US2697800A - Electric discharge device - Google Patents

Description

L. W. ROBERTS ELECTRIC DISCHARGE DEVICE Dec. 21, 1954 2 Sheets-Sheet 1 Filed Jan. 16, 1951 lNVENTOR LOUIS W. ROBERTS ATTORNEY Dec. 21, 1954 L. w. ROBERTS ELECTRIC DISCHARGE DEVICE Filed Jan. 16 1951 2 Sheets-Sheet 2 INVENTOR LOUPS w. ROBERTS fl aef Mia ATTORNEY I United States Patent fitice 2,697,800 Patented Dec. 21, 1954 ELECTRIC DISCHARGE DEVICE Louis W. Roberts, Roxbury, Mass., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application January 16, 1951, Serial No. 206,183

17 Claims. (Cl. 315-39) The present invention relates to gaseous electric discharge devices such as are used in guided transmission systems for control and for switching of signal propagation.

Devices of this general character have been known and widely used in specialized application, being formed as attenuator tubes, TR tubes (Transmit Receive) and ATR (Anti-Transmit-Receive) tubes, for example. These tubes have generally been constructed of a length of waveguide having an end opening for admitting high frequency energy while sealing the device hermetically so as to contain gas of a suitable composition at a suitable pressure. Such devices commonly include opposing pointed conductors so arranged as to form one or more narrow spark gaps. The combination of the gap and gas fill is so chosen that, upon the sudden incidence of radio frequency energy in excess of a given level, an ionized discharge forms in the gap in a very short period of time. Such a discharge will effectively block the transmission of this high-intensity radio frequency energy, reflecting nearly all of it back towards its source.

A very small gap is necessary for this purpose to permit the signal to establish a high voltage gradient within the gap, thereby inducing ionization in the gas between the gap electrodes. Such an arrangement of opposing pointed conductors forming a small gap will exhibit a capacitance in excess of that normally present between the two faces of the waveguide. This excess capacitance is, by this arrangement, effectively localized in a short length of the waveguide. This localized excess capacitance will, in the absence of any compensating effect, refleet radio frequency energy. It is desired that signals whose power is less than that required to create the aforementioned discharge be transmitted through the tube substantially without loss and reflection. Consequently, the localized capacitance is undesirable, and the usual arrangement of pointed conductors requires compensation.

In waveguide TR tubes, this compensation commonly takes the form of vanes which extend from the sides of the waveguide. These vanes may be considered as being equivalent to inductances extending across the waveguide. Normally, the vanes project into the waveguide to an extent which is just suflicient to allow their equivalent inductance to resonate with the excess capacitance of the gap electrodes at some particular frequency. For signals of this particular frequency, then, no reflection occurs. The vanes, together with the waveguide walls supporting the pointed conductors, form an opening or iris for transmission of low-level signals.

Frequently, it is desired to make the tube usable over a range of frequencies. The compensating vanes described serve only to eliminate reflection at some particular frequency. At some other frequency within the desired usable range, the vanes are not proper to compensate exactly for the excess capacitance of the gap. Further compensation is necessary, therefore, to achieve the desired performance.

In broad-band TR tubes (i. e. those which are expected to operate over band widths of more than a fraction of a percent) this is commonly achieved by combining a number of gaps and compensating vanes in one tube, these combinations being spaced from each other along the waveguide by intervals of a quarter wave-length of the center frequency of the operating band of frequencies for which the device is intended. ,The adjustment of the irises, effected. by adjusting the spacing becation of the parts is not as extreme.

tween the gap electrodes, become increasingly complex as the number of gaps is increased. Inasmuch as the tube cannot be taken apart and each combination of gap and iris adjusted separately, it is necessary to adjust each element while influenced by the other elements of the tube. Various complex methods for determining the approximately correct adjustment have been devised. These methods of adjustment have been successful, but they require highly skilled operators, and the elaborate adjustment method represents an important limitation on the present type of tubes.

It is highly significant, too, that it becomes increasingly diflicult to design tubes for shorter and shorter wave-lengths. The precision required in the construction of the gap electrodes, the irises and the space along the guide between the irises for proper broad-band performance, becomes extremely critical.

Furthermore, the bandwidth for which such a device can be designed is limited. This can be shown to be true theoretically and has proven true in practice. In general, multiple-element TR tubes have not been designed for more than about 13% bandwidth. This follows theoretically because the structure may be considered equivalent to impedances which are localized at different positions along the waveguide that are separated by significantly large fractions of the wave-length. Energy that is stored in each infinitesimal section of the waveguide varies widely and rapidly along the structure.

It has been noted that the irises are resonant-impedance structures localized at discrete positions along the waveguide. In contrast to this, the present invention introduces various forms of discharge-gap structure that are distributed along the waveguide so as to achieve broad-band performance without, however, requiring the extreme care and precision involved in forming and adjusting the separate irises and discharge gaps used heretofore in like broad-band devices. The discharge gap means will be seen from the illustrative forms disclosed below to be constituted of either a continuous blade or opposed continuous blades, or of what amounts to electrically a continuous blade or opposed blades formed of slender pointed conductors terminating in an extended discharge gap.

Since, in a distributed-gap device of this form, there is no dependence on the precise cancellation of the reflection introduced by one element by the reflections of the other elements, the exact adjustment of any small section of the tube described is clearly not as critical. For this same reason, the precision required in the fabri- Furthermore, because of the distributed nature of the device, the energy stored in any infinitesimal section of the structure, although it may be high compared to that in a like section of normal waveguide, will not vary rapidly along the structure. Hence the theoretical limitation on the achievable bandwidth previously noted for the usual type of device will not apply in the same manner to this tube. Bandwidths considerably in excess of what has been achieved can be obtained with the distributed form of discharge gap.

The effect of a distributed system of elements may be described in terms of what is called the characteristic impedance of the waveguide. If, for instance, a continuous blade, or two opposed continuous blades are introduced into a waveguide of, for example, rectangular cross-section, the resultant combination may be seen to be geometrically similar to the well-known type of waveguide known as ridged waveguide. Its effect then will be to lower the characteristic impedance of the waveguide into which the blade or blades have been introduced. No reflection will occur within this section of modified waveguide in spite of the increased capacitance per unit length of waveguide because this increased capacitance is uniform along its length. To avoid reflection'of a signal of any frequency, then, it is only necessary to include a suitable transition'between the normal waveguide (if of higher impedance) and this modified structure. Since, as has been shown, the transition required is in the nature of a single impedance. transformation, various forms of construction can be used. Some of these are shown in the specific embodiments to be described below.

might be thought to be undesirable sihee', for at giVeri' level of power input, the voltage developed across the structure will ,also be reduced. However, the use of one armors bla'd'es with camera-faintly sharp edges implies that the'di's'tanee acrossthegafisoform'ed deereases' much faster than the voltage developed across it. The level of radio' frequency power at which a discharge will be initiated is' determined by the voltage gradient; which is approximately the voitage across the gap divided by' the width of the gap. Therefore, this arrangement will'i'n fact have the desired characteristic that reasonably low power levels will be suffieient to cause a di sch'argejto oecur. The'voltag'e gradient for a given power level'will be high as is required. 1 H A I v In some" of the" embodiments described below, a sequenee of slender pointed c'onduetor's is shown,' or two such sequences are shown opposite each other. Such-an arrangement may be' coiisidereda's being breadly equivalent to th" continuous blade orblades discussed algqyq, provided that the interval between sh'ecessive members of the sequence is small compared. to the wave-length of the signal being considered. Where this interval is substantially less than a quarter of a wave legnth in; the case of the shortve end of its op'erating frequency band, the entire sequence will behaveas if it w'ere" acoutinilous blade or blades with the same average capa city. The use of one or more sequences of slender pointed conductors is advantageous fer applications l t does not introduce as much capacitance, and, hence; reduces the characteristic impedance to a lesser extent, than an actually continuo'ns' bla de or blades arrangedto obtain the same voltage gradient fer a given'inptit power level. With the series of closely spaced points, the design of the transition to the normal waveguide isfa'eilita The devices embodying various aspects oftheinvention to be described below will be recognized as. illustrative and for convenience, this disclosureof illustrative forms is devoted to the TR type of tube; Aspects of the invention will, of course, be recognized as haying other applications as in ATR tubes; I rr the aceompanying drawing, Figures 1, 3 5 and 7 areldngitudinal crosssections of TR tubes embodying various aspeets of the invention. Figures 2, 4, 6 and 8 are transverse cr sssections of the TR tubes along the lin es g 3 8 in Figures 1,- 3, 5 and 7 respectively. Figu a fragmentary longitudinal cross-section of an alte ve fdfin 9 devic. n a t l 'fi ts k ha jtii. i ures. 3; nd 4' e x'cept fdr the construction of pointed discharge ga'p ledrddes T i tta e sc u es sew rs R tu ssinc's ai li a fn. i t en velieat rsstb o he 'sas p devices will be apparent t8 those skilled iectne discharge in the art si kp thr 'm qn y tnna n the transmitter. 4, n .ter pul se, the as filling in he, R. tube i in nded t bln nse n ed n sq 'du tih 9 as t sh trs cu t earne transmitter energyaw ay from the reeeiver to the antenna. This conserves transmitter p o er and more vita ly prot'ects the sensitive receiver against damage. Immediately after each pulse, the gas becomes deionizedand the TR i eid mly b c mes 1 mo than a .q i uo seqt e W id Conn d o he css sr r a sm t n weak signals picked up by the antenna; ltjs desirable that the TR tube be etfective as a perfect transmission guide path overs broadband of frequencies Brpg l band TR tubes have a low voltage standing wa ratlo (VSWR) that is reasonably constant over the operating band of frequencies, an acceptable ratio being arbitrarily fixed at 1.4, for example. A perfect waveguide would be one having a VSWR of 1.0.

The broad bandin'g is accomplished in the illustrative embodiments by employing an eifectively continuous length of discharge gap along the waveguide that contains the ionizable gas so as to avoid resonant effeets,

seine po'int along the waveguide having a voltage peak forev'ery frequency in the operating band of frequencies. The discharge is promoted (as in comparable broad-band TR tubes known heretofore) by one or more so-c alled keep' aliye electrodes that have a charged electrode point near the discharge gap region; by acoiidu'ctor that hotman forms an extension of the wave-guide and in this effective to suppress; stapl r/e1 an n;

way confines and localizes a volume of gas that is continuously maintained in weakly ionizedcondition during operation. The degree of ionization maintained in the discharge region by these electrodes is not sufficient to interfere with the transmission of signals of low power. However, when a pulse of high power level arrives, the presence of this weak ionization promotes the resulting intense ionization that is required for preventing the transmission of thehigh power pulse: The speed with Which the tube acts toprevent the deleterious transmission of highfp'ower signals" is therefore enhanced by the presence of these electrodes;

Severalforms of broad-band tube embodying distributed constant discharge gap means are described'in connection with the accompanying drawings.

In Figure 1 a length of waveguide 10; of ridged form for most of its length, has a flange 12 at each end, and the longitudinal passage through wave-guide 10 is closed at each end bya window 1 4having a: metal fia'ng'e' portion 14a, 21' glass center- I417 and an edge I40 The windows' are 'r'es'ona'nt' at the mid-band frequency; andv'viridows of low-Q are desirable to avoid undue freqne'n'c'y sensitivity. ann-zine aband aroiidtheedg'e of the window is effective to tl'1*is*e "nd.- A pa r ot opposite-ridges 1-6, 18', are provided in" the widewalls 10b of the-w feguide, th' electric field ofthe modes-sea being parallel to the shorter walls 10a".- A series or slend'er poin'ted metal probes 20 extends fr'or'n' dneofneneges 18' close to but spacedfrom the other rid'ge' 1'6"so' as to fori'ha series of discharge gap one of these probes 200i; located mid-way" along waveguide 1'0, is disposed see-sire a keep-alive electrode 22'. This includes" a pointed probe 2 '4 having a" glass sheath 26, this press, and the glass she'a'th extending into a conical cavity 28' formed in ridge 16 of the'wavegnide. Ariiiiipedaric' transformer may be'neces's'aryin the tiansitio'ri between the waveguide of rectangular c'ro's'sfs'ectionl at the windows and of ridged cross se'etion at the c'enter' regiontd m'ir'iimiie reflections. This may take any of a wide variety of known forms; The ridge ends are sp ed item the win- (lows, and the pointed eon'duc't'ors' at th "nds of'tlie' series are spaced from the" ends of the supporting ridge, for irripedance transformingeffects.

The waveguide is filled with a twd to-oiie mixture of argon and water vapor at IO'mfri; totrl pressure, ref estample. Probe 24, together with the conical conductive wall surrounding its active end is ffe'e'tive during opera= tion tomaiiitain a siiiall volume of weakly ionized gas when the tube is essentially deioniied, so as to'pr'or'iiot'e instantaneous ionization in response to a burst of high level energy reaching the device. Keep-alive electrode 22 with the probe 24 constitutes one dischfifge gap and wall 16 of the wave'g'tiide together with pointed probes 20 constitute additional d-is'har'g'e gaps disposed at close intervals al'cifig the wave uide; separated by a distance substantially less than a quarter wave-length at the shortest wave-length of the operating f'reqiieney band. The substantially c'oritih o'iis gap enabnsa by the probes is y transmission over a broacl-bandof frequencies dti'r mg ionized discharges. Additional keep-alive electi-odes are desirable for more effective starting. When the gas opposite only the keep-f alive is slightly ioiiiZed; the prdbes 20 evidently street the waveguide only slightly, and are comparable in im pedance effects to the ridges-1.6 and 18. The keep=alives, contained within afidge, offer no appreciable diseoritinuity in impedanee df the ridged waveguide.

Thedevice inFigures 3 arid,4 is closely similar" to that in Figures 1 and 2' and for this reason the description arid the operation asse duplicated eiice'pt to point to the differences in details; Two sets er probles 30 are disposed in the central E-pl'ane perpendicular th the broad walls 34, 36 of the wave} uide section 32, these probes heiiig similarly nose; together as discussed in connection with Figures 1 2 so as to fes'e'mbl a pieket=fenee. Here, however, there are two fpicket f hes. These are ady ahtag'eotisly mutually 'opptised; with a close series of discharge gaps, effectively one discharge gap, formed betweeh points approiirhaiely mid a in the waveguide. in wall sir-,prmn pl ive electrodes 38 are formed, each iiiclu'dihg probe 40 having a glass sheath 42 and a con'e 4 This probe aiid, glass sheath extend close to but sep a ted from probe 30a', and cone 44 de'- fin'e's a lirri itedyvolu riie saga around priibe "40 that is weakly idnized by a pdtential applied "eehtit'i iions'ly Be= tween probe 40 and cone 44. The cone size is exaggerated, and should be designed to resemble the other probes 30 as nearly as is practical for minimizing impedance discontinuity effects.

The broad-band merit of a form of the invention in Figures 3 and 4 is in general the same as that in Figures 1 and 2, the form of waveguide having ridges 16 and 18 having certain operating advantages. Both forms of TR tubes are recommended for systems designed for very short wave-lengths. The picket-fence type of broadbanding is more readily fabricated than the multiplewindow and multiple-resonant-chamber type of. broadband TR tube heretofore known. The opposed picketfence of Figure 3 can, to advantage, be combined with ridged form of waveguide shown in Figure 1, the combination not being shown in the interest of succinctness.

In Figures 5, 6, 7 and 8 two forms of broad-band TR tubes embodying features of the invention are shown, these forms having discharge gap means formed not as a series of pointed wires but rather as a truly continuous gap, in the form of blades having sharpened opposed edges. In Figure 5, for example, the rectangular length of waveguide and the end windows are the same as in Figures 1 and 3 and the rectangular wave guide of Figure 6 is the same as that in Figure 4 While the rectangular waveguide of Figure 8 is the same as that in Figure 2.

In Figure 5 a pair of blades 46, 48 are supported by the broad walls of the rectangular waveguide and these blades have opposed sharpened edges. In the region of the windows, the characteristic impedance of the waveguide is the same as that of the waveguide system into which the device is to be connected, because the blade does not extend to the window. There is, however, a transition from that relatively high impedance to the reduced impedance in the region of the blades. This transition is effected in the form shown by gradually increasing the height of the blade from a minimum in the region of the end windows to a maximum where the sharp edges are opposed. This transition is identified in the drawings by numerals 50 and 52. The form and length of this transition will, of course, be modified to suit the voltage standing wave ratio requirements.

In Figures 5 and 6, the keep-alive electrodes are shown that appear in other forms'in Figures 1 to 4 inclusive. Keep-alive electrodes 54 are shown which are sheathed in glass or equivalent insulating material 56. It is desirable that the blade 46 be maintained of uniform width in order to avoid reflections that would disturb the voltage standing wave ratio over the operating band of frequencies, but some limited enlargement of the blade 46 as represented at 46a may be necessary in order to enclose the insulating sheath 56.

The continuous blades in Figures 5 and 6 have superior broad-band characteristics; but the truly continuous gap has the disadvantage of decreasing the characteristic impedance of the waveguide cross-section and of complicating the problem of impedance transformation in the region 52. The distinct probes in Figures 1 to 4 inclusive do not reduce the characteristic impedance to the extent that the blades in Figures 5 and 6 reduce that impedance, so the advantage of the picket-fence form of substantially continuous discharge gap will be apparent.

In Figures 7 and 8 another form of sharp-edged blade type of gap is shown in which a different type of im pedance transformation is effected. The blades 60 and 62 in this form are supported on ridges 64 and 66 respectively, the ridges eifecting an impedance transformation from that in the region where the rectangular waveguide is unconstricted adjacent the windows to that in the region where the blades are located. The blades and the keep-alive electrodes are constructed similar to those in Figures 5 and 6 and their description is not repeated here.

Figure 9 illustrates a further modification of the picketfence form of discharge-gap construction. The top and bottom walls of the waveguide (the walls between which the electric field lines extend in the operating mode) are shown in longitudinal cross-section in Figure 9. Top wall 70 is disposed opposite to bottom wall 72, and each of these walls supports one series of probes 74, 76, the probes being supported alternatelyrby the opposed walls. Probes 74 together with the opposed wall 72 form part of the elfectively continuous discharge gap whereas probes 76 with wall 70 contitute a further continuation of the efiectively continuous discharge gap. This form of construction can be used either with a rectangular waveguide or with the ridge waveguide, depending upon the impedance of the waveguide systems and of the impedance in the section where it is transformed by the physical inclusion of the probes themselves.

The several embodiments above specifically illustrate aspects of the invention as applied to TR tubes incorporating rectangular forms of waveguide. It will be selfevident to those skilled in the art that the features have application to other types of tubes, and that other shapes of waveguide or equivalent confined-space transmission lines can be employed so as to incorporate the novel discharge-gap structure. Also, the illustrative tubes are designed to operate with the TE10 mode; but the adaptation to use with other modes will be readily apparent to those skilled in the art. Therefore, it is appropriate that the appended claims be accorded that broad interpretation that is consistent with the spirit and scope of the invention.

What I claim:

1. An electric discharge device including an envelope containing an ionizable gas and having a lateral wall formed of a length of waveguide and an end wall embodying an opening for admission of signal propagation, said opening being hermetically sealed by a closure pervious to signal energy and means extending along the waveguide embodying portions forming discharge gaps at intervals along the length of waveguide substantially closer than quarter wave-length intervals.

2. A gaseous electric discharge device including an envelope containing a gaseous fill, said envelope having a length of waveguide constituting its lateral walls and a window constituting an end wall, and sharp discharge gap means distributed along the waveguide for a substantial extent and broadly constituting a continuous distributed discharge gap.

3. A device in accordance with claim 2 wherein said waveguide is rectangular, where said sharp discharge gap means includes opposing longitudinal ribs and sharp discharge means disposed along said ribs.

4. A device in accordance with claim 2 wherein said waveguide is rectangular, wherein longitudinally extending ridges are formed in said waveguide and wherein a series of pointed elements are disposed along one ridge at intervals substantially closer than quarter wave intervals extending closely adjacent but spaced from the opposite ridge.

5. A gaseous electric discharge device including an envelope having a lateral wall in the form of a length of rectangular waveguide and an end wall hermetically scaled and formed to admit signal propagation, sharp discharge gap means disposed medially along the waveguide for a substantial extent and broadly constituting a continuous distributed discharge gap.

6. An electric discharge device embodying a length of waveguide forming the lateral wall thereof and containing an ionizable gas, a window forming an end wall thereof, sharp discharge gap means disposed medially along the waveguide for a substantial extent and broadly constituting a continuous distributed discharge gap and displaced from said window, and a transition in said waveguide between said window and said sharp discharge gap means.

7. An electric discharge device in accordance with claim 6 in which said discharge means embodies a keepalive electrode for maintaining a restricted volume of said gas continuously weakly ionized.

8. A gaseous electric discharge device including a length of waveguide containing an ionizable gas, windows permeable to signal propagation hermetically sealed across the ends of said waveguide, and sharp discharge gap means distributed along the waveguide for a substantial extent and broadly constituting a continuous distributed discharge gap.

9. An electric discharge device in accordance with claim 8 in which said discharge gap means embodies insulated electrode means for maintaining a restricted volume of gas in weakly ionized condition.

10. An electric discharge device in accordance with claim 8 in which said discharge gap means is separated from said windows by waveguide impedance transformers.

11. An electric discharge device in accordance with claim 8 wherein said discharge gap means embodies a sequence of keep-alive electrodes displaced along the waveguide.

1-2; An electric discharge device in accordance with claim 8 wherein said discharge gap means embodies a sequence of slender pointed conductors disposed along saidwaveguide. at intervals substantially closer than quarter length intervals.

1.3. An electric discharge device in accordance with claim 12 wherein said pointed conductors are disposed in opposing pairs.

14.. An electric discharge device in accordance with claim 12 wherein said electric discharge device embodies a keep-alive electrode opposite one of said pointed condnctors.

15. An electric discharge device in accordance with claim 8 wherein said discharge gap means embodies 1ongitudinally extending bladeshaving opposed sharp edges.

16. An electric discharge device in accordance with claim 15 embodying a keep-alive electrode within one of said-blades closely adjacent the opposed edge of the other v blade.

17. An electric discharge device in accordance. with claim 8 wherein said waveguide is of rectangular cross section having narrow and wide walls, andwherein a portion of the length thereof is formed with longitudinally extending ridges and wherein the discharge gap means is disposed medially along the ridges.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,200,023 Dallenbach May 7, 1940 2,402,184 Samuel June 18, 1946 2,403,302 Richmond July 2, 1946 2,438,367 Keister Mar. 23, 1948 2,505,240 Gorn Apr. 25, 1950 2,524,268 McCarthy Oct. 3, 1950 2,556,881 McArthur June 12, 1951 2,587,305 Fiske Feb. 26, 1952

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