US2783414A - Electrode structure for transmitreceive switching devices - Google Patents

Description

Feb. 26, 1957 H. J. MCCARTHY 2,783,414

ELECTRODE STRUCTURE FOR TRANSMIT-RECEIVE SWITCHING DEVICES Filed June 1. 1954 2 Sheets-Sheet l INVENTOR.

HENRY J. MCCARTHY BY gm & W

ATTORNEY 1957 H. J. M CARTHY ELECTRODE STRUCTURE FOR TRANSMIT-RECEIVE SWITCHING DEVICES Filed June 1, 1954 2 Sheets-Sheet 2 REGULAR HOURS 'FIG.5

. INVENTOR. HENRY d. MCCARTHY BY ATTORNEY United States Patent "ice The present invention relates to gaseous discharge switching devices of the transmit-receive type, an d; more.

particularly to improved ignitor electrode structure for suehdevices. H W w Transmit-receive (TR) switching devices are employed in radio direetion and ranging systems to permit the use 2,783,414 fatented '-corporated in a radar system any change in the charac- 5 a us m h r ploye r in h art o ef r to thmhcof a common antenna for both transmission and receiving :1,

of radio frequency energy. During transmissiqn of the high power pulse such switching devices decouple the sensitive receiving apparatus by means of i oni zati on of a suitable gas filling finder reduced pressure eontained within the envelope. The position of theiliR switgh in th eireuit permits essentially all of thetr ansmitted enei' 'y' to be directed to the antenna. T he reflected-radio frequency signals will be received by the s aine ant enria and ais at too low apower level to 'eai se ioniaation of -the gas fSu'cli signalswill lie-permitted to pas's'ihioh n:

the TR s w'itoh tothe receiver. g p h J An auxiliary electrode eomnio'nly e'mplo d in the R switch to provide a source of electron aidfin the ionization of the gas mixture within, the e'nv 5; Such electrodes are now referred to in the art as the keepalive v orf ignlitor? electrode. Their operation, aswell as ween-ed description of various mbodiniehlS i pqea the text, MicrowaveDup1exers;,by'L. osmullin' and G. Montg omery, McGrawEl-IillBook Company, Ina,

Nefiqi york, 1942;, 5ages 199- 211 inclusive.

eofi'ductive metal, such as Kovar, 1a nickel iroric alloy, surrounded by an insulating sheath, over a;

pprtibn of its length and exposing onlyjthe tip surface. h ctro'deis introduced into the TR swi h, ube

1'1 axial passageiin a dis v afge "a con al A supported ftheouter env e A direct 'eur're'nt sustained glow discharge is ma ntained the egiofiextending fro'm the tip area by biasing the ign'i'tor electrode negatively with respe'c to the adja ealdisclia'rge' gap electrode; Since the direct an discharge is continuous, the tip area of the e H subject to surface pittingandztorma'tibn of metal 10 der i is q hfi h t hem Thefeaflhlbhsi giadaany red'uce the exposed electrodeitipsuriace and will result in momentary glow to are transitions which cau se extinguishing of the ignitor electrode act on With as partial ionizationthe dischargeigapilfargeijalhieh of radio fiequeiiey p'oyver will be permitted to pass through the TR switch before a gassbnslaisc'nf s 7 tiat'e'di This additional power ma'y exceed the tole bl e liin'its 6ft '6 receiving" apparatus andcause daniage to Briefly, the prior art electrodes comprise a ate of I v nomena which occurs in ignitor electrode characteristics during tube life is defined for the purposes of this wshahfan as ad fis ahae n n i stw h sriitor lectlrod e and adjacent discharge gap electrode when the -igi iitor current hasreached the value specified, 'increase n voltage drop will mean a drop in current with a fixed voltage supply. L v p H g .The mixture of gases now employed in the TR switch tub' e h a s been experimentally determined in the art to requirea small percentage of water vapor as aneces sary 'cempp em. The presence of this component has created aproble'm of tube life in such tubes employing th is tq'r electrode since the oxygen in the gas fillingthastens the 'foimatioii-ofmetallic oxide deposits on theelec'trodetip. Prevention of momentary glow to arc tran s itions or sud den inc rease of voltage drop above predetermined :limits is, therefore, a major aim of the electronicstt'ub'e inh t Y-V U In theprocess' of conducting investigations into various s utions; to t is P b I v FQY r A t e-uh? mation of a tenacious semi-conductive protective la y er aver the e h en e o e t s amefully -pisam ne electronemitting surface and yii' t la ly eliinin ages; of prior artelectrodes.

'th' dis th 12 ,9 et v ylay h sa l e h gh l'ct. cal 'sjstance, it provides .forthe formation of a unifpl 'm ad 6f .,.dit:. c r en g vvd s s and lso .p ev nlt momentary extinguishing of the ignitor action due to $5 95! iQ are t1ia si s-,. h et exact e k chmposition o'f the protective semiconductive layer cannot accurately determined because of the very 'small diirens ons, the method of fabricating this layer indicates that va 'i h hxids of e base me l ofthh l s ddh a wellas some oxides of the sheathing material, are present. It is), therefore, an object of the present invention to provide an improved ignitor electrode for gaseous dise e w ch n e c a. i. W .v lt is' a further object to provide an improved electrode 'p rh ly i z n as s qu m xt e n a ransm r 6 sh nthhe v a c ro eprov d n flld i r ihe life y eliminating glow to aretransiti9hS and increases in' ol'ta'ge' dropabovetolerable limits. q

"Other objects, advantages and featurcs'oi the present invention will be evident after consideration of thefql- 5o 'g' detaileddescription and reference to the accentpanyihg drawings, in which egg. 1 is a vertical ems; sectional view of the mustrative embodiment; V 4 v v is an emer es cross sectional view of aportion of prior art 'ignitor electrode structure; y

Fig. 3 is an enlarged cross sectional view of the improved ig'nit'or electrode of my invention; p

Fig; 4 is anenlarged cross sectional view ofan alternafiveeinbodiment of the present invention; and I Fig. 5 is a curve illustrating performance results during life.

sam; in the drawing's, ig; 1 illutratesa transmit rec 'e switching device 1, referred to in the art an fal cavity type. The outer envelope e mprise a lie body member 2 of a general cylinf V section 'witlian axial passagetherethrough de ng wall; 3; ssaeee within the axial passage and securd 1Q Wal 3' are a 'pair' of opposed conical discharge gap electrodes {and 5 having their large ends spaced apart to defi' therebet weeii resonant ca'vity 6. The oppositely dispo convergent ends of electrodes deride d efine therebet n disiiliaige "gap 7. Axial adjustment of the gap spiieihg to the frequency of operation may be made by deforming the flexible diaphragm end of electrode 5 with suitable difierential screw mechanism positioned within housing 8.

An exhaust tubnlation 9 extends into body member 2 to communicate with the resonant cavity 6 and is utilized for evacuation, as well as filling with the gas mixture under reduced atmospheric pressure. 1

Positioned at one end of axial passage is a reservoir plug 10 with an annular cup ll sealed thereto. A reservoir glass bulb 12 is sealed to cup 11 to form a storage chamber for the gas filling.

' The ignitor electrode 13 which forms the subject of my invention comprises a core of a conductive metal which is supported at one end by means of a glass to metal seal 14 to'the reservoir bulb 12. Electrical contact to a direct current voltage supply and external circuitry may be made by means of top cap 15 sealed to the outer end of electrode 13. The inner tip of said electrode extends axially within the device through a hollow passage 16 in'conical gap electrode 4 to a point a critically short distance from "the apex of said electrode. An insulating sheath 17 of a dielectric material surrounds electrode 13 over a major portion of its length to prevent arcing to the walls of hollow passage 16. j

Prior ant electrodes, as well as the improvement disclosed in this specification, will now be described'by referring to Figs. 2 and 3. The general construction-of the ignitor electrode comprises a dielectric sheath 17 extending to the tip of electrode 13 leaving only the tip surface'area 13A'exposed. Electron emission is, therefore, directed from this surface area in a continuous stream toward the discharge gap. Surface pitting of the tip' and sputtering of metallic deposits onto the sheath 17, aswell as tip surface, tends to restrict .the'emissive area; Momentary glow to are transitions may occur from thesemetallic deposits to adjacent cone structure.

According to the teachings of the invention, I envelope the tip surface area with a spherical semiconductive oxide layer 18. This not only protects the emitting surface, but assures the spread of the glow discharge uniformly over. the entire spherical oxide surface. The composition of this semiconductive layer will be better appreciated after consideration of the method of manufacture.

I first provide a lead wire of a conductive metal such as Kovar, an alloy containing approximately 29% nickel, 17% cobalt, 0.3% manganese and the remainder iron, and having a diameter of approximately forty thousandths (.040) of an inch and tapering'slightly to a v tip diameter of approximately twenty-five thousandths (.025) of an inch. To sheath this wire, an approximate beading glass having a wall thickness of approximately 25 mils (.025) of an inch, such as Corning 7052 glass, is sealed over a major portion of the length by conven tional sealing techniques. The tip surface is then ground, etched or sanded to expose the base metal. Next, the tip area is heated with a controlled oxygen-gas fire until the metal begins to oxide rapidly between approximately 1100 C. and the melting point of approximately 1450" Q By rotating while heating, a spherical oxide central layer 'is formed. The oxide layer thickness is approximately from three (.003) to five (.005) thousandths of an inch and the diameter is between thirty (.030) and thirtyseven (.037). thousandths of an inch.

" The Oxide protective layer must be dense, adherent and show no ruptures, cracks, pin holes or distortions.

The chemical composition appears to consist essentially of nickel, cobalt and iron oxides with some glass oxides alsovpresent. 1 Due to the small dimensions of the com pleted spherical oxide layer, accurate chemical analysis of the relative percentages of oxides present has been impossible. It is, however, the resultant structure, as well as the method of manufacture, that has contributed to the rzfiults obtained without the addition of any other chemic s.

An alternative embodiment as shown in Fig. 4- com;

application.

prises terminating the insulating sheath 17 at a point 19 approximately fifty thousandths (.050) of an inch or less from the tip. The spherical oxide layer 20 may then be formed according to the same steps previously described. In this embodiment there will be no fusing of the glass and metal oxides.

After preparation of the oxide ball tip ignitor electrode it is assembled in the complete device as shown in Fig. l.

The electrical characteristics of the improved ignitor electrode may be appreciated in Fig. 5. Since certain specified limits must be met according to rigid government inspection standards, I have indicatedv the lower and upper voltage drop limits for tubes of the type shown in Fig. 1. These limitsare respectively 325 to 450 volts. Tubes exceeding these limits during operation will result in erratic performance and, if continued high readings are noted, it means a replacement is in order to assure satisfactory operation. The tubes employing standard ignitor electrode structure have been observed .to rise initially, but then to fluctuate fairly close to the upper limit until approximately 700 hours of life'when they would rise steadily as shown by curve 21. l

Tubes employing the improved ignitor electrode have demonstrated a much desired stability during life up 'to 1000 hours and have stayed comfortably within the specified limits. Curve 22 illustrates these results which can be attributed primarily to protection against electrode surface deterioration, as wellas the providing of a high resistance immediately at the tip area which will result in very fine current paths all around the spherical protectivelayer rather than a concentration at one particular point. This dispersion of current paths results in a uniform direct current glow discharge all around the oxide sphere. Another advantage of the improved structure is to aid in the elimination of any relaxation oscillations of the discharge mentioned 'on'pages 201-206 of the aforementioned reference. It has been noted in the prior art that such oscillations can be eliminated by placing a high resistance directly at the TR switch tube, see Patent No. 2,582,202 issued January 8, 1952, to Carlyle W. Iacob.. It is the practice in the art to incorporate a resistance in the order of one half megohm. close to the top cap connection 15 in Fig. 1. My invention places additional high resistance at its most efiective position to eliminate reflex oscillations, namely, at the tip surface. Ohmic values of approximately 10,000 ohms have been measured for the spherical oxide protective layer.

There has thus been described an embodiment of an improved ignitor electrode structure which may be incorporated in all modifications of transmit-receive switches. Another embodiment to which the present invention is applicable is referred to as the band pass type and is illustrated in a disclosure Serial No. 299,895, filed July 19, 1952, by'Harold C. Booth, now Patent'Nol 2,680,207 assigned to the same assignee as the present What is claimed is: I

1. In a transmit-receive gaseous discharge switching device havingaresonant cavity and plural spaced conical electrodesaxially disposed therein defining a discharge gap, an ignitor electrode extending axially into one of said dischargegap electrodes comprising a core of a conductive'meta-l, a sheath of an insulating material covering a major portion of the lateral surfaces of said core and exposing only the tip area, a substantially spherical layer of a non-emissive high resistance material sure rounding the exposed tip area, said spherical layer being composed of oxides of said conductive core and insulating sheath without any additives.

2. In a transmit-receive gaseous discharge switching device having a resonant cavity and plural spaced conical electrodes axially disposed therein :defining a discharge gap, an ignitor electrode extending-axially into one of said discharge gap electrodes comprising a core of a con.- ductive metal, a sheath of an insulating material covering a major portion of the lateral surfaces of said core and terminating a short distance from the tip area of said core, a substantially spherical layer of a non-emissive high resistance material surrounding the tip area of said core, said spherical layer being composed of metallic 5 oxides of said conductive core without any additives.

References Cited in the file of this patent UNITED STATES PATENTS Mendenhall t- Nov. 9, 1937 Schwartz Oct. 25, 1938 Gaidies et a1 July 29, 1941 Booth June 1. 1954

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