US2814784A - Waveguide duplexers - Google Patents

Description

Nov. 26, 1957 H. J. RIBLET WAVEGUIDE DUPLEXERS 3 Sheets-Sheet 1 Original Filed April 28, 1948 7C RECEIVE/2 TENNA IGNAL R. E POWER IN FROM TRANSMITTER A NTENNA /NVENTO/? -/-/E/v/2v J. 17/51. ET BY 5% g ,M

ATTORNEY Nov. 26, 1957 J, RIBLET 2,814,784

WAVEGUIDE DUPLEXERS Original Filed April 28, 1948 3 Sheets$heet 2 Fla. 5

TO OSCILLATOR NON REFLECTIVE TERM/NAT! ON HEN/9V J. P/BL ET ATTOPNE'V NOV. 26, J, ET

WAVEGUIDE DUPLEXERS Original Filed April 28, 1948 3 Sheets-Sheet 5 Fla /O FIG.

FREQUENCY IN MEGAC'YCLES /NVEN7D/? HEN/2v J. R/BLET ATTORNEY United States Patent WAVEGUIDE DUPLEXERS Henry J. Riblet, Wellesley, Mass., assignorto Raytheon Manufacturing Company, a corporation er Delaware Original application April 28, 1948, Serial No. 23,822. Divided and this application August 25, 1952, Serial 13 Claims. (Cl. 333-10 This is a division of my copending application, Serial No. 23,822, filed April 28, 1948, now Patent No.

In radar systems, there is usually a single antenna for i use in both transmitting and receiving. To prevent the strong bursts of energy that are available during transmission from entering the receiver, an electronic switch is provided which has the effect of decoupling the receiver from the system and coupling the antenna to the trans mitter alone during transmission. When transmission ceases, the receiver is effectively recoupled to the system. This switch, generally known as a duplexen or a transmit-receive device, or simply, a T-R, includes as an element a spark gap, properly located in the tran'smission lines that interconnect the receiver and transmitter with the antenna, and adjusted to break down when power of the order of that available from the't'ransmitter is flowing through the transmission line between the transmitter and the antenna, but not when the lower order power such as is available during reception is present. It is now customary to provide a certain amount of auxiliary unidirectional potential, for example about 300 volts, as a keep alive in the vicinity of the spark gap, to maintain an amount of residual ionization of the gases surrounding the gap and thereby assure firing of the gap during transmission. During reception, some of the received energy attempts to enter the transmitter, 45

which is at that time poorly matched impedance-Wise to the transmission line, and is mostly reflected. To prevent a this energy from disturbing the system, a second spark gap device is employed, properly located in the transmission line system with respect to the first. This second device functions during receiving to decouple the transmitter from the system, and is known as an anti 1 R or R-T. The two spark gap devices are intended to have the effect of decoupling the transmitter from-the receiver at all times.

It is an object of this invention to provide an improved duplexer which is small, compact, and rugged in construction.

It is a further object to provide an improved duplexer which reduces the number of discharge tubes required in radar systems and generally simplifies the construction of such systems.

It is a still further object to provide such a dnplexer which is readily installed in a radar system with known installation techniques.

The invention consistsessentially of a four-terminal network composed of two waveguides directionally coupled together, for example, at a common wall by means ofslo'ts in said wall, and is based on the discovery that the directional coupler character of the network is not destroyed when the coupling slots are short-circuited, for example,

2,814,784 Patented Nov. 26, 1957 by an ionic discharge. The network is connected in a radar system by coupling three of the terminals thereof to the radio frequency power source or transmitter, the antenna, and the receiver, respectively, and providing the fourth terminal with a non-reflective termination. Other and further advantages and characteristics of the invention, as well as the details of certain particular embodiments thereof, will become apparent from the discussion that follows, reference being made to the accompanying drawings, wherein:

Fig. sis a side-sectional view of a duplexer like that of' Fig. 1 showing the manner of connecting it into a 'radarsys'tem; I

Fig. 4illu'strates another form of duplexer constructed in accordance with the invention;

Fig. 5 illustrates a preferred form of the duplexer shown in Fig. 1;

Fig. 6 is a top plan view of the duplexer shown in -1.

Fig. 7 is a view taken on the line 7--7 of Fig. 6; Fig; 8. is. a top plan view of a modification of the "duplexe'r of Fig. 5;

9 is a top plan view of another modification of "the duplexer shown in Fig. 5;

, Fig. 10 is a view along line 1010 in Fig. 9; and Fig. 11 is a performance curve illustrating a certain broad band characteristic of the invention. Referring now to Fig. 1, two parallel waveguides 10 and 11' are joined together at a common wide wall 12. A pair of rectangularly crossed slots 13 and 14 is cut in this emamofi wall, in a region to one side of the g tudinal center line thereof. At each of the four pointswhere the walls of the slots intersect, the material of' the wall 12 is continued in the form of a pointed extension 15 inthe direction of a diagonally oppositely disposed" intersection point. The extensions 15 extend toward-the geometric centerof the slot configuration, and all terminate just short thereof, so that their ends are all very near to. but do not touche'ach other. This construction is shown more clearly in Fig. 2. The slots 13 and 14 are each resonant to the mid-band frequency of the operative frequency band, and consequently tend to erect practically percent transfer from one waveguide to the other ofpower at that frequency. The upper waveguide 10 is sealed at both sides of the slot arrangement with suitable gas impervious but electromagnetic wave pervious windows 16, 17 made of glass, mica, or the. like, anda similar window 18, shown in dotted line in Fig 1-but more clearly in Fig. 3, is provided in the other waveguide 11, mountedon the common wall 12 and covering the slots 1-3 and 14. There is thus provided a sealed chamber 19 in the upper waveguide 10, and the extensions 15 are in-this chamber. The chamber 19 is partially evacuated, and may contain anysuitable gas, for example hydrogen or argon, and water-vapor at a pressure-below tha't ofpthe atmosphere.

The four. terminals of the duplexer of Fig. l are denominated 1,2, '3, and 4,-the ends of the lower, unsealed 'waveguide 11 being terminals 1 and 3, respectively, and 5 the ends of the u'pp'er, se'aled waveguide 10 being terminal's 2 and 4, respectively. This duplexer is connected witlr a; complete system as shown in Fig. 3. Terminal 1 is connected to the transmitter, or source of radio frequency power (not shown), which is usually the mago netron. Terminal 2 is connected to the receiver (not shown). Terminal 3 is connected to the antenna (not shown). Terminal 4' is' connected to a non-reflective terw shown) which does not reflect energy. In Fig. 3, the

window 18 at the slots is shown in cross section, and it should be noted that this window is spaced away from the 14 and the extensions 15. The purpose of this will be presently explained.

As has already been set forth, the present duplexer is essentially a directional coupler. Considering first the from the antenna, this power enters terminal .3, in the direction of line 22, and proceeds to the left along the lower waveguide 11. When the received signal arrives at the resonant slots 13 and 14, it is directionally coupled over into the upper waveguide 11, and proceeds through terminal 2 to the receiver (not shown). A small fraction of the received signal fails to be coupled over into the upper waveguide 10, and, as indicated by a dotted line 22', proceeds toward terminal 1 and the transmitter.

Here however there is a marked impedance mismatch, due to the fact that the transmitter is dormant, and this stray signal power is reflected back toward terminal 3. On arriving at the slots 13 and 14, however, the stray signal power is directionally coupled into the upper waveguide 10, and therein to the non-reflective termination 21, where it is completely absorbed for all practical purposes.

, This action of the present duplex rendersunnecessary the special anti T-R circuits that have heretofore been employed in radar systems, for, with the present duplexer, the transmitter is effectively decoupled from the receiver.

Considering now the relatively high radio frequency power that is available from the transmitter, this power enters the. duplexer at terminal 1 and proceeds to the right along the lower waveguide 11 as indicated by line 23. When this power arrives at the slots 13 and 14, it tends normally to proceed through the slots into the upper waveguide and toward terminal 4 and the non-reflective termination 21. However, the voltage produced across the slots is so great that the gaps between the ends of the antenna through terminal 3. A small amount .of transmitter power is coupled into the upper waveguide 10, however, and proceeds toward the non-reflective termination 21. I have discovered and verified experimentally that the directional coupler nature of the present device is not destroyed by the firing of the gaps between the ends of the extensions 15, and that the portion of the transmitter power that is coupled through the slots 13 and 14 during firing of the gaps is coupled directionally, as indicated by the dashed line 23', as though the gaps were not fired. In other words, that the present duplexer has high level directivity as well as low level directivity is an experimental fact which distinguishes the present duplexer in performance from conventional balanced duplexers. The nonreflective termination 21 absorbs practically all of the stray transmitter power that is coupled into the upper waveguide 10, so that only a very small amount of such stray power is reflected therefrom. The small amount of this power that is reflected proceeds to the receiver, as indicated by the dot-dashed line 23", since the gaps are still fired and it cannot be coupled back in any appreciable quantity to the transmitter.

The voltage level of the transmitter power that eventually finds its way to the receiver is so low that it cannot burn out the crystal detector that is ordinarily found wall material in the immediate vicinity of theslots 13 and relatively low power that is available as received signal 1 there. Thus the present duplexer combines in one device the functions of the T-R and the anti T-R circuits of the known art.

In general and as will be discussed in detail in connection with Fig. 11, the amount of received signal power that is not coupled into the receiver is no greater than 2 db of total power furnished at terminal 3 over a band of frequencies of the order of 500 megacycles wide at 9300 me. mean operating frequency and may be as low as 0.2 db thereof in a narrower band. Thus within the wider band, at least 63 percent of the received signal power arrives at the receiver. On the other hand, the transmitter power that is coupled out of the lower waveguide 11 into the upper waveguide 10 is at a level approximately 40 db below that in the lower waveguide 11, so that nearly percent of the transmitter power is furnished to the antenna. The fraction of the transmitter power that is furnished to the receiver by reflection from the practically reflectionless termination 21 is at the still lower level of approximately 60 db below that of the original power in the lower waveguide 11, and this small fraction cannot damage the receiver crystal even when energy at a 30 kilowatt level is furnished by the transmitter.

While the crossed arrangement of the two slots 13 and 14 is preferred, as will be explained below, the invention may be practiced with directional couplers of other forms, such as the directional coupler shown and described in my copending application Serial No. 784,277, filed November 5, 1947. Fig. 4 shows a slot arrangement in accordance with the disclosure of said copending application. Thus the common wide wall 12 may be provided with one or more pairs of individual mutually perpendicular slots 26 and 27. The transversely directed slot 26 is centered at the longitudinal center line of the wall 12, and the longitudinally directed slot 27 is centered to one side thereof, so that when the excited waveguide is excited in the fundamental mode, each will be in a position of advantage for coupling energy, all as is explained in said copending application. The present arrangement differs from that of said copending application, however, in that the slots are resonant to the mid-band frequency of the operating frequency band, for, in the present invention, it is desired to secure, as nearly as possible, 100 percent coupling from one waveguide to the other. Each slot is provided with confronting projections 28 disposed in the long walls thereof so that they originate at points of high potential difference, which function as discharge electrodes in a low pressure atmosphere, as do the extensions 15.

When the slots 26, 27, and discharge electrodes 28 are all enclosed in a proper low pressure gaseous atmosphere, as in Fig. 1, the directional coupler in accordance with Fig. 4 may be employed in place of the crossed slot arrangement of Fig. 1 to provide a duplexer which behaves basically like that of Fig. 1. It is possible, however, that one of the slots 26 or 27 of a pair will fire prior to the other during the T-R function, and, if that happens, the directional coupler action will be impaired and the output signal pulse at the antenna will not be sharp and distinct, but may be irregular, with some power lost through one of the slots into the termination 21 or perhaps even into the receiver, where damage might result to the crystal. Hence, one reason for preferring the crossed slot configuration shown in Fig. 1 is that, because the gaps of the two slots 13 and 14 are practically coextensive, the firing of one slot necessarily fires the other. In this way, the directional coupler behaves the same each time the gap is fired. The crossed slot configuration is set somewhat to one side of the longitudinal center line in the common wide wall 12 so that each slot will by itself transfer the same amount of power from one waveguide to the other.

Referring now to Figs. 5, 6, and 7, the embodiment of the invention there shown employstwo waveguides 30 each other and joined together in a common wide wall "preferably maintained as short as possible.

5 section 32. The junction may be made by removing a portion of, "for'example, the lower wide wall of the top waveguide 31 and soldering together the 'two waveguides so that a portion"32 of the upper wide wall of the lower waveguide 30 is common to both waveguides. A crossed slot configuration similar to that of Fig. 1, made of two slots 33 and 34, is provided in one corner of the common wall area 32, so that it is to one side of the longitudinal center line of each waveguide. The lower waveguide 3c is sealed at 'both endsiwith the window 16 and the non-reflecting termination 21, and a sealing window 38, corresponding to the window 18 in Figs. 1 and 3, is provided in the upper waveguide covering the Slts33 and 34. A gap 36 is provided among four extensions 35, which correspond to'the extensions 15 in Fig. l. The covering window 38 is. preferably arched as shown in Fig. 7 so that sparking across the gap 36 will not burn it. As will be readily appreciated, the lower waveguide 30 of Fig. corresponds to the upper waveguide 10 of Fig. 1, and the upper waveguide 31 of Fig. 5 corresponds to the lower waveguide 11 of Fig. 1. The various terminals 1,2, 3, and 4 of the dupleXer shown in Fig. 5 are denominated the same as the corresponding terminals in Fig. 1, and are intended to be connected in a system in the same manner as shown in Fig. 3.

The crossed waveguide configuration of Figs. 5, 6, and

7 provides an arrangementwherein equal powers are transferred bythe two slots at all frequencies in the operative frequency band, thereby enhancing the bandwidth and overall performance of the device.

Referring now to Fig. 8, the embodiment there shown employs a replaceable slot and gap element 40, which is mounted in the common Wall section '32. This element may be soldered or screw-threadedly engaged in the wall 32, as desired, and consists of a rim portion 41 and equally spaced around the circumference thereof four radially inwardly projecting arms 42, which extend toward the center to provide a gap 43. The arms define a-pair of somewhat wing-shaped crossed slots 44 and 45, which together have theappearance of a four-leaf clover. In fact,-the slots 13 and 14 or 34 and35 may also be rounded at the outer ends'to-provide a similar appearance. The crossed-slot arrangement of the invenand 38 and a'suitable matched load or non-reflective termination like termination 21 are provided as set forth above.

The embodiment of the invention that is shown in Figs. 9 and 10 is electrically the same as those of Figs. 1,

5, and 8, but provides a different mechanical structure wherein the gap is adjustable and the electrodes aremore easily renewed. Two waveguides 50 and 51 are disposed at right angles to each other, with wide walls confronting, and a smallspace 52 between them. Each waveguide is provided with an aperture 53, 54, respectively, in the wide wall confronting the other. The apertures are collinear and each is near an edge of its wall, so that, as shown in the top plan view of Fig. 9, they are disposed in a corner of the square region where one waveguide 51 is directly above the other 50. This disposition is the same as that of the slots in the embodiments shown in'Figs. 5 and 8. A tube 55 of electrically conductive material connects the two apertures. This tube is in effect a short length of waveguide, and is The tube is provided with four radially inwardly directed screws 56 which approach but do not meet at the center. The

space between their inner endspr'ovides a spark .gap '57.

The lower'waveguide 50 .is sealed at one end with the non-reflecting termination 21 .or .the like and .at the otherend with the window 16 (not shown). The .aperture'54 of the upper waveguide 51 is closed inside that waveguide with a window 58. The spark gap 57 is thus completely sealed off from theatmosphere and can be surrounded withgas at .a low pressure. The screws 56 provide for adjustment of the gap and can be .re-

placed. The threaded fit of the screws into the tube 55 must be such that low pressure can be-m'aintained around the spark gap. This can be assured by providing a solder seal (not 'shown) after the gap '57 is adjusted, thus been built and tested, none has been provided with a keep-alive potential, and yetnone has been known tohold off firing.

Inthe so-callcd X-band, which is in the region of 3 centimeter wavelengths,-the duplexer accordingto Figs. 5 a'nd'8 occupies a'space which -can be made as small as one cubic inch. This small space includesboth'the'T-R and the anti T-R equipment, whichis a'vast saving in Space over the prior knownarrangements. As has been pointed out above .in the'discussion-of Fig. 3, the duplexer of the present-inventionis actually a directional coupler atboth highandlowlevels, with a practically non-existent amount of loss at high levels, and the directionalcoupler featureathigh levels provides for protection vof the receiver. For example, the high levelloss-due-to'the duplexer has beenfoundon some samples to be from 0.1 to 0.3 idb. At low=levels,'however, that is, during receiving, the duplexer'is -a directional coupler inthe ordinary sense, the slots 'beingin the unfired condition, and thereis a loss of energy "due to the insertion thereof between the antenna and theftreceiver. Fig. 11 shows the amount of'the insertionloss for atypical duplexer'operating in the X-band. The curve 60 shows that over a wide :band, from 8900'to greater than 9400 megacycles per second, the insertion loss'was never greater than 2.1 db. In: the band from 9-180 to 9375 megacycles, the loss was.0;5 db oriless.

Fnrther'measurements that have'been made indicate that the recovery time-of the present duplexer isat least as good'as'that of prior known duplexers. In other charac-teristics,-s'uch as leakage power,iit is likewise-as suitable =as other duplexers. In its lower losses at both high andlow levels,- its savings in space, cost, and weight,'and in many other features that have been pointed. out-above, it is vastlysupe'rior to .the prior known duplexers.

Duplexers constructed inaccordance with the' present invention may take many physical forms, and no attempt has been 'madeherein to illustrate more than but'a few examples.

Accordingly, it is intended that the claims that follow shall not be limited by the herein described details, but only by the prior art. In the claims thatfollow,

which, when there exist waves travellingin both directions in the main line, delivers to one end of theauxiliary line a voltage which is largely a function of .the amplitude of the wavegOing inone-preferred direction in themain line, and relatively independent of the wave going in the opposite direction in the main line. Also, in the claims that follow, the word annulus is to be understood as defining either the rim 41 of Fig. 8, the tubular eyelet 55 of Figs. 9 and 10, or any equivalent closure element, of whatever contour, that may serve as a bounding anchorage -for the radially extending electrode elements whose inner ends form the described spark gap or gaps, and whose lateral surfaces combine with the inner periphery of the annulus to form the described energy-admitting passage-ways.

What is claimed is:

1. A duplexer comprising a pair of conduits for directing electromagnetic wave energy, an energy coupling means including an annular member providing a directional wave conductive passage between said conduits and a plurality of electrode elements extending radially inward from the peripheral wall of said annular member toward the center thereof and terminating short of said center to form a spark gap therebetween, said electrode elements combining with said annular member to form energy-admitting passage-ways whose transverse width exceeds substantially the thickness of said electrode elements.

2. A duplexer comprising a pair of conduits for direct ing electromagnetic wave energy, an energy coupling means including an annular member providing a directional wave conductive passage between said conduits and a plurality of electrode elements extending radially inward from the peripheral wall of said annular member toward the center thereof and terminating short of said center to form a spark gap therebetween, said electrode elements combining with said annular member to form energyadmitting passage-ways of clover-leaf contour whose transverse width exceeds substantially the thickness of said electrode elements.

3. A duplexer comprising a pair of conduits for directing electromagnetic wave energy, an energy coupling means including an annular member providing a directional wave conductive passage between said conduits and a plurality of electrode elements extending radially inward from the peripheral wall of said annular member toward the center thereof and terminating short of said center to form a spark gap therebetween, said electrode elements combining with said annular member to form energy-ad mitting passage-ways whose transverse width exceeds substantially the thickness of said electrode elements, each of said electrode elements having its radially extending longitudinal axis disposed in parallelism with the longitudinal axis of one of said wave directing conduits.

4. A duplexer comprising a pair of conduits for directing electromagnetic wave energy, an energy coupling means including an annular member providing a directional wave conductive passage between said conduits and a plurality of electrode elements extending radially inward from the peripheral wall of said annular member toward the center thereof and terminating short of said center to form a spark gap therebetween, said electrode elements combining with said annular member to form energyadmitting passage-ways of clover-leaf contour whose transverse width exceeds substantially the thickness of said electrode elements, each of said electrode elements having its radially extending longitudinal axis disposed in parallelism with the longitudinal axis of one of said wave directing conduits.

5. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of wave guides each having two terminals and having a common side wall, a circular aperture in said common wall intermediate said terminals, an annular member adapted to fit into said aperture, a plurality of elongated electrodes mounted on said annular member and extending radially inwardly with their free ends confronting to provide a spark gap therebetween and with their. lateral surfaces combining with said annular member to form energy adt1) mitting passages whose transverse width exceeds substantially the thickness of said electrodes.

6. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides each having two terminals and having a common side wall, a circular aperture in said common wall intermediate said terminals, an annular member adapted to fit into said aperture, a plurality of elongated electrodes mounted on said annular member and extending radially inwardly with their free ends confronting to provide a spark gap therebetween, and with their lateral surfaces combining with said annular member to form directional energy-admitting passages whose transverse width exceeds substantially the thickness of said electrodes.

7. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides each having two terminals and having a common side wall, a circular aperture in said common wall intermediate said terminals, an annular member adapted to fit into said aperture, four elongated electrodes mounted on said annular member and extending radially inwardly with their free ends confronting to provide a spark gap between them, and with their lateral surfaces cooperating with said annular member to form energyadmitting passages of clover-leaf contour, and means supporting said annulus from said wall in said aperture, said electrodes defining a pair of crossed slots resonant to said wave energy, said slots being disposed to directionally couple said waveguides, said electrodes being so dimensioned that said spark gap breaks down when the coupled energy exceeds a prescribed value thereby effectively closing said slots.

8. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides each having two terminals, and havinga common side wall, a circular aperture in said common wall intermediate said terminals, an annular member adapted to fit into said aperture, a plurality of elongated electrodes mounted on said annular member and extending radially inwardly with their free ends confronting to provide a spark gap between them, and with their lateral surfaces cooperating with said annular member to form energy-admitting passage-ways of clover-leaf contour, means supporting said annular member from said wall in said aperture, said electrodes defining a wave passage through said aperture which is disposed to directionally couple said waveguides, and being so dimensioned that said spark gap breaks down when the coupled energy exceeds a predetermined value thereby efiectively closing said passage, and means providing a hermetically-sealed cell about said annular member and electrodes.

9. A duplexer to be used with ultra-high frequency electromagentic wave energy comprising a pair of waveguides having confronting spaced-apart side walls, each of said waveguides having two terminals, a third waveguide connected between said side walls in directional energy coupling relationship between said waveguides of said pair, and electrode means combining with said third wave guide to form energy-admitting passage-ways of clover-leaf contour.

10. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides having confronting spaced-apart side walls, each of said waveguides having two terminals, a third waveguide connected between said side walls in directional energy coupling relationship between said waveguides of said pair, and electrode means combining with said third waveguide to form energy-admitting passageways of relatively wide extent, said electrode means having confronting end surfaces forming a gap of relatively narrow extent for emergency shunting of said relatively wide passage-ways.

11. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides having confronting spaced-apart side walls, each of said waveguides having two terminals, a third waveguide connected between said pair of waveguides in directional energy coupling relationship between said waveguides of said pair, said third waveguide being disposed at a region intermediate said terminals, and electrode means extending transversely into said third waveguide with inner ends confronting to provide a spark gap of relatively narrow extent, said electrode means 00- operating with said third waveguide to define at least two wave passage-ways of relatively wide extent.

12. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides having confronting spaced-apart side walls, each of said waveguides having two terminals, a third waveguide connected between said pair of waveguides in directional energy coupling relationship between said waveguides of said pair, said third waveguide being disposed at a region intermediate said terminals, and four elongated electrodes extending transversely into said third waveguide with inner ends confronting to provide a spark gap of relatively small area, said electrodes cooperating with said third waveguide to define at least two wave passage-ways of relatively large area.

13. A duplexer adapted to be used with ultra-high frequency electromagnetic wave energy comprising a pair of waveguides having confronting spaced-apart side walls,

each of said waveguides having two terminals, a third waveguide connected between said pair of waveguides in directional energy coupling relationship between said waveguides of said pair, said third waveguide being disposed at a region intermediate said terminals, electrode means extending transversely into said third waveguide with inner ends confronting to provide a spark gap, said electrode means being radially movable with respect to said third waveguide, and cooperating with said third waveguide to define wave passage-ways of clover-leaf contour.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,728 Southworth Apr. 11, 1939 2,407,069- Fiske Sept. 3, 1946 2,432,093 Fox Dec. 9, 1947 2,473,274 Bradley -a June 14, 1949 2,479,650 Tiley Aug. 23, 1949 2,602,859 Moreno July 8, 1952 2,606,248 Dicke Aug. 5, 1952 2,644,139 Hunter June 30, 1953 FOREIGN PATENTS 592,224 Great Brita-in Sept. 11, 1947

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