WAVEGUIDE WINDOW FOR TRANSMISSION OF ELECTROMAGNETIC WAVES
United States Patent 3675165
A window for sealing a waveguide gas pressurized to transmit microwave signals comprising: a dielectric plate permeable to the microwave signals, and a mechanical structure hermetically sealing the dielectric plate in the interior of the waveguide and including four openings dimensioned to provide the window with a substantially flat voltage standing wave ratio versus frequency characteristic over a predetermined frequency range, two relatively movable members to permit the dielectric plate to move to compensate for unequal gas pressures on opposite surfaces thereof, and a space to allow movement of the two members to compensate for thermal expansion thereof due to microwave signal loss in the dielectric plate.
US Patent References:
Glass closure for metal containers
Smith - June 1937 - 2085277
Transmission line windows having high voltage breakdown characteristic
Clark, Jr. - April 1955 - 2706275
Waveguide window
Bondley - April 1959 - 2881401
Microwave switching device
Brewster - August 1959 - 2900568
Envelope structures for electron tubes
Preist et al. - September 1959 - 2903614
Glass closure for metal containers
Smith - June 1937 - 2085277
Transmission line windows having high voltage breakdown characteristic
Clark, Jr. - April 1955 - 2706275
Waveguide window
Bondley - April 1959 - 2881401
Microwave switching device
Brewster - August 1959 - 2900568
Envelope structures for electron tubes
Preist et al. - September 1959 - 2903614
Inventors:
Ueda, Isao (Tokyo, JA)
Kageyama, Takao (Tokyo, JA)
Inomata, Isao (Tokyo, JA)
Kageyama, Takao (Tokyo, JA)
Inomata, Isao (Tokyo, JA)
Application Number:
05/066040
Publication Date:
07/04/1972
Filing Date:
08/21/1970
View Patent Images:
Export Citation:
Assignee:
Nippon Electric Company, Limited (Tokyo, JA)
Primary Class:
Other Classes:
333/33
International Classes:
H01P1/08; H01P1/30; H01P1/08; H01P5/00
Field of Search:
333/98P,98R,33
US Patent References:
| 2964719 | Electronically controlled microwave attenuator | December 1960 | Hatch | |
| 3101460 | Hermetically sealed waveguide window with non-sputtering iris | August 1963 | Walker et al. | |
| 3163835 | Voltage-tuneable microwave reactive element utilizing semiconductor material | December 1964 | Scott | |
| 3387237 | Microwave window | June 1968 | Cook |
Other References:
Vogl, et al., "Vacuum Tight Windows with Wide Band Transmission Characteristics," Rev. of Scientific Instruments, Vol. 36 -10-10-65, pp. 1439-1440 .
Chen RCA, "Broadbanding of Resonant Type Microwave Output Windows," RCA Review 6-1954, pp. 204-229 .
Chen; T. S. "Output Windows for Tunable Magnetrons," Electronics 5-1954, p. 170-173.
Primary Examiner:
Saalbach, Herman Karl
Assistant Examiner:
Punter, Wm H.
Claims:
What is claimed is
1. A window for sealing a waveguide filled with a preselected gas under pressure and transmitting microwave signals without adversely affecting a transmission frequency characteristic of said waveguide, comprising:
2. The window according to claim 1 in which said collar means includes:
3. The window according to claim 1 in which said collar means includes:
4. A window for sealing a waveguide filled with a preselected gas under pressure and transmitting microwave signals without adversely affecting a transmission frequency characteristic of said waveguide, comprising:
5. A window for sealing a waveguide filled with a preselected gas under pressure and transmitting microwave signals without adversely affecting a transmission characteristic of said waveguide, comprising:
1. A window for sealing a waveguide filled with a preselected gas under pressure and transmitting microwave signals without adversely affecting a transmission frequency characteristic of said waveguide, comprising:
2. The window according to claim 1 in which said collar means includes:
3. The window according to claim 1 in which said collar means includes:
4. A window for sealing a waveguide filled with a preselected gas under pressure and transmitting microwave signals without adversely affecting a transmission frequency characteristic of said waveguide, comprising:
5. A window for sealing a waveguide filled with a preselected gas under pressure and transmitting microwave signals without adversely affecting a transmission characteristic of said waveguide, comprising:
Description:
This invention relates to an input or output waveguide window for sealed microwave tubes and to a pressuring window for sealed high-power waveguide means filled with gas such as nitrogen gas.
Several types of waveguide windows for sealing waveguide systems for high power use are known. Among them, there are (1) half-wavelength dielectric resonant windows; (2) cone windows; (3) slanted-disk windows; and (4) circular windows.
All these types have been widely in use as input or output windows for high-power microwave tubes for use in ground-stations of satellite communications systems. Their broad-band characteristics and high-power-withstanding property have been fully demonstrated in diversified application fields.
Nevertheless, all of these window types have caused technical difficulties particularly when operated at relatively low microwave frequencies, i.e., in the UHF band.
For instance, a ceramic disk of 400 mm in diameter, is needed when a waveguide window designed for a 500 MHz band is operated as a circular window. This requires overall window assembly dimensions as large as 410 mm in diameter and 350 mm in thickness. In case of the half-wavelength resonant window, a ceramic block as large as 105 mm by 190.5 mm by 381 mm would be required. Since the axial length of the window becomes as long as 105 mm, the window assembly becomes bulky and heavy. In addition, low-loss dielectric materials used for the window such as mica, ceramic, or quartz are expensive, making both manufacture and finishing of these dielectric materials difficult.
The dielectric-filled oval-aperture windows for low-power use are advantageous compared with other types of windows such as the circular window, in that both the overall size of the window assembly and the amount of the dielectric material can be reduced. It must be noted however that they have a narrow bandwidth and are weak against thermal expansion as compared with the aforementioned conventional types suited for high-power applications. In other words, it is substantially impossible to manufacture the oval-aperture resonant windows suited for high-power use or large windows of this type for UHF band use.
Accordingly, it is the principal object of this invention to provide a compact, light, and inexpensive waveguide window adapted to sealing waveguide means for handling high UHF band power, without affecting the frequency response of the waveguide itself.
The high-power, broad-band waveguide window of this invention is placed in a rectangular waveguide with the TE 10 mode as the dominant mode. The waveguide window arrangement consists essentially of; (i) a low-loss ceramic plate capable of transmitting high microwave power and of providing excellent sealing action (i.e., nonporous, etc.) for the waveguide system; (ii) a pair of metallic members bonded to the opposite sides of the ceramic plate. (By the provision of the metallic members on both sides of the plate, aperture dimensions are precisely determined and, at the same time, window failures can be prevented. If the metallic member were bonded only to one side of the ceramic plate as in the conventional window structure, window failures due to the difference in the thermal expansion characteristics of the two different materials would be inevitable); (iii) an assembly of metallic members jointly serving to clamp and support the window assembly in a rectangular waveguide, and to provide a hermetic sealing joint for the window aseembly; and (iv) means composed of a combination of a pair of apertures formed on the opposite sides of the ceramic element, effectively separated with an electrical distance corresponding to the guide wavelength in the ceramic plate, and the other combination of two or more apertures formed in the assembly of the metallic elements.
The above-mentioned and other features and advantages of this invention will be fully understood from the following detailed description taken in conjunction with the accompanying drawings, wherein:
FIG. 1 shows a cross section of a conventional ceramic-bonded oval-aperture resonant window in a rectangular waveguide;
FIG. 2 is a plan view of the resonant window shown in FIG. 1;
FIG. 3 shows a cross section of a resonant window for use in a rectangular waveguide according this invention;
FIG. 4 shows a plan view of a ceramic plate for use in the embodiment of FIG. 3;
FIG. 5 shows a cross section of another embodiment of this invention; and
FIG. 6 shows a characteristic curve of the embodiment of FIG. 3.
Referring to FIG. 1, it will be seen that a waveguide window of this type is disclosed, for example, in G.L. Ragans paper published on page 221 of "Microwave Transmission Circuit," Vol. 9, M.I.T. Radiation Laboratory Series.
With this window structure, the periphery of the surface of the ceramic plate 12 to form a hermetic seal with the apertured metallic plate 13 is metallized so that the ceramic plate 12 can be brazed to the plate 13. The metallic plate 13 is securely bonded to a metallic flange 14 which is securely clamped to the internal wall of the rectangular waveguide by brazing.
The ceramic resonant window with a resonant opening 15 in the metallic plate 13 gives a resonance characteristic centered at a desired operating frequency.
Referring to the plan view of the oval-aperture resonant window of FIG. 2, the aperture 15 has a shape as shown by the broken line. With this window structure, a pair of confronting portions of the metallic flange 14 disposed transverse to the waveguide axis and perpendicular in the direction of the electric field acts as a parallel capacitive susceptance element in the waveguide. In like manner, the other pair of confronting portions of the metallic flange 14 disposed parallel to the direction of the electric field acts as a parallel inductive susceptance in the waveguide. Accordingly, the resonant opening 15 surrounded by the two pairs of confronting portions may be represented equivalently by a parallel resonant circuit.
A ceramic plate in a waveguide window is known to behave so as to increase the above-mentioned parallel capacitance when the waveguide is shown by an equivalent circuit. Therefore, the ceramic resonant window can be matched to a desired resonance frequency by making the width of the window narrower than that of a window with an opening which is not filled with dielectric material. Since the equivalent circuit of such a resonant window containing a dielectric element is a simple resonant circuit without exception, such ceramic windows are narrower in bandwidth than other types of windows such as circular windows. Furthermore, with the increase in the cross sectional area of waveguides adapted to the UHF band use, the size of ceramic plates becomes large.
Consequently, excessively large sealing thermal stresses are produced in the ceramic plate due to the difference in the thermal expansion characteristics of the dielectric element 12 and the metallic plate 13. This tends to cause deformation or breakdown of the ceramic plate when the plate is in the process of fabrication or in high power operation.
Referring to the embodiment of FIG. 3, the window assembly is sealed in a rectangular waveguide 11. The periphery of each of the side surfaces of the ceramic plate 16 is metallized, and a pair of frame members having outer frame 17 and inner frame 18 are brazed to the opposite metallized surfaces of the plate.
To the outer frame 17 one end of an inner collar member 19 is brazed substantially in perpendicular relations to each other. One end of an outer collar member 21 outwardly tapered is brazed to the other end of the inner collar 19, and the other end of the outer collar is hermetically fixed to an outer flange member 22 hermetically fixed to the internal wall of the waveguide to form a space 28. That surface of the inner metallic frame 18 which is not brazed to the ceramic plate is slidably mounted on the surface of an inner flange member 23.
This window structure may be considered to contain a combination of four resonant apertures -- that is, a pair of apertures 24 and 25 on opposite sides of the plate 16, an aperture 26 formed in the inner collar 19, and an aperture 27 formed in the inner flange 23.
Extensive experiments conducted by the present inventors using this embodiment have shown the following results: (i) A maximally flat voltage standing wave ratio (VSWR) vs. frequency characteristic obtained, which is the overall resonance characteristics of the four apertures; (ii) despite the anticipated disadvantages such as the increase in the overall length of the window in the axial direction of the waveguide, optimum VSWR ratio can be obtained by suitably selecting the dimensions of the ceramic plate, the area of two apertures 24 and 25 defined respectively by the frame members 17 and 18, and the supporting member assembly and thereby, controlling the resonance characteristics of the four apertures; and (iii) the apertures 24 and 25 on the opposite sides of the ceramic plate are separated with a distance equal to the thickness t of the plate. Since the dielectric coefficient of the ceramic is larger than unity, however, the electrical length for this spacing becomes longer than t (One of the apertures may be greater in area than the other). Therefore, the resonance characteristic of the bandpass filter formed by the two apertures is governed not by the smaller of the two, but by both. Thus, a broadband VSWR characteristic can be obtained.
Features of this window structure may be specified as follows; (i) The window is securely held and sealed in place with the aid of the inner collar 19 and the outwardly tapered outer collar 21, contiguous ends of which collars are brazed or welded together. (ii) The free surface of the inner frame 18 is not firmly fixed to the inner flange 23 but only pressed thereto. Thus, the deformation of the window assembly is prevented, that may be caused due to the pressure difference on both sides of the window when the left-hand side of the ceramic plate is evacuated. (iii) Inner collar 19 and outer collar 21 are so arranged as to form space 28 which can accommodate thermal expansion of the outer frame 17 inner frame 18 and ceramic plate 16 when the ceramic plate 16 is heated due to the dielectric loss. For this feature, a compact window structure suited for high power operation can be realized. (iv) The outer and inner frame members 17 and 18 brazed to the opposite metallized surfaces of the ceramic plate are of the same metallic material and thickness. Consequently, a relatively uniform thermal stress distribution is obtained and the danger of deformation or breakdown of the ceramic plate due to the difference in the thermal expansion characteristics of the metal and the ceramic can be prevented.
It will be obvious from these features that a ceramic plate of sufficiently large size can be brazed to the frame member and that a ceramic window capable of transmitting high power can be manufactured.
A still further advantage of the window structure of FIG. 3 is that the geometry of the plate can be arbitrarily selected. This is because the plate needs to be simply sandwiched between the frame members by brazing and is not subject to limitations of the prior art such as the geometry of the window opening or recessed part of the metallic plate.
According to the inventors' experiments, the size of the ceramic plate used for a WR-1500 type rectangular waveguide operated at 500 MHz was as small as 5 × 160 × 340 mm and the overall length of the window assembly in the axial direction of the waveguide was of the order of 30 mm.
This demonstrates that the high-power, broad-band waveguide window for operation in the UHF band can be made much more compact, lighter, and less expensive than the conventional windows.
In the embodiment shown in FIGS. 3 and 4, the outer metallic collar 21 may have an outer portion extending to and hermetically fixed to the inner wall of flange member 22, replacing the inner cylindrical collar 19 and the outer collar 21.
Referring now to FIG. 4, the ceramic plate 16 used in the window assembly of FIG. 3 is of elongated-octagonal shape. The broken line represents the shape of the aperture 24. It is to be understood that such an octagonal shape facilitates manufacture and finishing of ceramic plates and reduces the manufacturing costs as compared with the conventional oval-shaped ceramic plates, especially when the plate size is made large.
Referring to FIG. 5, the peripheral portion of each side surface of the ceramic plate 31 is metallized similarly to the previous embodiment. An inner frame 33 and an outer frame 32a formed integrally with an outwardly flared collar 32 are brazed to the opposite metallized surfaces of the ceramic plate. The other end of the collar 32 and one end of outer collar 34 are brazed or welded together. The other end of the outer collar 34 is anchored to the metallic flange 35 to form a hermetic sealing joint. The free surface of the inner frame 33 is slidably mounted on the surface of the metallic flange 35. As in the case of the foregoing embodiment, this window assembly contains four resonant apertures in all -- that is, apertures 36 and 37 on the opposite sides of the ceramic plate, another aperture 38 in the outwardly flared metallic collar 32, and still another aperture 39 in the metallic flange 35. It is possible with this window assembly to obtain a desired broad-band response by suitably varying the dimensions and mutual relationships of these apertures and thereby, controlling their individual rosonance characteristics.
The embodiment of FIG. 5 may be considered to be an improved version of the first embodiment in that the outer frame 17 and the inner collar 19 in the structure of FIG. 3 have been replaced with a single-piece outwardly flared collar 32 and that by the adoption of this outwardly flared collar, the flat portion of the response could be further extended.
Outstanding features of this embodiment are briefly summarized below. (i) Formation of space 40 surrounded by the outwardly flared inner collar 32, the outer collar 34, and the metallic flange 35 may be mentioned first. This space can accommodate thermal expansion of the inner frame 33, the outer frame 32a and the ceramic plate 31 in the radial direction. (ii) The outer frame 32a and the inner frame 33 are brazed to the opposite metallized surface of the ceramic plate 31. This contributes to make uniform the thermal stress distribution in the plate caused by the different thermal expansion properties of the ceramic and metallic materials and enables the ceramic plate to withstand the flow of high peak power. (iii) Both the combination of outer frame 17 and inner collar 19 and the combination of inner flange 23 and outer flange 22 of the embodiment of FIG. 3 are united into a single-piece unit in the window structure of FIG. 5. This structure facilitates mounting of the ceramic plate assembly from the left-hand side waveguide section in the window fabrication and, at the same time, helps prevent movement of the ceramic plate assembly, due to the pressure difference on both sides of the window when the left-hand side is evacuated.
An exact formula treating the design problem of such windows with mathematical precision does not seem to have been published. Therefore, in order to obtain an optimum broad-band characteristic from various design data such as the thickness of the ceramic plate, the dimensions of the apertures on both sides of the plate, the axial length of the inner collar, the dimensions of the apertures in the supporting member assembly and the like of the window of FIG. 3, the trial and error method is the only approach to rely on.
Numerical data obtained with the waveguide window assembly of FIG. 3 containing a high alumina ceramic plate (95 percent pure) are as follows:
Center frequency: 520 MHz Rectangular waveguide: Model WR-1500 Ceramic plate: 5 × 160 × 340 mm Aperture of outer frame: 140 × 316 mm Aperture of inner frame: 140 × 300 mm Aperture of inner flange: 150 × 325 mm Aperture of inner collar: 150 × 330 mm Length of inner collar in the axial direction: 20 mm
Thus, a broad-band response with VSWR smaller than 1.20 in the usable frequency range 470 - 570 MHz as shown in FIG. 6 has been obtained by the combination of the four resonant apertures. A power performance test of this window was also conducted to evaluate the high power withstanding property. The temperature rise of the ceramic plate at room temperature of 26° C was as mall as two or three degrees centigrade for the continuous power flow of 30 kilowatt.
Although the invention has been described with particular reference to the two preferred embodiments, each containing a combination of four apertures, it will be obvious to one skilled in the art that the VSWR characteristic vs. frequency of a desired bandwidth can be obtained by suitably designing the window assembly so as to contain only one pair of apertures, four apertures, or more. The two metallic frame members brazed on the opposite metallized surfaces may be of any suitable construction other than those described in the embodiments, so far as they contribute to prevention of excessive thermal stresses as mentioned previously and reduce the possibility of consequent deformation or destruction of the ceramic plate. Furthermore, there is provided a space in the window assembly of any one of the embodiments for accommodating thermal expansion of the ceramic plate assembly. There is no disadvantage, however, in incorporating any other suitable means for safely taking up the thermal expansion. For instance, provision of a pair of apertured, corrugated diaphragms of resilient metallic material employed so as to sandwich the ceramic plate will eliminate the necessity of the provision of such space.
It is apparent from the foregoing description that various modifications, and addtions of constituents of the disclosed embodiments can be made without departing from the scope and spirit of this invention.
Several types of waveguide windows for sealing waveguide systems for high power use are known. Among them, there are (1) half-wavelength dielectric resonant windows; (2) cone windows; (3) slanted-disk windows; and (4) circular windows.
All these types have been widely in use as input or output windows for high-power microwave tubes for use in ground-stations of satellite communications systems. Their broad-band characteristics and high-power-withstanding property have been fully demonstrated in diversified application fields.
Nevertheless, all of these window types have caused technical difficulties particularly when operated at relatively low microwave frequencies, i.e., in the UHF band.
For instance, a ceramic disk of 400 mm in diameter, is needed when a waveguide window designed for a 500 MHz band is operated as a circular window. This requires overall window assembly dimensions as large as 410 mm in diameter and 350 mm in thickness. In case of the half-wavelength resonant window, a ceramic block as large as 105 mm by 190.5 mm by 381 mm would be required. Since the axial length of the window becomes as long as 105 mm, the window assembly becomes bulky and heavy. In addition, low-loss dielectric materials used for the window such as mica, ceramic, or quartz are expensive, making both manufacture and finishing of these dielectric materials difficult.
The dielectric-filled oval-aperture windows for low-power use are advantageous compared with other types of windows such as the circular window, in that both the overall size of the window assembly and the amount of the dielectric material can be reduced. It must be noted however that they have a narrow bandwidth and are weak against thermal expansion as compared with the aforementioned conventional types suited for high-power applications. In other words, it is substantially impossible to manufacture the oval-aperture resonant windows suited for high-power use or large windows of this type for UHF band use.
Accordingly, it is the principal object of this invention to provide a compact, light, and inexpensive waveguide window adapted to sealing waveguide means for handling high UHF band power, without affecting the frequency response of the waveguide itself.
The high-power, broad-band waveguide window of this invention is placed in a rectangular waveguide with the TE 10 mode as the dominant mode. The waveguide window arrangement consists essentially of; (i) a low-loss ceramic plate capable of transmitting high microwave power and of providing excellent sealing action (i.e., nonporous, etc.) for the waveguide system; (ii) a pair of metallic members bonded to the opposite sides of the ceramic plate. (By the provision of the metallic members on both sides of the plate, aperture dimensions are precisely determined and, at the same time, window failures can be prevented. If the metallic member were bonded only to one side of the ceramic plate as in the conventional window structure, window failures due to the difference in the thermal expansion characteristics of the two different materials would be inevitable); (iii) an assembly of metallic members jointly serving to clamp and support the window assembly in a rectangular waveguide, and to provide a hermetic sealing joint for the window aseembly; and (iv) means composed of a combination of a pair of apertures formed on the opposite sides of the ceramic element, effectively separated with an electrical distance corresponding to the guide wavelength in the ceramic plate, and the other combination of two or more apertures formed in the assembly of the metallic elements.
The above-mentioned and other features and advantages of this invention will be fully understood from the following detailed description taken in conjunction with the accompanying drawings, wherein:
FIG. 1 shows a cross section of a conventional ceramic-bonded oval-aperture resonant window in a rectangular waveguide;
FIG. 2 is a plan view of the resonant window shown in FIG. 1;
FIG. 3 shows a cross section of a resonant window for use in a rectangular waveguide according this invention;
FIG. 4 shows a plan view of a ceramic plate for use in the embodiment of FIG. 3;
FIG. 5 shows a cross section of another embodiment of this invention; and
FIG. 6 shows a characteristic curve of the embodiment of FIG. 3.
Referring to FIG. 1, it will be seen that a waveguide window of this type is disclosed, for example, in G.L. Ragans paper published on page 221 of "Microwave Transmission Circuit," Vol. 9, M.I.T. Radiation Laboratory Series.
With this window structure, the periphery of the surface of the ceramic plate 12 to form a hermetic seal with the apertured metallic plate 13 is metallized so that the ceramic plate 12 can be brazed to the plate 13. The metallic plate 13 is securely bonded to a metallic flange 14 which is securely clamped to the internal wall of the rectangular waveguide by brazing.
The ceramic resonant window with a resonant opening 15 in the metallic plate 13 gives a resonance characteristic centered at a desired operating frequency.
Referring to the plan view of the oval-aperture resonant window of FIG. 2, the aperture 15 has a shape as shown by the broken line. With this window structure, a pair of confronting portions of the metallic flange 14 disposed transverse to the waveguide axis and perpendicular in the direction of the electric field acts as a parallel capacitive susceptance element in the waveguide. In like manner, the other pair of confronting portions of the metallic flange 14 disposed parallel to the direction of the electric field acts as a parallel inductive susceptance in the waveguide. Accordingly, the resonant opening 15 surrounded by the two pairs of confronting portions may be represented equivalently by a parallel resonant circuit.
A ceramic plate in a waveguide window is known to behave so as to increase the above-mentioned parallel capacitance when the waveguide is shown by an equivalent circuit. Therefore, the ceramic resonant window can be matched to a desired resonance frequency by making the width of the window narrower than that of a window with an opening which is not filled with dielectric material. Since the equivalent circuit of such a resonant window containing a dielectric element is a simple resonant circuit without exception, such ceramic windows are narrower in bandwidth than other types of windows such as circular windows. Furthermore, with the increase in the cross sectional area of waveguides adapted to the UHF band use, the size of ceramic plates becomes large.
Consequently, excessively large sealing thermal stresses are produced in the ceramic plate due to the difference in the thermal expansion characteristics of the dielectric element 12 and the metallic plate 13. This tends to cause deformation or breakdown of the ceramic plate when the plate is in the process of fabrication or in high power operation.
Referring to the embodiment of FIG. 3, the window assembly is sealed in a rectangular waveguide 11. The periphery of each of the side surfaces of the ceramic plate 16 is metallized, and a pair of frame members having outer frame 17 and inner frame 18 are brazed to the opposite metallized surfaces of the plate.
To the outer frame 17 one end of an inner collar member 19 is brazed substantially in perpendicular relations to each other. One end of an outer collar member 21 outwardly tapered is brazed to the other end of the inner collar 19, and the other end of the outer collar is hermetically fixed to an outer flange member 22 hermetically fixed to the internal wall of the waveguide to form a space 28. That surface of the inner metallic frame 18 which is not brazed to the ceramic plate is slidably mounted on the surface of an inner flange member 23.
This window structure may be considered to contain a combination of four resonant apertures -- that is, a pair of apertures 24 and 25 on opposite sides of the plate 16, an aperture 26 formed in the inner collar 19, and an aperture 27 formed in the inner flange 23.
Extensive experiments conducted by the present inventors using this embodiment have shown the following results: (i) A maximally flat voltage standing wave ratio (VSWR) vs. frequency characteristic obtained, which is the overall resonance characteristics of the four apertures; (ii) despite the anticipated disadvantages such as the increase in the overall length of the window in the axial direction of the waveguide, optimum VSWR ratio can be obtained by suitably selecting the dimensions of the ceramic plate, the area of two apertures 24 and 25 defined respectively by the frame members 17 and 18, and the supporting member assembly and thereby, controlling the resonance characteristics of the four apertures; and (iii) the apertures 24 and 25 on the opposite sides of the ceramic plate are separated with a distance equal to the thickness t of the plate. Since the dielectric coefficient of the ceramic is larger than unity, however, the electrical length for this spacing becomes longer than t (One of the apertures may be greater in area than the other). Therefore, the resonance characteristic of the bandpass filter formed by the two apertures is governed not by the smaller of the two, but by both. Thus, a broadband VSWR characteristic can be obtained.
Features of this window structure may be specified as follows; (i) The window is securely held and sealed in place with the aid of the inner collar 19 and the outwardly tapered outer collar 21, contiguous ends of which collars are brazed or welded together. (ii) The free surface of the inner frame 18 is not firmly fixed to the inner flange 23 but only pressed thereto. Thus, the deformation of the window assembly is prevented, that may be caused due to the pressure difference on both sides of the window when the left-hand side of the ceramic plate is evacuated. (iii) Inner collar 19 and outer collar 21 are so arranged as to form space 28 which can accommodate thermal expansion of the outer frame 17 inner frame 18 and ceramic plate 16 when the ceramic plate 16 is heated due to the dielectric loss. For this feature, a compact window structure suited for high power operation can be realized. (iv) The outer and inner frame members 17 and 18 brazed to the opposite metallized surfaces of the ceramic plate are of the same metallic material and thickness. Consequently, a relatively uniform thermal stress distribution is obtained and the danger of deformation or breakdown of the ceramic plate due to the difference in the thermal expansion characteristics of the metal and the ceramic can be prevented.
It will be obvious from these features that a ceramic plate of sufficiently large size can be brazed to the frame member and that a ceramic window capable of transmitting high power can be manufactured.
A still further advantage of the window structure of FIG. 3 is that the geometry of the plate can be arbitrarily selected. This is because the plate needs to be simply sandwiched between the frame members by brazing and is not subject to limitations of the prior art such as the geometry of the window opening or recessed part of the metallic plate.
According to the inventors' experiments, the size of the ceramic plate used for a WR-1500 type rectangular waveguide operated at 500 MHz was as small as 5 × 160 × 340 mm and the overall length of the window assembly in the axial direction of the waveguide was of the order of 30 mm.
This demonstrates that the high-power, broad-band waveguide window for operation in the UHF band can be made much more compact, lighter, and less expensive than the conventional windows.
In the embodiment shown in FIGS. 3 and 4, the outer metallic collar 21 may have an outer portion extending to and hermetically fixed to the inner wall of flange member 22, replacing the inner cylindrical collar 19 and the outer collar 21.
Referring now to FIG. 4, the ceramic plate 16 used in the window assembly of FIG. 3 is of elongated-octagonal shape. The broken line represents the shape of the aperture 24. It is to be understood that such an octagonal shape facilitates manufacture and finishing of ceramic plates and reduces the manufacturing costs as compared with the conventional oval-shaped ceramic plates, especially when the plate size is made large.
Referring to FIG. 5, the peripheral portion of each side surface of the ceramic plate 31 is metallized similarly to the previous embodiment. An inner frame 33 and an outer frame 32a formed integrally with an outwardly flared collar 32 are brazed to the opposite metallized surfaces of the ceramic plate. The other end of the collar 32 and one end of outer collar 34 are brazed or welded together. The other end of the outer collar 34 is anchored to the metallic flange 35 to form a hermetic sealing joint. The free surface of the inner frame 33 is slidably mounted on the surface of the metallic flange 35. As in the case of the foregoing embodiment, this window assembly contains four resonant apertures in all -- that is, apertures 36 and 37 on the opposite sides of the ceramic plate, another aperture 38 in the outwardly flared metallic collar 32, and still another aperture 39 in the metallic flange 35. It is possible with this window assembly to obtain a desired broad-band response by suitably varying the dimensions and mutual relationships of these apertures and thereby, controlling their individual rosonance characteristics.
The embodiment of FIG. 5 may be considered to be an improved version of the first embodiment in that the outer frame 17 and the inner collar 19 in the structure of FIG. 3 have been replaced with a single-piece outwardly flared collar 32 and that by the adoption of this outwardly flared collar, the flat portion of the response could be further extended.
Outstanding features of this embodiment are briefly summarized below. (i) Formation of space 40 surrounded by the outwardly flared inner collar 32, the outer collar 34, and the metallic flange 35 may be mentioned first. This space can accommodate thermal expansion of the inner frame 33, the outer frame 32a and the ceramic plate 31 in the radial direction. (ii) The outer frame 32a and the inner frame 33 are brazed to the opposite metallized surface of the ceramic plate 31. This contributes to make uniform the thermal stress distribution in the plate caused by the different thermal expansion properties of the ceramic and metallic materials and enables the ceramic plate to withstand the flow of high peak power. (iii) Both the combination of outer frame 17 and inner collar 19 and the combination of inner flange 23 and outer flange 22 of the embodiment of FIG. 3 are united into a single-piece unit in the window structure of FIG. 5. This structure facilitates mounting of the ceramic plate assembly from the left-hand side waveguide section in the window fabrication and, at the same time, helps prevent movement of the ceramic plate assembly, due to the pressure difference on both sides of the window when the left-hand side is evacuated.
An exact formula treating the design problem of such windows with mathematical precision does not seem to have been published. Therefore, in order to obtain an optimum broad-band characteristic from various design data such as the thickness of the ceramic plate, the dimensions of the apertures on both sides of the plate, the axial length of the inner collar, the dimensions of the apertures in the supporting member assembly and the like of the window of FIG. 3, the trial and error method is the only approach to rely on.
Numerical data obtained with the waveguide window assembly of FIG. 3 containing a high alumina ceramic plate (95 percent pure) are as follows:
Center frequency: 520 MHz Rectangular waveguide: Model WR-1500 Ceramic plate: 5 × 160 × 340 mm Aperture of outer frame: 140 × 316 mm Aperture of inner frame: 140 × 300 mm Aperture of inner flange: 150 × 325 mm Aperture of inner collar: 150 × 330 mm Length of inner collar in the axial direction: 20 mm
Thus, a broad-band response with VSWR smaller than 1.20 in the usable frequency range 470 - 570 MHz as shown in FIG. 6 has been obtained by the combination of the four resonant apertures. A power performance test of this window was also conducted to evaluate the high power withstanding property. The temperature rise of the ceramic plate at room temperature of 26° C was as mall as two or three degrees centigrade for the continuous power flow of 30 kilowatt.
Although the invention has been described with particular reference to the two preferred embodiments, each containing a combination of four apertures, it will be obvious to one skilled in the art that the VSWR characteristic vs. frequency of a desired bandwidth can be obtained by suitably designing the window assembly so as to contain only one pair of apertures, four apertures, or more. The two metallic frame members brazed on the opposite metallized surfaces may be of any suitable construction other than those described in the embodiments, so far as they contribute to prevention of excessive thermal stresses as mentioned previously and reduce the possibility of consequent deformation or destruction of the ceramic plate. Furthermore, there is provided a space in the window assembly of any one of the embodiments for accommodating thermal expansion of the ceramic plate assembly. There is no disadvantage, however, in incorporating any other suitable means for safely taking up the thermal expansion. For instance, provision of a pair of apertured, corrugated diaphragms of resilient metallic material employed so as to sandwich the ceramic plate will eliminate the necessity of the provision of such space.
It is apparent from the foregoing description that various modifications, and addtions of constituents of the disclosed embodiments can be made without departing from the scope and spirit of this invention.