US3467883A - Microwave switching device - Google Patents

Description

Sept. 16, 1969 o m ET AL MICROWAVE SWITCHING DEVICE Filed Sept. 2, 1966 INVENTORS HARRY GOLDIE 8| [3 ATTORNEY THEODORE M. N E LSON WITNESSES WWW %WJ@ United States Patent US. Cl. 31539 3 Claims ABSTRACT OF THE DISCLOSURE A microwave switching device in the form of a resonant discharge device in which a tuning post of a resonant iris cooperates with a sidewall of a waveguide to constitute a discharge gap within the waveguide. A keep-alive electrode, located in a chamber external to the waveguide, is positioned in alignment with the tuning post and an aperture in the waveguide.

This invention relates generally to microwave frequency gaseous discharge switching devices and more particularly to those devices incorporating a keep-alive electrode.

In prior art microwave switching tubes, more commonly referred to as TR tubes, the devices consisted of two truncated cones geometrically opposed within a wave guide portion or a cavity resonator. The space between the two cones provided the discharge gap region in which breakdown resulted on the transmission of adequate microwave frequency energy. It is desirable to provide a keep-alive electrode structure which will furnish a residual supply of electrons adjacent to this discharge gap region to ensure reliable operation; that is to obtain a consistent breakdown level with each incident radio frequency pulse.

In the prior art design utilizing the two cone electrodes, the most common practice was to provide an internal keep-alive electrode within a hollowed out portion of one of the cone electrodes. The geometrical constraints on the design of such a cone electrode including the keepalive electrode provide a multitude of problems. For example, an insulating sleeve is normally provided between the hollowed out portion of the cone and the keepalive electrode. The insulating sleeve was therefore in intimate contact with the negative keep-alive electrode and also in the region adjacent to the discharge gap. The close proximity of the insulator to the keep-alive electrode is found to be one of the significant reasons for short lifetime in TR devices. This comes about because the heavy positive ions are accelerated toward the negative keep-alive electrode, bombarding it and causing the liberation of metallic particles. These metallic particles diffuse toward the insulating sleeve and coat it with the metallic deposit. This coating causes a continuous and slow change in the static field distribution between the cone tip and the negative discharge electrode. This leads at first to unreliable radio frequency gap priming with consequent fiuxation in the leakage characteristics, and ultimately to a complete short circuit between the keepalive electrode and the cone electrode.

It is accordingly the principal object of this invention to provide an improved switching device to overcome the problems of the prior art.

It is another object to provide an improved switching device in which an improved keep-alive electrode system is provided.

It is another object to provide an improved keep-alive electrode system for a switching device in which bom- 3,467,883 Patented Sept. 16, 1969 bardment of the keep-alive electrode by positive ions does not reduce the lifetime of the device.

'It is still another object of the invention to provide an improved keep-alive electrode system for a switching device in which the insulating member holding the keepalive electrode from the switching device is remote from the discharge region.

It is still another object to provide an improved baflle structure for a single cone device.

Briefly, the present invention accomplishes the abovecited objects by providing a microwave switching device utilizing a single cone electrode, full height wave guide with an aperture provided in the wall of the wave guide opposite the apex of the single cone electrode. The keepalive electrode is positioned exterior of the aperture and supported within a tubular metallic member with an in sulating sealing member remote from the aperture positioning and retaining the metallic electrode within the tubular member and providing a long path therebetween. An improved baffle is also provided to obtain tuning in conjunction with the single cone.

These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is a perspective view of an embodiment of the invention with a portion of the switching device broken away to reveal internal structure; and

FIG. 2 is an enlarged sectional view of the embodiment shown in FIG. 1 taken along the line II-II.

Referring now to the drawings, the embodiment shown in FIGS. 1 and 2 is a TR switching tube comprising a section of a rectangular wave guide 2 of good electrical conductive material such as copper defining parallel broad upper and lower walls 4 and 6 and narrow side walls 8 and 10. Secured adjacent the ends of the wave guide 2 are conventional mating flanges 12 and 14. The flanges 12 and 14 are utilized to secure the switching device to suitable microwave energy wave guides. A resonant window assembly 16 closes each end of the wave guide section 2 to provide a hermetically sealed envelope 3. Each window assembly 16 includes a dielectric member transmissive to microwave energy. The envelope 3 defined by the wave guide section 2 and window assemblies 16 contains a gaseous ionizable atmosphere such as argon gas or water vapor at reduced pressure such as 10 torrs to provide a high intense gaseous discharge when high power microwave frequency energy is transmitted therethrough. In the case of an X-band switching device the dimensions of the rectangular wave' guide 2 are 1 inch by /2 inch.

Positioned within the wave guide 2 is a baffle 21 which includes two symmetrical electrically conductive vanes 22 and 24 transverse to the wave guide 2. Mounted between the inner edges of the vanes 22 and 24 is a tubular member 33 of a suitable electrically conductive material such as copper. The tubular member 33 is secured to the bottom wall 6 and extends about /5 of the distance to the upper wall 4. The inner edges of the vanes 22 and 24 are secured to the outer surface of the tubular member 33. The edges of the vanes 22 and 24 between the member 33 and the upper wall 4 are recessed back so as to provide an iris 31.

An electrode 26, having a cylindrical cross-section, extends through the lower wall 6 of the wave guide 2 and thorugh the tubular member 33 to the iris 31. The elec trode 26 is in sliding engagement with the member 33. The lectrode 26 includes a pointed portion 28 with the apex 29 of the pointed portion 28 adjacent the wall 4. The pointed portion 28 of the electrode 26 is within the iris 31 and constitutes a tuning post for the latter. The

electrode 26 is of a suitable electrically conductive material such as Kovar alloy. The electrode 26 is positioned and retained within the wave guide section 2 by means of an annular member 32 which is of a suitable material such as nickel which is brazed to a seat portion in the outer surface of the wall 6. A flexible diaphragm 34 which is also annular in shape and of a suitable material such as copper has its outer periphery brazed to the member 32 and its inner periphery to the electrode 26. The diaphragm 34 hermetically seals the electrode 26 within the wave guide 2 and also allows movement of the electrode 26 within the member 33 so as to provide the proper space for a discharge gap. The spacing of the apex 29 of the pointed portion 28 of the electrode 26 from the wall 4 may vary considerably. An aperture 40 is provided in the wall 4 opposite the apex 29 of the pointed portion 28. The diameter of the aperture 40 is about 20 mils.

Positioned exteriorly of the wave guide 2 and secured to the wall 4 is the keep-alive electrode system consisting of a tubular member 42 of a suitable material such as Kovar alloy which is secured to the outer surface of the wall 4 by brazing. A second tubular member 44 is in turn secured to the opposite end of the member 42. The tubular member 44 is of a suitable material such as Kovar alloy which is sealed to an insulating ring seal 46 of a suitable material such as glass which in turn supports a keep-alive electrode 48. The keep-alive electrode 48 is of an electrically conductive material such as Kovar alloy and extends perpendicularly to the wall 4 and to a point adjacent the aperture 40. The insulating material of the seal 46 extends down along the surface of the keep-alive electrode 48 to the end portion adjacent the aperture 40.

In the operaiton of the device, microwave energy is connected to the wave guide 2 in a well known manner and a DC voltage is applied to the keep-alive electrode 48 such that in most cases the electrode 48 is negative with respect to the wave guide 2. The application of the voltage to the keep-alive 48 causes the generation of electrons which causes ionization of the gas in the envelope 3 in the immediate region between the apex 29 of the electrode 26 and the keep-alive electrode 48. The result is that when sufficient microwave energy is impressed across the space discharge gap between the apex 29 of the electrode 26 and the wall 4 to cause electrical breakdown.

A table of representative values and data obtained from the described embodiment is shown below in which w is the width of the iris 31 between the vanes 22 and 24. l indicates the height of the iris 31 between the two vanes 22 and 24 and the upper wall 4. The letter d indicates the various vertical spacings of the apex 29 of the electrode 26 from the inner surface of the wall 4. The negative value indicates that the apex 29 is inside the aperture 40. Q indicates the frequency bandwidth of the cavity. VSWR indicates the reflected microwave voltage of the unfired device. The letter f indicates the frequency in gigahertz and L indicates the insertion losses.

LOW LEVEL PARAMETERS OF SINGLE-STAGE SINGLE CONE TR W, millil, millid, milli- VSWR in L1, dbs. inches inches inches QL f0 gHz t.

It is found that in restricting the discharge to the region between the keep-alive electrode 48 and wall of the aperture 40 where no insulation exists and providing a. long insulating path from the exposed portion of the electrode 48 to the member 44 that the lifetime of the device is substantially extended. Portions of the insulator 46 are also remote from the discharge and the products of sputtering will difiuse to the metallic surfaces of the wall 4, tubular member 42 and tubular member 44 where .4 there will not be an adverse effect. This design leaves all of the critical DC gap lengths and electrode dimensions of the keep-alive unchanged. The essential parameters, consistent with effective TR design, is to obtain low Q, simultaneously with small R.F. gap lengths. The small R.F. gap lengths are essential for low firing power which is a prerequisite to low leakage power characteristics. Extensive low level measurements as indicated above were made to obtain families of curves to provide a wide latitude in design.

Other advantages of the structure are that the tuning diaphragm does not see any of the R.F. currents and that the very small aperture leads to high values of fired isolation. The bafile 21 enables the building of a single cone full height resonant element stage of low Q and wide bandwidth.

Various modifications may be made within the spirit of the invention. For example, for a fixed tuned TR tube the vanes 22 and 24 could be secured to the electrode 26 and the tubular member 33 would not be required.

We claim:

1. A microwave switching device in the form of a resonant type discharge device comprising a section of waveguide; a conducting baffie plate extending transversely of said waveguide to constitute a transverse iris plate having an iris notch in one edge thereof adjacent one wall of said waveguide, the remaining edges of said baffie plate being in ohmic contact with the walls of said waveguide; an aperture in said wall intermediate the sides of said notch; a tuning post in said notch adjustable toward and away from said aperture; a keepalive electrode adjacent said aperture but on the side of the waveguide wall opposite that of said tuning post for supplying free electrons in the volume between said tuning post and the edges of said aperture; an auxiliary chamber formed of a cylindrical member of electricallyconducting material disposed exteriorally of said waveguide encompassing said aperture and having one of its edges ohmically connected to the wall of said waveguide; and insulation means carried by the other end of said cylindrical member and supporting said keep-alive electrode, whereby said insulation means is removed from the vicinity of the plasma discharge region around said aperture and tuning post.

2. The microwave switching device as set forth in claim 1 in which said tuning post is substantially circular in cross-section with a pointed end portion adjacent said aperture.

3. The microwave switching device as set forth in claim 1 in which said keep-alive electrode is a rod positioned substantially perpendicular to said waveguide wall, said insulating means is annular in shape to provide a seal between said keep-alive electrode and said cylindrical member, and said seal is positioned a greater distance from said aperture than the distance from said keepalive electrode to said cylindical member.

References Cited UNITED STATES PATENTS 2,611,109 9/1952 Casellini 333-13 XR 2,697,800 12/1954 Roberts 3 l5-39 2,903,623 9/1959 Walker 33313 XR 3,310,706 3/1967 Woermbke 33313 XR OTHER REFERENCES Microwave Duplexers, Smullin and Montgomery,

McGraw-Hill, 1948, Tk 6590, M49, S5. Pages 69, 70, 107 and 109 relied upon.

HERMAN K. SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 333-13

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