US2472274A - High-frequency coaxial cable switch - Google Patents

Description

June 7, 1949. s. M. BELESKAS HIGH-FREQUENCY COAXIAL CABLE SWITCH Filed April 23, 1946 1 N V EN TOR. I?! IV. 52/54 45 I Arron [y Patented June 7, 1949 HIGH-FREQUENCY COAXIAL CABLE SWITCH Stanley M. Beleskas, Oaklyn, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 23, 1946, Serial No. 664,160

1 Claim. 1

This invention relates to switches for connecting the inner and outer conductor of one coaxial cable, selectively, to the corresponding conductors of a plurality of similar cables.

The prior art recognizes two general types of switches for coaxial cables, i. e. the rotary type described by Goddard in U. S. Patent 2,360,219 and the sliding type disclosed in U. S. Patent 2,235,010 to Chafi'ee. Irrespective of the relative advantages of the two it may be said generally that the complete shielding of the inner conductors (which is essential in a high frequency system) is achieved at considerable sacrifice in operating convenience. That is to say, if complete shielding is to be achieved in a rotary type switch the rotor and stator elements must have a tight fit and, as a result, the torque required to move the rotor is considerably greater than is desirable in a device designed to handle relatively small currents. Electrically operated coaxial switches of the sliding type are also cumbersome and expensive since duplicate motors or relay armatures are usually provided for the inner conductor of each branch of the main transmission line. Accordingly, the principal object of the present invention is to obviate the foregoing disadvantages of present day coaxial switches and to provide an improved switch which shall be characterized, particularly, by its ease of operation.

Another and important object of the present invention is to provide a simple and reliable coaxial switch and one which presents a minimum mismatch in impedance to the transmission lines which it is designed to serve.

Stated generally, the foregoing and other objects are achieved in accordance with the invention by making one of the inner conductors of the switch in the form of a simple lever and by so shaping its surrounding outer conductor that when the lever is moved to either of its closed positions the capacitance, and hence the characteristic impedance of the switch is uniform at all points along the lever and corresponds to that of a standard section of the transmission line which the switch is designed to serve. The peculiar shape of the said outer conductor or bore of the switch, and the complete shielding afforded by the switch casing, minimizes any mismatch in impedance occasioned by the presence of the switch in the transmission line. The force required to operate the lever-like switch arm of the present invention is considerably less than is required to move the tight fitting rotors and slidable connectors of the prior art and this reduction in operation permits the use of a very low-power,

inexpensive and compact actuating device when automatic operation is required.

Other objects and advantages will be apparent and the invention itself will be best understood upon reference to the following specification and to the accompanying drawing wherein:

Fig. 1 is a view in perspective, with the cover removed, of a coaxial switch constructed in accordance with the principle of the invention, Fig. 2 is a sectional view taken on the line II--II, and Fig. 3 is a similar view taken on the line IIIIII of Fig. 1.

In the drawing, I designates generally a metal casing which, as indicated by the line la in Figs. 2 and 3, is preferably of split construction so that access may be had to the interior of the switch for purposes of assembly and inspection. This casing l is provided with a longitudinal bore 3 of a special tapered shape, later described, and with a switch cavity 5 at one end of the bore. Two inner conductors l and 9 terminate, within the cavity, in contact elements la, and 9a, respectively, which are fixedly mounted, as by means of insulating inserts 8, in spaced relation along an axis substantially normal to the longitudinal axis of the bore 3. A third contact Ila is supported for movement, between the contacts Ia and 9a, on the end of an arm or lever II which comprises the inner conductor of the third or main branch of the switch. Three suitable jacks or other coupling units Tb, 9b, llb of the coaxial type are provided on the outer surface of the casing for connecting the switch in circuit. Thus, if the switch is to be incorporated in a high frequency radio transceiver the inner conductor of the coaxial cable to the antenna (not shown) will ordinarily extend through the jack llb to the movable switch arm II. In this case, the inner conductors I and 9 of the coaxial cables for the receiving and transmitting units should extend through the other jacks 1b and 9b to the stationary switch terminals la and 9a, respectively.

In any event, it will be apparent that since the outer shells of the 'jacks 1b, 9b, llb are conductively connected to the switch casing I, the casing itself comprises an electrically common outer conductor, or shield, for all of the inner conductors to which the switch is connected.

As previously pointed out, the branch conductor II is the movable arm of the switch. Its movement may be facilitated by a localized reduction in its diameter or by means of flexible insert or pivot llc adjacent to the narrow end of the tapered bore or conduit 3. The force for moving this conductive arm or third-class lever H is applied by an insulating plunger I3 which extends through a suitable aperture I in the casing I and is connected within the bore 3 to the said lever at a point intermediate its fulcrum Ho and weight Ila. Since the force required to move the lever arm I I to a position whereat its contact I Ia engages one or the other of the contacts Ia or 9a is very small, the source of the actuating force may likewise be small and may in fact comprise any convenient device such, for example, as the low powered electro-mechanical relay shown at IT.

The shape of the bore or conduit 3 is an important feature of the invention. That this is so will be apparent When it is appreciated that if the inner diameter of this conduit 3 were uniform throughout its length, the inner conductor II would approach quite close to the inner surface of the conduit when said conductor is moved ofi the central axis of the conduit. This would materially alter the capacitance and hence the characteristic impedance of this branch of the switch and attenuate the high frequency current which flows therethrough. Such a mismatch in impedance is minimized, in accordance with the invention, by so shaping the inner wall of the conduit 3 that when the lever or inner conductor I I is moved to bring its contact I Ia into engagement with one or the other of the contacts Ia or 9a any increase in capacitance (or decrease in impedance) occasioned by the approach of the lever I I to the top portion of the inner surface of the outer conductor or bore 3 is vcompensated, sufiiciently, by a decrease in capacitance (or increase in impedance) occasioned by a corresponding increase in the spacing between the said lever and the bottom portion of the inner surface of the said bore.

The shape of the conduit best suited for achieving the above described impedance compensating effect is the tapered elliptical shape shown in cross-section in Figs. 2 and 3, though good results may be achieved with a conduit which tapers outwardly from a circle to an oblong rectangle. In either event the long axis of the enlarged end of the conduit 3 (i. e. adjacent to the switch cavity 5) should be substantially parallel to the axis along which the con.- tact i la moves to engage the contacts Ia and 9a. In one successful embodiment of the invention wherein the switch was designed to connect coaxial cables wherein the inside diameter of the outer conductor was one-half of an inch and theout-side diameter of the inner conductor was one-eighth of an inch, the length of the bore 3 was approximately two inches and the axial dimensions of its elliptical or cavity end were; approximately five-eighths of an inch as measured on its longest axis and one-half of an inch as measured on its shortest axis,

It will now be apparent that the present invention provides a coaxial switch which presents a minimum mismatch in impedance to the transmission lines which it is designed toserve, and one characterized by the lower power required to actuate it.

What is claimed is:

A switch for selectively connecting two of three coaxial cables, said switch comprising a conductive casing constituting an electrically common outer conductor for all three of said coaxial cables and having a longitudinally tapered bore terminating at its larger end in a cavity within said casing, a pair of cable contacts mounted in spaced relation within said cavity along an axis substantially normal to the direction of extension of said bore, an inner conductor for the other one of said coaxial cables mounted for move.- ment within said bore and terminating in the space between said pair of cable contacts, and means for moving said inner conductor in a direction substantially parallel to said axis whereby to bring its said terminal end into engagement with a selected one of said pair of cable contacts, said tapered bore being elliptical in cross-section adjacent to said cavity, the long axis of said ellipse being substantially parallel to the axis along which said spaced apart cone tacts mounted whereby when said inner conductor is brought into engagement witheither of said cable contacts any increase in capacitance occasioned by the approach of said conductor to. one surface portion of said bore is compensated by a decrease in capacitance occasioned by an increase in the spacing between said inner conductor and the opposite surface portion of said bore.

STANLEY M. BE-LESKAS.

CITED The following references are of recordin the file or this patent:

UNITED STATES PATENTS l-lrunber Name Date 2,251,997 Gcldmann Aug. 12, 1941 2,425,010 Smith Aug. 5, 1947 FOREIGN PATENTS Number Country Date 852,646 France Oct. 30, 1939

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