US3632941A

US3632941A - Constant impedance coaxial switch

Info

Publication number: US3632941A
Application number: US28776A
Authority: US
Inventors: Norman Wasserman
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1970-04-15
Filing date: 1970-04-15
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04

Abstract

A coaxial cable switch is disclosed which maintains a constant impedance through the switch by providing a contact configuration with the same ratio of inner conductor diameter to outer conductor diameter as is provided by the cable. The switch is operated by inserting a conductive liquid into a cavity resulting from a discontinuity in the inner conductor of the switch, thereby bridging the discontinuity.

Description

Unite States Patent Norman Wasserman Columbus, Ohio Apr. 15, 1970 Jan. 4, 1972 Bell Telephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.

Inventor Appl. No. Filed Patented Assignee CONSTANT IMPEDANCE COAXIAL SWITCH 8 Claims, 2 Drawing Figs.

U.S. Cl 200/182, ZOO/81.6, 200/153 S, 200/195, 200/211, 200/214 Int. Cl H0111 29/28 Field of Search 200/81 .6,

[56] References Cited UNITED STATES PATENTS 2,762,881 9/1956 Brockwell et al. 200/153.18 X 3,184,693 5/1965 Lanctot ZOO/153.18 X 3,278,713 10/1966 Crupen ZOO/152.9

Primary Examiner-Herman J. Hohauser Attorneys-R. J Guenther and Edwin B. Cave ABSTRACT: A coaxial cable switch is disclosed which maintains a constant impedance through the switch by providing a contact configuration with the same ratio of inner conductor diameter to outer conductor diameter as is provided by the ca ble. The switch is operated by inserting a conductive liquid into a cavity resulting from a discontinuity in the inner conductor of the switch, thereby bridging the discontinuity.

i/RELEASE ATE PATENTEDJRN 4:972

525ml @25 5 A l l l I I I ll INVENTOR M WASSERMAN 1 ATTORNEY CONSTANT IMPEDANCE COAXIAL SWITCH This invention relates to fluidically actuated switching devices and more particularly to such devices as are suitable for use in communication switching networks, specifically networks used for the switching of high-frequency signals, such as radio and video frequency signals and coaxial cable transmission lines.

Background of the Invention The communication systems widely used at the present time employ large numbers of switches to provide the numerous interconnections required within the system. The desirability of miniaturizing these switches is apparent and requires no explanation. However, miniaturization of conventional switches used to carry high-frequency signals in a coaxial cable introduces a problem. Since the conventional contact configuration is that of two overlapping blades, commonly known as the reed switch," appreciable capacitive coupling is introduced between the contacts in an open condition when high-frequency signals are present. A good example of this switch configuration, and an early attempt to solve the capacitive coupling problem, is US. Pat. No. 3,355,684 issued Nov. 28, 1967 to D. S. Church and R. W. Kordos. In addition, a relatively high contact resistance is obtained from the operated switch, and perhaps as disturbing, the resistance is of a variable nature. This necessarily affects the return loss characteristics as well as the insertion loss.

A further serious problem results from the change in shape between the contacts of the switch and the conductors in the coaxial cable. Since the impedance in coaxial cables is a function of the ratio between the inner and outer conductors, it is apparent that switches of this type will necessarily introduce impedance variations into the transmission system.

It is therefore an object of my invention to provide a switching device which introduces essentially no impedance variation into the transmission path.

It is also an object of my invention to provide a switching device having very low contact resistance.

It is a further object of my invention to provide a switching device which has its overall size miniaturized yet permits a large spatial separation between the switch contacts to minimize capacitive coupling when the switch is in the open state.

SUMMARY OF THE INVENTION In an illustrative embodiment of my invention a switch is provided to selectively connect two discontinuous ends of the center conductor of a coaxial cable. The two contacts of the switch have a circular cross section with the same diameter as that of the inner conductors of the cable. To operate the switch, a conductive liquid from a reservoir within the switch is forced into a chamber between the two switch contacts. Since the outside diameter of the chamber is the same as that of the inner conductor of the cable, no impedance variations are introduced. Also, since a large separation may be obtained between the contacts in an open condition, and a wetted contact is obtained in a closed condition, capacitive coupling and contact resistance are minimized in the switch.

DESCRIPTION OF THE DRAWING FIG. I is a cross-sectional view of an unoperated switch embodying my invention; and

FIG. 2 is a cross-sectional view of the device of FIG. 1, shown operated.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT The switch shown in FIG. 1 comprises

contacts

101 and 107, corresponding to the inner conductor of a coaxial cable, which are separated from each other by chamber 105. Coaxial with contact 101, and located at its inner end, is cavity 102. Contact 107 also has a coaxial cavity 108 at its inner end. This cavity is substantially larger than cavity 102 and acts as a reservoir for a supply of liquid mercury. Surrounding the contacts and chamber is dielectric I10 and outer conductor III of the coaxial cable. Cavity I02 connects through a fluid port to a source of fluid actuating pulses (not shown). Reservoir I08 connects through fluid port II6 to a second source of fluid actuating pulses (also not shown). Ports I15 and 116 are, of course, constructed from an insulating material to prevent the shorting of the inner and outer conductors. The inner diameter of ports 115 and I16 is small enough so that, due to its high surface tension, mercury from reservoir 108 and cavity I02 will not enter the ports.

To operate the switch, as shown in FIG. 2, a fluid pulse is applied to the reservoir through port 116. Mercury is forced out of cavity 108 and into chamber 105 and cavity 102. When chamber 105 is completely filled, a wetted contact having almost zero resistance is obtained between contact I01 and contact 107. Since the outside diameter of chamber 105 is the same as that of

contacts

101 and 107 which act as inner conductors in the coaxial cable, it should be apparent to those understanding the problems of switching high-frequency signals that the impedance of the switch is the same as that of the coaxial cable, since the ratio between the diameters of inner and outer conductors is the same for both.

To release the switch assembly and return it to the condition shown in FIG. I, a fluid pulse is applied to cavity 102 through port 115. The mercury in chamber 105 is forced back into reservoir I08, interrupting the contact established between conductors I01 and 107.

In order to minimize the capacitive coupling between conductors I01 and 107, the width of chamber 105 may be made quite large. Naturally, cavity 108. and the supply of mercury in the reservoir must be correspondingly increased. However, no matter how large chamber 105 is made, the impedance through the switch remains unchanged since the ratio between the diameters of the inner and outer conductors is unchanged. Also, increasing the size of chamber 105 has little effect on the resistance of the switch. Since a wetted contact is obtained at both contacts, the change in switch resistance is merely the resistance of the additional mercury, which is effectively zero regardless of the quantity necessary to fill chamber 105.

Although the switch described is fluidically actuated, other actuation modes are equally applicable. For example, the switch may be gravity actuated or magnetically actuated using magnetic mercury" consisting of a colloidal suspension of iron particles in a mercury base. Any skilled designer could devise a number of actuation modes and a variety of switch configurations without departing from the inventive principles disclosed. However, the switch described is probably the simplest, smallest, most economical and effective structure possible.

What I claim is:

l. A coaxial switch comprising a continuous cylindrical outer conductor,

a discontinuous cylindrical inner conductor, coaxial with the outer conductor and having a first end and a second end, wherein the length of the discontinuity is sufficient to prevent capacitive coupling between the first and second ends when the switch is unoperated,

a reservoir of conducting liquid,

means for operating the switch by inserting liquid from the reservoir into the discontinuity between the first and second ends of the inner conductor, thereby establishing a conducting path between the first and second ends, and

means for constraining the inserted liquid to impart a peripheral configuration thereto which is substantially the same as that of the inner conductor,

whereby the impedance of the switch is essentially equal to that of a coaxial cable of similar size since the ratio between the diameters of the inner and outer conductors is the same through the switch as it is through the cable.

2. A coaxial switch in accordance with claim I wherein the operating means comprises means for applying a pressurized fluid pulse to the liquid reservoir.

3. A coaxial switch in accordance with claim I wherein the reservoir is located within the first end of the discontinuous inner conductor.

4. A coaxial switch in accordance with claim 1 wherein the constraining means comprises a dielectric sleeve between the inner and outer conductors.

5. A coaxial switch in accordance with claim 1 further including means for releasing the switch by evacuating the discontinuity and returning the inserted liquid to the reservoir, thereby interrupting the conducting path between the first and second ends.

6. A coaxial switch in accordance with claim 5 wherein the releasing means comprises means for applying a pressurized fluid pulse to the discontinuity.

7. A coaxial switch in accordance with claim 6 wherein the operating means comprises means for applying a pressurized fluid pulse to the liquid reservoir.

8. A coaxial cable switch comprising a hollow cylindrical insulator;

an outer conductor circumscribing the insulator;

a first cylindrical inner conductor with a coaxial cavity, the

conductor filling one end of the hollow in the insulator;

a second cylindrical inner conductor with a coaxial reservoir, separated from the first conductor and filling the other end of the hollow in the insulator, wherein said separation has a length sufficient to prevent capacitive coupling between the first and second conductors when the switch is unoperated;

conductive liquid normally contained in the reservoir;

a nonconductive fluidic conduit communicating with the reservoir for delivering thereto a fluidic actuation pulse causing liquid from the reservoir to fill the separation between the inner conductors and establish conductivity therebetween, the liquid being constrained by the hollow insulator so that the outer diameter thereof is equal to that of the inner conductors; and

a nonconductive fluidic conduit communicating with the cavity for delivering thereto a fluidic actuation pulse causing the liquid filling the separation between the inner conductors to return to the reservoir, thereby interrupting the conductivity established between the inner conductors.

Claims

1. A coaxial switch comprising a continuous cylindrical outer conductor, a discontinuous cylindrical inner conductor, coaxial with the outer conductor and having a first end and a second end, wherein the length of the discontinuity is sufficient to prevent capacitive Coupling between the first and second ends when the switch is unoperated, a reservoir of conducting liquid, means for operating the switch by inserting liquid from the reservoir into the discontinuity between the first and second ends of the inner conductor, thereby establishing a conducting path between the first and second ends, and means for constraining the inserted liquid to impart a peripheral configuration thereto which is substantially the same as that of the inner conductor, whereby the impedance of the switch is essentially equal to that of a coaxial cable of similar size since the ratio between the diameters of the inner and outer conductors is the same through the switch as it is through the cable.

2. A coaxial switch in accordance with claim 1 wherein the operating means comprises means for applying a pressurized fluid pulse to the liquid reservoir.

3. A coaxial switch in accordance with claim 1 wherein the reservoir is located within the first end of the discontinuous inner conductor.

8. A coaxial cable switch comprising a hollow cylindrical insulator; an outer conductor circumscribing the insulator; a first cylindrical inner conductor with a coaxial cavity, the conductor filling one end of the hollow in the insulator; a second cylindrical inner conductor with a coaxial reservoir, separated from the first conductor and filling the other end of the hollow in the insulator, wherein said separation has a length sufficient to prevent capacitive coupling between the first and second conductors when the switch is unoperated; conductive liquid normally contained in the reservoir; a nonconductive fluidic conduit communicating with the reservoir for delivering thereto a fluidic actuation pulse causing liquid from the reservoir to fill the separation between the inner conductors and establish conductivity therebetween, the liquid being constrained by the hollow insulator so that the outer diameter thereof is equal to that of the inner conductors; and a nonconductive fluidic conduit communicating with the cavity for delivering thereto a fluidic actuation pulse causing the liquid filling the separation between the inner conductors to return to the reservoir, thereby interrupting the conductivity established between the inner conductors.