US2498720A - High-frequency protective circuits - Google Patents

Description

Feb. 28, 1950 N. R. wu ETAL HIGH-FREQUENCY PROTECTIVE CIRCUITS Filed Feb. 6, 1947 RN T M \MWQ E mm \m llllllllllllll n AURMAN R. WILD l/US7DNWCOC/(P/LL flm Q yr.

ATTORNEY Patented Feb. 28, 1950 HIGH-FREQUENCY PROTECTIVE CIRCUITS Norman R. Wild, East Natick, and Huston W.

Cockriii, Auburndale, Mass., assignors to Raytheon Manufacturing Company, Newton, Mass a corporation of Delaware Application February 6, 1947, Serial No. 726,914

2 Claims. (Cl. 250-17) This invention relates to electrical circuits, and

i more particularly to a system for automatic protection of a high-radio-frequency generator or microwave generator.

An object of this invention is to devise a system for automatic protection of a microwave generator against high-voltage breakdown at the window or seal thereof.

Another object is to devise a protective system which will automatically afford protection against standing wave amplitudes in a radio-frequency transmission line which are in excess of a predetermined value, such value being manually variable at will.

A further object is to provide a protective system for a microwave transmission system which will afford protection against an unduly high ratio between standing wave maxima and minima herein.

The foregoing and other objects of the invention will be best understood from the following description of some exemplifications thereof, reference being had to the accompanying drawing, wherein:

Fig. 1 is a diagrammatic representation of one form of protective system according to our invention; and

Fig. 2 is a diagrammatic representation of a modified protective system.

Now referring to the drawing, and particularly to Fig. 1 thereof, the numeral I generally designates an electron-discharge device of the magnetron type, which includes, for example, an evacuated envelope 2, made of highly conductive material, such as copper, and provided with a plurality ofanode vanes 3. The arrangement is such that each pair of adjacent anode vanes 3 forms, together with that portion of the envelope lying therebetween, a cavity resonator whose natural resonant frequency is, as is well known to those skilled in the art, a function of the geometry of the physical elements making up the same. The dimensions of each such cavity resonator are preferably such that the wavelength of the electrical oscillations adapted to be generated therein has a predetermined value lying in the highradio-frequency or microwave region of the frequency spectrum.

Centrally located in envelope 2 is a highly electron-emissive cathode member 4, for example of the well-known alkaline-earth metal-oxide type, said cathode member being provided with conventional means (not shown) for raising the temperature thereof to a level sufficient for them ionic emission.

The electron-discharge device I is completed by magnetic means (not shown) for establishing a magnetic field in a direction transversely of the electron path between the cathode and anode members thereof.

In order to supply an operating voltage to magnetron I, to energize the same and to thereby cause it to act as a source of radio-frequency energy,cathode 4 is connected, by means of a lead 5, to the negative side of a suitable direct current source, while envelope 2 is connected, by means of a lead 6, through the normally-closed contacts I of arelay 8, to the positive side of said direct current source. As long as contacts I are closed, magnetron I will be energized, and hyperfrequency oscillations will be generated in the cavities provided betweenadjacent vanes 3, but when contacts I are open, the operating voltage is removed from said magnetron and such oscillations are no longer produced.

Theouter conductor 9 of a coaxial output pipe III has one end sealed into envelope 2 and has its opposite end fastened securely to the end wall of a hollow metallic waveguide II which may be termed a transmission line, since it is adapted to transmit radio-frequency energy therealong. Inner conductor I2 of coaxial pipe III has a loop I3 formed on one end thereof, this loop extending into one of the cavities between adjacent vanes of magnetron I and serving to couple the radio-frequency energy output of said magnetron to output pipe I0. The end of conductor I2 opposite loop I3 extends beyond the end wall of waveguide II into said waveguide to serve as an exciting rod or probe therefor. A disk I4 of dielectric material is mounted betweenouter conductor 9 and inner conductor I2 of pipe II! at the end of theouter conductor 9 or at the end wall of waveguide II, this disk providing a hermetic seal between these two conductors and being termed the window or seal of the magnetron I.

Transmission line or waveguide II may have any desired length, and a first coupling loop I5 extends into the interior of said guide, through a suitable opening I6, at a point spaced a distance of an integral number of quarter-wavelengths from the end wall of said guide, or from the window or seal of magnetron I, the term wavelength referrin to the wavelength of the oscillations of oscillator I. The end of loo I5, which extends through opening I6 to the exterior of the guide II, is connected to one electrode or element I! of a discharge device I8 of the cold-cathode gaseous diode type, such as anordinary neon 3 bulb, the second element of device [8 being denoted by [9.

Asecond coupling loop 20 extends into the interior of guide ll through a suitable opening 2|,loop 20 being located at a point spaced a distance of a quarter-wavelength beyond loop I in the direction therefrom opposite from the magnetron window or seal. The end ofloop 28, which extends through opening 2| to the exterior of guide I l, is connected to one electrode orelement 22 of;

adischarge device 23 which is similar to discharge device l8, the second element of device "23 being denoted by 24. Although discharge devices set up in the guide.

' devices it or 23 will break down at any predel8 and 23 are shown as diodes, gaseous-tubes of 7 the multiple-element type may also be-used.

The input terminals of a full-wave bridge recti--fier 25, which is adapted to serve as a current and voltage source, are connected to a suitable alternating current source, for example the ordinary 1l5-volt, GO-cycle commercial power lines. Asuitable smoothingoi filtering condenser 26 is connected-across the direct current output terminalsof rectifier -25. Alead'i'l isconnected at one'end to one ofthe output terminals of rectifier and is also connected, in series with the operating winding 260i relay '8, to the electrode ii and 22 of devices it and 23.Alead 29 is connected at one end to the other output terminal ofrectifier 25 andis also connected to one end-oi aresistor 30 having amovable tap 3| thereon, this tap being in turn connected to theelectrodes IQ and 24 of devices it and 23.

It will be apparent, from the above-described circuit, that devices it and 23 are connected in parallel across thedirect voltage source 25,

and that the discharge paths between the elements of therespective devices 18 and 23 are connected in parallel with each other, each of such paths being in series between theD. C. source 25 and the windingit of relayB. Therefore, when the gaseous atmosphere in -either of thedevices 18 or 23 ionizes orbreaks-down to provide a current-conducting path between'the twoelements of the ionized device, current can flow from the current source .25, through such conducting path and relay winding-m energizing relay 8 and opening contacts l. Radio-frequency energy of a predetermined wavelength is -supplied by source I to-"waveguide ll and is'propagated therealong to some reactive load (not shown), causing standing waves to be established inthe radio-frequency transmission system including waveguide l i. Tubes of the magnetron type have the inherent characteristic that, for Stable'operation of the tube, there-is an optimum-standing wave phase value which is desired at the window or seal thereof. In other words,iorstableoperation, the tube wants to look at either avstande ing wave maximum or a standingwave minimum at the seal thereof. It is also desirable tolim-it the amplitude of the standing waves in the guide to a certain value, in other words, to in efiect maintain the standing wave ratio in the radiofrequency transmission line below a predetermined value, in order to prevent high-voltage breakdown at the window or seal of the tube or source.

The radio-frequency energy in gguide II is coupled withdevices 18' and 23 by means of the respective coupling loops [5 and.20. A .direct voltage bias is applied todevices 18 "and 23 from the directvoltage biasing source 25, thisbias being adjustable bytap 3|. Theatmosphere in either one of the devices l8 or 23 is ionizableiby the radio-frequency energy if thestanding wave jtermined standing Wave amplitude, or any predetermined standing wave ratio, in the guide. Whensuch a predetermined standing wave ratio has been reached, due to changes in the load orv other reasons, the atmosphere in device it or indevice 23 will be ionized to provide a conducting path in the ionized device, resulting in the energization ofrelay 8, the opening of contacts-l, and the consequent deenergization of magnetron i. Itwillbe seen, therefore, that the system of this invention automatically in effect maintains the standing wave ratio in a radioirequency transmission line below a predetermined manually-adjustable value, since if it goes above this value the magnetron l is deenergized.

Two ionizable devices l8 and 23 areprovided, connected inparallel and spaceda quarter-wavelength apart, so that one or the other of said devices will be ionized, zno matter What standin wave phase relations may be-established by the rmknown characteristics of the load, provided, of course, that the standing wave. ratio in the transmission line'rises above the predetermined value. Of course, the relay Bis energized to deenergize the magnetron when eitherone-of the devices ie or 23 is ionized.

If a standing wave maximum is desired and'is known to be atthe tube window Miwhen. excessive standing wave ratios arelikely tooccur,.theiactor n in Fig. 1 should be made an even number, so thatloop i5 is spaced an even number ofquarterwavelengths fromseal l 4, while if a standing wave minimum is desired randis known to .be at'the tube window when excessivestanding wave: ratios are likely to occur, the factor n shouldbe: made an oddnumber, so that loop I5 is. spaced an odd number of quarter-wavelengthsfrom seal It. In both these known phase cases,thereiore, loop [5 is located at a standingwavemaximumand functions, when such maximum becomesgreater than a predetermined value, to deenergize magnetron! by causing the ionization of device l8 and the consequent energ'mation ofrelay 8, thus affording protection against excessive standing wave amplitude in these cases of known phase, as well as in the conditions of unknown phase relations. in the guide. If desired, the bias ondevices 18 and 23 so adjusted that any increaseabove the voltage present in a flat line, that is, one'in which there are no standing waves, will cause breakdown of one or the other of such devices. In this case, the system of our invention maybe made to give protection of the magnetron at an absolute minimum value of standing wave amplitude in the transmission system.

Now refering to Fig. 2, which shows'a modified protective system, elements similar to those of Fig. 1 are denoted by the-same reference numerals. In this modification, only a.single discharge device 32 is used, this device being similar todevices l8-iand23 ofFig. 1 and including a pair of electrodes orelements 33 and 34. A-pair of coupling loops l5 and 20, spaced a quarter-wavelength apart, project through the respective openings l3 and El into waveguide II and are connected, outside of the guide, to therespective elements 33 and 34 ofdevice 32, thus serving to couple the radio-frequency energy of the transmission line or waveguide l i to said device.

Movable tap 3! on resistor 39 is connected toelement 33 ofdevice 32, whilelead 21 is connected, through relay winding 28, toelement 33 ofdevice 32. From the above connections, it will be seen thatrectifier 25 serves as a variable direct voltage biasing means fordevice 32, and that, when the atmosphere indevice 32 is ionized, current flow fromcurrent source 25 through winding 23 and current path 3334 in series, energizingrelay 8 to open contacts I and deenergize magnetron I.

As in Fig. 1, standing Waves of radio-frequency are set up in guide ii, and a direct voltage bias is applied to dischargedevice 32. Since loops l5 and 20 are a quarter-wavelength apart, the standing wave maxima may be considered as coupled to one loop and the standing wave minima to the other. When the ratio between said maxima and said minima becomes high enough to produce, in combination with the bias voltage applied todevice 32, a total voltage equal to or greater than the ionization voltage of said device, the gaseous atmosphere in said device becomes ionized, or said device breaks down, allowing current from the source to pass through the path 33-34 of said device to energizerelay 8, opening contacts 7 to deenergize magnetron I.

It is thus apparent that when the ratio between standing wave maxima and standing wave minima in the guide is above a predetermined but adjustable value, the magnetron is automatically deenergized. In this case, as in Fig. 1, the discharge device becomes ionized by radio-frequency energy in the guide, but this ionization can occur only when the ratio between standing wave maxima and minima in the guide is above a predetermined value which is variable at will by varying the bias applied from the biasing source ,todevice 32. It is to be further understood that reliable operation of this sytem is obtained only when the standing wave phase in the transmission line is fixed and known approximately, and when the factor n is made such as to placecoupling loop 15 in the vicinity of a standing wave voltage maximum or a standing wave voltage minimum, depending on the phase that the tube I wants to see in order to operate in a stable manner. Of course, ifloop 15 is at a minimum,loop 29 will be at a maximum, and vice versa, because of the quarter-wavelength spacing between these loops.

Of course, it is to be understood that this invention is not limited to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. For example, the predetermined point at which the discharge devices will break down may alternatively be varied by adjusting the coupling of these devices to the waveguide, rather than by adjusting the direct voltage bias thereon. Also, alternating voltage bias may be used on the discharge devices, rather than direct voltage, if desired, and if this is done, an alternating voltage relay would be needed in the controlled part of the system. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

Some features of invention which are disclosed but not claimed herein are disclosed and claimed in copending application Serial No. 753,180, filed June 7, 1947, and owned by the same assignee as the present application.

What is claimed is:

1. A protective system, comprising a source of radio-frequency energy of a predetermined wavelength, controllable means for supplying an operating voltage to said source, a transmission line connected to said source, a pair of gaseousdischarge devices each having a plurality of elements, said devices being coupled to said line at points spaced a quarter of said wavelength apart in such a way that the gaseous atmosphere of either of said devices is ionizable by standing wave peaks in said line to provide a current-conducting path between the elements of the ionized device, means independent of said transmission line for biasing said pair of devices in parallel so that said ionization can occur only above a predetermined value of standing wave ratio in said line, and means coupling said devices in parallel to said controllable means for controlling the same.

2. A protective system, comprising a source of radio-frequency energy of a predetermined wavelength, controllable means for supplying an operating voltage to said source, a transmission line connected to said source, a gaseous-discharge device having a plurality of elements, two of said elements being coupled to said line at points spaced a quarter of said Wavelength apart in such a way that the gaseous atmosphere of said device is ionizable by standing wave peaks in said line to provide a current-conducting path between said elements, means independent of said transmission line for biasing said device so that said ionization can occur only when the ratio between standing wave maxima and minima in said line is above a predetermined value, said biasing means being variable at will to vary said predetermined value, and means coupling said device to said controllable means for controlling the same.

NORMAN R. WILD. HUSTON W. COCKRILL.

REFERENCES CITED The following references are of record in the file or this patent:

UNITED STATES PATENTS Number Name Date 1,180,075 Pichon Apr. 18, 1916 1,779,259 Meissner Oct. 21, 1930 1,913,588 Francis June 13, 1933 2,165,848 Gothe et al July 11, 1939 2,262,932 Guanella -i Nov. 18, 1941 2,304,015 Peterson et a1 Dec. 1, 1942 2,366,660 Uselman Jan. 2, 1945 OTHER REFERENCES Radio News, article by McQuay, pp. 36 and 37, February 1946.

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