US3194921A

US3194921A - Radio frequency mercury switch

Info

Publication number: US3194921A
Application number: US192827A
Authority: US
Inventors: Jr Chester B Watts
Original assignee: Scanwell Laboratories Inc
Current assignee: Scanwell Laboratories Inc
Priority date: 1962-05-07
Filing date: 1962-05-07
Publication date: 1965-07-13
Anticipated expiration: 1982-07-13

Description

,July 13, 1965' c. B. WATTS, JR

RADIO FREQUENCY MERCURY SWITCH Filed May '7, 1962 ATTORNEYS United States Patent 3,194,921 RADIO FREQUENCY MERCURY SWITCH Chester B. Watts, Jr., Annandale, Va., assiguor to Scanwell Laboratories, Inc., Springfield, Va., a corporation of Virginia Filed May 7, 1962, Ser. No. 192,827 5 Claims. (Cl. 200-112) This invention relates to radio frequency switching, and more specifically to an electrically actuated, twoposition, mercury-wetted reedtype switch for microwave and lower frequencies.

It is an object of the invention to provide a switch having a novel and useful combination of properties. These include an operable frequency band extending from DC. to more than 10,000 megacycles, together with an actuating time of about one milli-second and the long life expectancy characteristic of prior reed switches. I

An embodiment of this invention is described with reference to the accompanying drawings wherein:

FIG. 1 is a longitudinal section, taken through the center of the switch.

FIG. 2 is an exploded view of the switch to clarify the relationship of the parts.

The switch of the present invention may be classed as a single-pole double-throw type in a coaxial transmission line structure. The switch is of T-shape with a housing having a leg and side arms, the housing being made in symmetrical halves, a front half 2, and a rear half 4. The housing halves are clamped together by three spring rip rings 6, 8 and 10. Raised projections 12 or other means are provided for keeping the housing halves separated everywhere except in the regions near the extremities of the T -shape where the grip rings are located, for reasons to be explained later.

A tubular glass envelope 14 is placed vertically within the leg of the housing. Iron

alloy side members

16 and 18 are sealed through the left and right sides of envelope 14 near its upper end, and serve as alternate inner-conductor paths. It is, of course, desirable that the electrical capacitance between the alternate innerconductor paths be kept low so that the path being switched ott is not appreciably coupled to the path which is switched on. Prior double-throwreed switches have had the alternate conductors sealed out the end of the envelope essentially parallel to one another. This resulted in a coupling capacitance which was much too large to allow good switching performance at microwave frequencies. I have found that by bringing the alternate conductors in from opposite sides as shown, and by conically tapering the facing ends, it isv a practical matter to construct a switch with an isolation ratio as great as 20 decibels at 10,000 megacycles. The parts of the

side members

16 and 18 which project outside envelope 14 are preferably plated with silver toreduce microwave losses,

, but the base material must be an iron-alloy to provide a magnetic flux path for actuating the reed.

Welded to the facing ends of

side members

16 and 18 are the left and right contacts 20 and 22 which are preferably made small in comparison with

side members

16 and 18 so as not to contribute appreciably to the coupling capacitance. Contacts 20 and 22 must be made of platinum or some other material which is readily wet by mercury.

The upper end of envelope 14 is sealed or domed over with glass. The lower end is sealed to an ironalloy filler tube 24 which ha welded to its upper end the resilient reed 26. Reed 26 extends centrally upwards far enough to extend between and reach slightly beyond contacts 20 and 22. Reed 26 is made of a strip of iron-alloy, a practical embodiment being approximatep 7 3,194,921 Patented July 13, 1965 ICC 1y .007 inch thick, .060 inch wide, and one inch long.

It is finely grooved vertically and plated with platinum i capillary attraction up the reed until the weight of mercury on the reed balances the capillary forces.

1 In operation, reed 26 is magnetically deflected to bear 7 against contact 20 or contact 22 and in doing so transfers sufiicient mercury to wet the contact. The remainder of the envelope is filled with dry hydrogen.

Insulation beads 30 and 32, made preferably of Teflon, I

give support to the

side members

16 and 18 and help to keep them centered in their respective side arms of the housing.

Filler tube 24, mercury 28, reed 26, contact 20 and side member 16 together constitute an inner RJF. conductor of the switch, while the housing halves 2 and 4 constitute the outer RF. conductor. The inside dimensions of the housing are chosen with respect to the outside dimensions of the various parts constituting the inner conductor and are so spaced therefrom as to keep as nearly as possible a constant coaxial characteristic impedance of 50 ohms throughout the switch. In addition, the diameter of glass envelope 14 must be small enough to prevent propogation of modes of higher order than the desired TEM coaxial mode. This requires that the mean circumfernce of the glass tube be somewhat less than the wavelength divided by the square root of the dielectric constant. I have found that a glass tubing with an outer diameter of .200 inch modes at about 8000 megacycles but it the outer diameter is reduced to .150 inch, the moding does not occur until a frequency of about 11,000 megacycles is reached. However, the use of such a small tube creates a problem with the mercury. The high surface tension of mercury pro duces a meniscus of relatively large radius which prevents free flow through a constricted passage which is not wet by the mercury. This applies to the filler tube and to the weld area around the base of the reed. The mercury can be transferred through the constriction by shaking the switch like one does a fever thermometer,

but since one cannot predict just where the mercury will always be, it is not possible to design the housing shape to match the position of the mercury. Specifically, the

the bottom end of the reed and extending down into the filler tube. The wire 36, being in contact with the mercury inthe envelope, and being wet by it, provides a free-flowing path between the envelope and the filler tube. The result of this is that although the switch may be turned upside down and shaken as in shipment, nevertheless, when it is set upright in its normal position for operation the mercury will flow down and assume its proper position. I

Numeral 38 denotes a permanent bar magnet located above, adjacent and parallel to the

side members

16 and 18. Actuating electromagnetic coil 40 surrounds the vertical part of the housing and acts in cooperation with magnet 38 to magnetically deflect reed 26 to the left or right. Magnet 38 provides a fixed polarizing field in

side members

16 and 18. Application of current to coil 40 produces the actuating field which adds to the polarizing field in one member and subtracts from the polarizing field in the other member causing the reed to move to the stronger side. Which side is the stronger depends on the polarity of the polarizing magnet 38 and on the direction of current flow in the actuating coil 40.

I have found that by dividing the housing as shown into the two essentially non-contacting halves 2 and 4, the actuating time is significantly reduced as compared to the time obtained with a solid housing. This is because the actuating time depends among other things upon the time required for the magnetic field distribution in the reed and side members to change. A solid housing has a short-circuited-turn effect, producing an induced field wihch opposes the change.

In any event the housing must be divided in some manner in order to be able to assemble it around the envelope. A further advantage to dividing it in the manner shown is that the plane of division does not cut through any lines of R.F. current flow. This fact eliminates the necessity for contact between the two parts from the RF. standpoint and thus removes the possibility of RF. loss associated with contact resistance in the housing.

While a single specific embodiment has been shown and described, the invention may take other forms within the scope of the appended claims.

I claim:

1. A two-position, mercury wetted, radio frequency switch comprising; a generally T-shaped conductive housing having a central leg and side arms; a tubular glass envelope disposed within said central leg; ferromagnetic side members extending laterally through opposed sides of said envelope at one end thereof and into said side arms, the inner ends of said side members being tapered toward each other and defining end faces of very small area adjacent but spaced from each other within said envelope and defining spaced contacts; a ferromagnetic filler tube extending axially into the other end of said envelope; a resilient ferromagnetic reed secured to the inner end of said filler tube and extending within said envelope to a position between said spaced contacts; a body of mercury in said filler tube and extending into said envelope; the surface of said reed and said spaced contacts being covered with material wettable by mercury; a permanent magnet disposed adjacent and parallel to said side members; and an electromagnetic coil surrounding said leg of said housing and said envelope; said ferromagnetic side members being elongated rod-like members arranged in coaxial relation whereby to minimize capacitance therebetween.

2. A two-position, mercury wetted, radio frequency switch comprising; a generally T-shaped conductive housing having a central leg and side arms; a tubular glass envelope disposed within said central leg; ferromagnetic side members extending laterally through opposed sides of said envelope at one end thereof and into said side arms, the inner ends of said side members being adjacent but spaced from each other within said envelope and defining spaced contacts; a ferromagnetic filler tube extending axially into the other end of said envelope; a resilient ferromagnetic reed secured to the inner end of said fillter tube and extending within said envelope to a position between said spaced contacts; a body of mercury in said filler tube and extending into said envelope; the surface of said reed and said spaced contacts being covered with material wettable by mercury; a permanent magnet disposed adjacent and parallel to said side members; and an electromagnetic coil surrounding said leg of said housing and said envelope; said housing comprising two longiwith material wettable by mercury;

tudinally extending halves, each defining a half of said leg and of each of said side arms, the adjacent edges of said halves being spaced apart throughout substantially their entire lengths.

3. A two-position, mercury wetted, radio frequency switch comprising; a generally T-shaped conductive housing having a central leg and side arms; a tubular glass envelope disposed within said central leg; ferromagnetic side members extending laterally through opposed sides of said envelope at one end thereof and into said side arms, the inner ends of said side members being tapered toward each other and defining end faces of very small area adjacent but spaced from each other within said envelope and defining spaced contacts; a ferromagnetic filler tube extending axially into the other end of said envelope; a resilient ferromagnetic reed secured to the inner end of said filler tube and extending within said envelope to a position between said spaced contacts; a body of mercury in said filler tube and extending into said envelope; the surface of said reed and said spaced contacts being covered with material wettable by mercury; a permanent magnet disposed adjacent and parallel to said side members; and an electromagnetic coil surrounding said leg of said housing and said envelope; said means wettable by mercury including a filamentary portion thereof extending from said reed into said filler tube.

4. A switch as defined in claim 3 wherein said filamentary portion comprises a platinum wire secured to said reed inwardly of the inner end of said filler tube.

5. A two-position, mercury wetted, radio frequency switch comprising; a generally T-shaped conductive housing having a central leg and side arms; a tubular glass envelope disposed within said central leg; ferromagnetic side members extending laterally through opposed sides of said envelope at one end thereof and int-o said side arms, the inner ends of said side members being tapered toward each other and defining end faces of very small area adjacent but spaced from each other within said envelope and defining spaced contacts; a ferromagnetic filler tube extending axially into the other end of said envelope; a

. resilient ferromagnetic reed secured to the inner end of said filler tube and extending within said envelope to a position between said spaced contacts; a body of mercury in said filler tube and extending into said envelope; the surface of said reed and said spaced contacts being covered a permanent magnet disposed adjacent and parallel to said side members; and an electromagnetic coil surrounding said leg of said housing and said envelope; said leg of said housing comprising an outer conductor and said filler tube, body of mercury and reed comprising a coaxial inner conductor, the leg of said housing having a relatively large inner diameter from the end thereof to the inner end of said body of mercury and a smaller diameter from there to said side arms, to provide a substantially constant characteristic impedance along the length of said switch.

References Cited by the Examiner UNITED STATES PATENTS 2,264,124 11/41 Schreiner 200-153 X 3,008,021 ll/61 Pollard 2001 12 X BERNARD A. GILHEANY, Primary Examiner. ROBERT K. SCI-IAEFER, Examiner.