially identical in this embodiment only one will be described in detail.

The conductor 8- is a single throw crank having end shafts 10 and 11. The throw of the conductor comprises the shaft 12 mounted eccentrically of the common axis of the shafts 10 and 11 between the transverse member 13 carried by the shaft 10 and the transverse member 14 caressence ried by the shaft 11. The end shaft 10 is journaled in a suitable bearing in the support 2 and the end shaft 11 is similarly journaled in a suitable bearing in the support 4. Any convenient means may be employed to prevent longitudinal displacement of the end shafts 10 and '11 in their respective bearings.

Annular discontinuities in the surface of the shaft 12 of the crank-like conductor 8 are provided by a series of fins or plates 15 mounted transversely of the shaft 12 at spaced intervals along its length. In the embodiment of Fig. 1 these plates were formed from basically circular blanks as best seen in Figs. 2 and 3. They have been especially shaped, however, by removing diametrically opposed sectors as evidenced by the straight portions of their peripheries indicated at 16 and 17. Each plate was mounted on the shaft 12 at a location very near its straight portion 17 by drilling through the plate and assembling it on the shaft before the

transverse members

13 and 14 of the crank-like conductor were assembled on the shaft. As shown, the plates have been identically angularly oriented about the longitudinal axis of the shaft 12 and have been regularly spaced therealong. Finally, the plates were welded or brazed to the shaft 12.

As previously stated the crank-like conductor 9 is identical to the conductor 8 with the exception that the plates 15 of the conductor 9 are offset with respect to the plates 15 of the conductor 8 so that, when the conductors are rotatably mounted in their respective supports and the crank throws are longitudinally aligned, the plates 15 may interleave or mesh with the plates 15' as the conductors are rotated or as the spacing between the supports 2.and 3, and between the supports 4 and is varied.

Any suitable means may be used to connect the phase shifting device of Fig. 1 into a balanced-type transmission line. The device is completely reversible and either end may be considered the load end or the input end.

The embodiment of my invention just described was designed to operate at a frequency of approximately 50 to 1,000 megacycles per second. As an illustration of the material and dimensional characteristics of one particular embodiment designed for operation at approximately 200 megacycles, the following is given:

The material of the

shafts

10, 11, and 12 and plates 15 is brass.

The diameter of the shafts 10 and 11 is A".

The diameter of the shaft 12 is A1".

The length of the shaft 12 is 12".

The offset of the axis of the shaft 12 with respect to the axis of the shafts 1G and 11 is A.

The radius of the plates 15 is The longitudinal displacement of the plates 15 with respect to the plates 15' is A The height of the sector cut off along the line 16 is approximately 33 percent'of the radius.

The height of the sector cut off along theline 17 is approximately 33 percent of the radius.

The thickness of the plates is It is to be understood, however, that the foregoing characteristics are exemplary, and should not be regarded as limiting my invention.

The operation of my new phase shifter is based on two transmission line formulas; namely, the impedance formula, Z= /Z7, and the phase shift formula,

where Z is the impedance, L is the inductance and C is the capacitance.

It is apparent from a consideration of thesetwo equations that it is possible to maintain Z, the impedance, constant and, yet, to achieve a phase shift. This result is realized if the values of L, the inductance, and C, the capacitance, are changed so that their ratioL/C remains constant. My new phase shifting device achieves this result.

It is known that very high frequency and ultra high frequency electric currents travel substantially on the surface of a conductor. It is also known that, generally, the inductance and the capacitance of any given circuit are de termined by the geometrical characteristics of the circuit elements. According to my invention I provide apparatus which combines the effect of these several factors to provide a phase shifting apparatus which maintains a substantially constant impedance throughout the range of phase shift.

Specifically, in the unmeshed position substantially as illustrated in Fig. 3, the impedance of the device approaches the impedance of the two

parallel shafts

12 and 12 since the inductance is primarily determined by the most nearly adjacent current elements traveling along the surfaces of the two conductors. Likewise, the capacitance of the device is determined primarily by the effective spacing between the adjacent surfaces of the rods and their areas and curvatures. As the conductor 8 is rotated counterclockwise, as seen in Figs. 2 and 3, and the conductor 9 is rotated clockwise the plates begin to mesh. Now the current elements traveling along the surfaces of those portions of the plates 15 which are most nearly adjacent the plates 15', being more closely spaced than the previously most nearly adjacent current elements, increase the inductance of the device. Likewise, the common projected area between plates 15 and 15' and the decrease in spacing between the plates increases the capacity. Shapes of the plates have been so designed that, as the meshing of the plates progresses, the inductance and the capacitance increase, and, therefore, the product of the inductance and capacitance increases. Accordingly, the phase shift characteristic of the device decreases in accordance with The plate shape is also adapted to keep the variations in inductance and capacitance substantially proportional so that their ratio, L/C, remains constant. The impedance of the device, Z= /L/ C, will, therefore, remain substan- '18, are straight shafts rather than cranks and the irregularities in the surfaces of the conductors are provided by wholly

circular plates

19 and 19'. In the embodiment the conductors 18 and 18' pass through the plates at locations closely adjacent to a portion of the circular periphery of the

plates

19 and 19, respectively. As in the previous embodiment the conductors are journaled in hearings in the pairs of upright supports 20, 21 and 22, 23. Also in Fig. 4 adjustment knobs 24 and 25 are shown on the near ends of the conductors 18 and 18', respectively. In this embodiment the space between the pairs of

upright posts

20, 21, and 22, 23 is not adjustable but may be made so if the additional mode of adjustment is considered to be desirable.

Two particular embodiments of my invention have been illustrated and described in some detail. The wide variety of possible modifications within the scope of my invention is indicated in part by the structural differences between the two embodiments shown and described here. In general, to achieve the best possible results for any particular application the following characteristics must be considered: plate thickness, spacing of the plates along the conductors, the shape of the plates, the diametersof the conductors, the separation between the conductors, and the eccentricity of rotation of the crank throw where that feature is incorporated. The maximum amount of phase shift which any particular apparatus can introduce into a transmission line will of course be directly related to the increase in electrical length of the conductors as contrasted with the electrical length of ordinary two wire transmission line. Again, generally speaking, thernaximum possible phase shift and the maintenance of most nearly constant impedance throughout 360 degrees of rotation of the conductors are achieved where crank-like conductors ofthe type shown in Fig. l are used.

In view of the diversity of possible modifications which can be incorporated in phase shifting apparatus according to my invention, I do not propose that the scope of my invention be limited to the details described and illustrated in connection with the two particular embodiments given here. The scope of my invention is defined by the subjoined claims.

I claim:

1. Phase shifting apparatus for a balanced, two-conductor transmission line, which apparatus comprises a pair of substantially parallel crank-like conductors mounted for rotation about their respective crank axes and for variable separation between the axes, a first input terminal and a first output terminal connected to one of said conductors, a second input terminal and a second output terminal connected to the other of said conductors, said input and output terminals thereby forming a fourterminal network, each conductor having a plurality of basically circular plates fixed thereto in spaced relation along the throw of the conductor, the plates on one conductor being longitudinally otfset with respect to the plates on the other conductor so that the plates on the one conductor may be meshed with the plates on the other conductor by rotating the conductors and by varying the spacing between them, the position of said conductors and the degree of mesh of said plates determining the amount of phase shift of signals appearing at the said output terminals with respect to signals introduced to said input terminals according to the equation a JZE where a=phase shift, L=inductance, and C=capacitance,

while the impedance of said apparatus remains constant according to the equation rotated with the plates of said conductors being arranged 6 to interleave to a degree dependent upon their angle of rotation, said mounting means rotatably retaining said conductors and consequently said plates in spaced relation to produce a substantially proportionate variation of the inductance and capacitance between said conductors to accomplish phase shift, while keeping the impedance constant. 1

3. Constant impedance, electromagnetic wave phase shifting apparatus for balanced two-conductor transmission lines, comprising a pair of elongated conductors each rotatably mounted about spaced parallel axes, said con ductors having input terminals for receiving input signals and output terminals at which output signals are produced, said input and output terminals thereby forming a four-terminal network, a plurality of conductive plates mounted in spaced relation along each conductor, the plates on one conductor being ofiset with respect to the plates on the other conductor so that said plates may be meshed, means for varying the rotational position of said conductors to alter the degree of mesh of said plates and thereby vary the inductance and capacitance therebetween, the phase shift of said output signals relative to said input signals being determined by the relative positions of said conductors and said plates according to the formula vre where a=phase shift, L=inductance, and C: capacitance, while the impedance of said apparatus remains constant according to the equation 4. Constant impedance, electromagnetic wave phase shifting apparatus for balanced two-conductor transmission lines in accordance with claim 3 which includes means for varying the separation of said parallel axes of said conductors, and the said conductive plates are of a basically circular configuration.

References Cited in the file of this patent UNITED STATES PATENTS