US2640878A - Switch for high-frequency electrical oscillations - Google Patents

Description

June 2, 1953 D. c. ESPLEY k 2,640,878

SWITCH FOR HIGH-FREQUENCY ELECTRICAL. OSCILLATIONS Filed Oct. 11. 1948 26 F: H 2/01/623 25 1 5 3 3 55 .31 j l PM, L r 5 g? 1 2 C3 T c4 19' 0 .INVEN OR 2 1 DEA/-15 Y ESPI-EY TTORNEY Patented June 2, 1953 UNITED STATES PATENT OFFICE SWITCH FOR HIGH-FREQUENCY ELEC- TRICAL OSCILLATIONS Application October 11, 1948, Serial No. 53,868 In Great Britain July 29, 1947 8 Claims. 1

The present invention relates to switches for high frequency electrical oscillations. By a switch is meant a device by which the impedance between a pair of terminals can be made alternatively very large and very small.

The invention is concerned with switches in which the desired change of impedance is obtained without making and breaking electrical contacts. One such switch, which forms the subject of Patent No. 2,404,832, comprises a uniform transmission line with distribution inductance and capacity, with substantially zero attenuation constant, of characteristic impedance Z and of length substantially equal to M 4 where A is the wavelength (or the mean wavelength) in the line of the oscillations to be switched and p is an odd integer. The outer and inner members of the line are connected at one end to the pair of terminals between which the impedance is to be changed, means are provided for approaching a conductor to the other (open) end of the line to provide a capacity C shunting the open end and a condenser of capacity C, connected permanently between the said pair of terminals, is made substantially equal to l/41r f ZoC, 1 being the frequency of the oscillations. Preferably p is made equal to unity.

When the capacity C shunts the open end, the impedance between the said pair of terminals is substantially infinite and when the conductor is withdrawn to reduce C to a negligibly small value the said impedance is substantially zero.

It is found diflicult in practice to arrange that the air gap between the conductor and the open end of the transmission line is small enough, and hence that C has a large enough value, to avoid the necessity for an inconveniently large value for C. Moreover, the path of current at the open end is somewhat indeterminate and may include parts of the mechanism by which the conductor is moved towards and away from the open end. This presents difficulties in design. The present invention has for its object to provide an improved switch in which these difficulties are avoided or substantially reduced.

According to the invention a switch for high frequency electric oscillations of wavelength or mean wavelength i comprises a substantially uniform line of electrical length ilk/4, where p is an integer, connected at one end to a pair of terminals between which switching is required, a second transmission line which at the wavelength x has a reactance X and a conductor by which the sion line can be coupled to the second line by a reactance which at the wavelength A is substantially equal to X, the conductor being capable of being withdrawn to render the coupling between the lines negligibly small. In this way, if p is an odd integer, when the conductor is in the approached position in which coupling between the lines is effected, the reactance at the open end of the first-named line is substantially zero and hence the impedance between the said terminals is substantially infinite. If on the other hand the conductor is withdrawn and the coupling between the lines is made negligibly small, the impedance at the open end of the first-named line is substantially zero.

If p is an even integer, when the conductor is in the approached position the impedance between the said terminals is substantially zero and when the conductor is withdrawn the impedance at the said terminals is substantially infinite.

The invention will be described, by way of example, with reference to the accompanying drawing in which Figure 1 is a diagrammatic illustration of one embodiment of the invention, 7

Figure 2 contains at (a) and (b) two curves illustrating the determination of the appropriate length for the said second transmission line,

Figure 3 is a diagrammatic illustration in crosssection of one embodiment of the invention,

Figure 4 is a diagrammatic illustration in crosssection of a part of a further embodiment of the invention and Figure 5 shows diagrammatically an application of the invention to two-way switching.

Referring to Figure 1, the first co-axial transmission line It] has a length A/i, that is to say 22 is made equal to unity, and the second co-axial transmission line II has its outer conductor continuous with that of the line iii. A capacitive coupling C1/wX, where w is the pulsatance corresponding to A and X is positive, can be provided by bringing a member 12 close to the inner conductors of the two lines Hi and H and C can be made very low by removing the member 12. The end l3 of the line H remote from the coupling C is shown open-circuited. The end [4 of the first transmission line H] has terminals l5 and I6 between which switching is desired.

In Figure 2 there is shown at (a) a curve of reactance of the line i i plotted against its length as a function of the wavelength A. Since the line H is required to have a positive reactance X, its length must be between V4 and V2, if a minimum length is required as is usually convenient. It could, have a value between 3M4 and or even a greater length if desired. The length, if between \/4 and V2, will usually be chosen fairly near to \/2 since, as is seen in Figure 2(a), the value of the positive reactance X is then relatively large and consequently the required value of C can be made relatively small.

If the line ..II .is' short-circuited at the end I3, its reactance will be related to its length as shown in Figure 2(b) and the length may then be chosen below V4 and preferably near to M4. If this length is too short for convenience a value between \/2 and 3M4 may be used.

A convenient form which the device of Figure 1 may take is illustrated in Figure 3. In this form the inner conductors of the two lines In and II at their adjacent ends I1 and I8 are made of increased area in order to increase the capacity between them for a given spacing. At the lower end of the outer conductor of the line [I is. provided a bellows I9 of conducting material and the inner conductor 26 of this line is mounted upon an insulating support 2|, since it is again assumed that the end I3 is to be open-circuited. A guide plate 22 of insulating material is provided to guide the inner conductor 20. By applying suitable tension or compression at the end |3,.the capacitance C between and [8 can be made negligibly small or equal to -1/wX respectively. An insulating plate 4| is provided between the opposing faces of I! and I8 to ensure that the desired value of C is obtained when pressure is applied to the end I3. When a shortcircuited line is desired for the line H, the support 2| may be made of conducting material.

The desired increased area of the opposing ends of the inner conductors of the lines l and Il may be obtained by simply attaching plates to these ends.

The outer .conductor of the line H may be extended below the support 2| as shown at 24 in Figure 3 in order to prevent appreciable energy loss from the open end l3 and its lower end may be closed. The design of lines to attain this result is well understood. The space within the bellows and the remainder of the outer conductor of the line H (and if desired also the space within the outer conductor of the line H], the end I4 being, if necessary, suitably closed with insulating material), may be filled with any desired atmosphere or liquid.

Many other means for varying the coupling at C may be used. Thus for instance the inner conductor 20, or an insulating extension thereof, may be made slidable through an aperture in the member 2| and the bellows I9 may be dispensed with. The two inner conductors may be fixed and a third member may be approached to the gap between their adjacent ends when coupling is desired as described with reference to Figure 1.

In the embodiment shown in Figure 4, the

capacity C is formed between a spigot on one end of the inner conductor 23 of the line H and a recess in the adjacent end of the inner conductor of the line In. There is fixed to the inner conductor 23 a contact member 34 having a number of spring fingers 35 making contact with the inner surface of the outer conductor of the line H. The inner conductor 23 passes through a guide 36 in which it is slidable and has a flange 31 to which is fixed one end of a bellows 38, the other end of which is fixed to the outer conductor. The bellows may be of metal or insulating material such for example as'rubber. The guide 36 is preferably of metal.

The inner conductor 23 may be moved, when the capacitive coupling is desired, by means of an eccentric 39 rotatable on a spindle 40, or otherwise. The bellows 38 forms a hermetic seal for the arrangement.

Figure 5' shows how theinvention may be applied to provide two-way switching whereby a line 25 may be placed in effective connection with either of two lines 26 and 21. In this example a switching system comprising four switches according to the invention is provided as shown at 2B, 29, 30 and 3|. The line 25 is connected to the lines26 and 21 by two lines 32 and 33 respectively, each )\/2 in length. The switches 29 and 30 are arranged at a distance A/ l on either side of the line 25 and the switches 28 and 3|are arranged at a distance of M4 from the switches 29 and 36 respectively. The couplings C in the four switches are represented by condensers C1, C2, C3 and C4 respectively.

If C1 and C2 are operative and C3 and C4 are withdrawn, the impedanoes at the terminals I51 and |5z in parallel across the line 26 are approximately infinite and those at the terminals I53 and I54 are approximately zero. Since the points [52 and I53 are spaced x/ i from the line 25, the impedance presented by the line '32- to the adjacent terminals of the line 25 is the normal impedance of the line 26 and the impedance presented by the line 33 to the same terminals is approximately infinite. Hence the two warv switch is set to couple line 25 to line 26 and to disconnect it effectively from line 21. .Obviously a reversal of the conditions of the four switches reverses the two-way switch,

Suitable means are preferably provided for approaching the conductors to form the capacitances C1 and C2 and simultaneously withdrawing those forming the capacitances C3 and C4.

The effectiveness of switching is, as already known, greatly increased by employing two or more switches 28, 29 or 30, 3| spaced by M4 (or an odd multiple of M4) rather than only a single switch in each line 26 and 27.

I claim:

1. A switch for high frequency electric oscillation of mean wave length A, comprising a first transmission line of electrical length gal/4 where p is an integer, a second transmission line having such an electrical length, and being terminated at one end in an impedance of such a nature, that at the wave length x it has an input impedance measured at its second end which is substantially a pure positive reactance, a coupling element, means guiding said coupling element for movement over a predetermined fixed path, and means for limiting the normal movement of said coupling element over said path so that at one limit of the normal movement of the coupling element said second end of the second transmission line is coupled to one end of the first transmission line by an impedance which at the wave length A is substantially a pure negative reactance of equal magnitude to said positive reactance and at the other limit of the normal movement of the coupling element said second end of the second transmission line is substantially decoupled from said one end of the first transmission line.

2. A switch according to claim 1, in which said first and second transmission lines are both coaxial lines and the outer conductors of the two coaxial lines are continuous.

3. A switch for high frequency electric oscillations of mean wave length x, comprising a first coaxial line of electrical length px/ i, where is an interger, a second coaxial line having an outer conductor continuous with the outer conductor of said first coaxial line, said second coaxial line having such an electrical length, and being terminated at the end remote from said first coaxial line in an impedance of such a nature, that at the wave length A it has an input impedance measured at the end adjoining said first coaxial line which is substantially a pure positive reactance, means guiding the inner conductor of said second coaxial line for longitudinal movement, and means for limiting the normal longitudinal movement of the inner conductor of the second coaxial line so that at one limit of the normal movement of said inner conductor the effective capacity between adjacent ends of the inner conductors oi the two coaxial lines has at the wave length x a negative reactance substantially equal in magnitude to said positive reactance and at the other limit of the normal movement of the inner conductor of the second coaxial line said effective capacity is substantially zero.

4. A switch according to claim 3, in which one of said adjacent ends is of smaller cross-sectional area than the remainder of the inner conductor of which it forms part, and the other of said adjacent ends is bored to permit entry of the first named adjacent end without electrical contact between the adjacent ends.

5. A switch according to claim 3, in which both said adjacent ends are of greater cross-sectional area than the remainder of the respective inner conductors of which they form part.

6. A switch according to claim 3, in which said means guiding the inner conductor of the second coaxial line comprises a bellows providing a mechanical coupling between the inner and outer conductors of the second coaxial line.

'7. A switch according to claim 6, in which part at least of the space enclosed by the outer conductors of the first and second coaxial lines is hermetically sealed from the exterior thereof, said bellows forming part of the hermetic seal.

8. A two-way switch system for switching high frequency electric oscillations of mean wave length A between a main transmission line and either of two subsidiary transmission lines, ends of all said lines being electrically connected at a single junction, said switching system com- 8 prising a first switch device connected to one of said subsidiary lines at a pair of terminals at an electrical distance pix/4 from said junction where m is an odd integer, and a second switch device connected to the other of said subsidiary lines at a pair of terminals at an electrical distance pix/4 from said junction where 202 i an odd integer, said first and second switch devices each comprising a first transmission line of electrical length gal/4 where p is an integer, connected at one end to the appropriate pair of terminals, a second transmission line having such an electrical length, and being terminated at one end in an impedance of such a nature, that at the wave length x it has an input impedance measured at its second end which is substantially a pure positive reactance, a coupling element, means guiding said coupling element for movement over a predetermined fixed path, and means for limiting the normal movement of the coupling element over said path so that at one limit of the normal movement of the coupling element said second end of the second transmission line is coupled to the end of the first transmission line remote from said terminals by an impedance which at the wave length A is substantially a pure negative reactance of equal magnitude to said positive reactance and at the other limit of the normal movement of said coupling element said second end of the second transmission line is substantially decoupled from the end of the first transmission line remote from said terminals.

DENNIS CLARK ESPLEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,404,832 Espley July 30, 1946 2,408,895 Turner Oct. 8, 1946 2,411,299 Sloan Nov. 19, 1946 2,510,064 Bryan June 6, 1950 2,530,089 Smith Nov. 14, 1950 OTHER REFERENCES Practical Analysis of Ultra High Frequency, by Meagher and Markley. Published by R. C. A. Service Co. Inc., Camden, New Jersey. Copyright 1943.

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