US2881401A - Waveguide window - Google Patents

Description

April 7, 1959 R. .1. BONDLEY 2,881,401

WAVEGUIDE WINDOW Filed'Jan. 12, 1954 IN V EN TOR.

Ralph J B ane leg Q/4 71 2/ H115 Attorney.

United States Patent O WAVEGUIDE WINDOW Ralph J. Bondley,

tric Company,

a corporation of New York My invention relates to dielectric windows for use in hollow pipe type waveguides which are commonly used for transmitting electromagnetic waves of ultra high frequency and more particularly, to a window which may be used to sectionalize such a hollow pipe type waveguide to permit operation of adjacent sections under different pressure conditions.

It is customary to isolate the vacuum portion of electron tubes used for generating or amplifying energy at high power levels at microwave frequencies from the remainder of the system by means of a hermetically saled dielectric window. To permit maximumpower to be transmitted through a waveguide it is necessary to keep the standing wave ratio in the guide at a minimum; therefore, a window to be used in the transistion section from the evacuated electron tube to a waveguide transmission line must be designed to present a minimum discontinuity to the transmission of the high frequency waves. Since the dielectric constant of all suitable dielectric window materials is greater than unity, it is necessary to alterthe geometry of the waveguide system at such a window to achieve a low standing wave ratio. This results in the cross-sectional area of the window being less than the cross-sectional area of the waveguide itself. Electrical failure in the system usually manifests itself at the window. On the air or high pressure side of the window, failure may occur by arc-over across the face of the dielectric window. On the vacuum or low pressure side, failure is usually due to erosion of the metal at the junction of the dielectric window and the metal frame to which the dielectric is hermetically sealed. Electrical failure is particularly troublesome when a waveguide is operated at a high power level. For purposes of this description a high power level is considered to be that condition under which the electrical field gradient across the waveguide is very nearly that value which would result in arc-over between two metal plates spaced the same distance apart in air.

It is an object of my invention to provide a new and improved waveguide window, for use in a hollow pipe type waveguide system, that has a circular form in a plane perpendicular to the direction of propagation of high frequency energy in the waveguide system, that provides minimum attenuation to high frequency energy passing through the window and that provides a hermetical seal between two portions of a waveguide system.

Another object of my invention is to provide a window for use in a waveguide system operating at different pressures on either side of said window that has a configuration which minimizes the tendency to arc-over on the Scotia, N .Y., assignor to General Elec- I high pressure side of the window and the tendency to erode the dielectric to metal seal on the low pressure side of said window.

A further object of my invention is to provide a new and improved structure for sealing and supporting a dielectric window in a waveguide system.

A still further object of my invention is to provide a very strong dielectric to metal seal as a portion of the structure for sealing and supporting a waveguide window in a hollow pipe waveguide system.

One of the featrues of my invention is the utilization of a circular window in the rectangular waveguide system. Several advantages result from this shape window. Since the cross-sectional area of the window must be smaller than the waveguide itself, this particular symmetry results in maximum distance between the regions of highest voltage stress. That is, in a rectangular waveguide of conventional design the maximum voltage stress will be across the narrow dimension of the waveguide. This circular configuration decreases the likelihood of arc-over across the face of the window and provides a window having a cross-sectional area less than the crosssectional area of the hollow pipe waveguide. To obtain a maximum creepage distance across the face of the window and thereby further reduce the likelihood of arc-over, and to reduce the accumulation of electrical charge on the low pressure side of the dielectric to metal seal, two dielectric flanges are provided which extend beyond and on either side of the seal. The volume of the dielectric inside the window supporting frame or ring determines the resonant properties of the window. Therefore, the flanges or projections outside the window area reduce the probability of electrical failure without altering the resonant frequency characteristics of the window.

It is well known that a rectangular metal waveguide of material such as copper or brass may be utilized as a path for the transmission of high frequency electromagnetic energy. Many types of high frequency electromagnetic waves may be transmitted through a hollow pipe type waveguide although my invention is particularly adapted to systems for transmitting waves of a mode where the electrical field occurs across the narrow dimension of the guide such as the TE mode. It is recognized that this window will also operate and transmit energy occurring in other modes.

For a better understanding of my invention, reference may be made to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims. Figure 1 is an end view of a wave guide window in accordance with my invention. Figure 2 illustrates, by way of a crosssectional view along section 2-2 of Fig. 1, one manner in which a window may be sealed into a waveguide structure in accordance with my invention.

Referring to Figures 1 and 2 of the drawing in which the same parts have like reference numbers, there is illustrated a preferred structure for achieving the objects of my invention. Figure 2 illustrates in detail one manner of assembling a window in a hollow waveguide. There is shown disk 1 and ring 2 both of which may be of any suitable dielectric material. In this case I used highfired alumina. Sandwiched between disk 1 and ring 2 is an expansion member 3 made of pure nickel sheet. The expansion member 3 is bonded between disk 1 and ring 2 by means of a high melting-point solder. Such a solder may be formed by painting the parts with titanium hydride, applying a sheet of silver eutectic alloy between the parts, and heating to a temperature of approximately 825 -850 C. This produces a strong and hermetically tight seal between ring 2, nickel member 3 and disk 1. This method of soldering is more fully described and claimed in Kelley Patent No. 2,570,248 dated October 9, 1951 and assigned to the assignee of this application. The nickel expansion member is chosen because it is very soft and flexible and therefore makes an ideal member to absorb the different rates of thermal expansion between waveguide -flange 7 and the dielectric window disk 1 and ring 2. A strong joint is formed by placing the expansion member 3 between alumina disk -1 and ring --2 since ring 2 reduces the tendency of a thin sheet of metal, such as nickel member 3, to tear away from a flat surface.

Expansion member 3 is welded to support ring 4 and support ring 4 is electrically welded to support ring 5 by weld 6. Support ring 5 is connected to waveguide flange 7 in any suitable fashion, in this case by electric brazing a ring 8 over support ring 5, and thereby completes the structure for hermetically sealing the waveguide window to the waveguide flange. .Waveguide flange '7 is designed to be connected to a similar flange by means of bolts which pass through holes 9. Support rings 4 and 5 are, in this case, constructed of stainless steel having a coeflicient of thermal expansion approximating that of the copper or copper alloy waveguide flange. The differences in thermal expansion between the supporting structure and the dielectric window are absorbed byflexible expansion member 3.

Disk 1 is cast and machined with flange 10 which has surfaces ll-a'nd 12 forming a pair of faces against which faces 13 and 14, respectively, of alumina ring 2 may rest, Face 12 is incontact with nickel expansion membe! '3 over that portion'opposing surface 14. Face 15 isin contact with the'expansion fold of member 3 only and does not contact theremainder of member 3 or the support ring 4. There is thereby formed a long path around flange 10 which tends to limit the possibility of are-over across the face of the disk 1. Alumina ring 2 is incontact with member 3 over the area of face 14 only. Face 16 tapers away from the nickel member 3 and support ring 4 and makes no contact therewith. Face 17 of ring-2 forms an effective protective shield for thebond between face 14, nickel member 3 and face 12. This shield protects and prevents direct bombardment of the joint by the electrodeless discharge within the waveguide. Furthermore, the "long path from surface 17, along surface. 16, and finally to thejoint between disk 1 and ring 2 tends to limit the build-up of electrostatic charge and thus erosion of the joint.

Itisnotedthat the resonant frequency of the window 1-is determined by the amount of dielectric material inside of surface 11. Therefore, the flanges formed by flange 10 of disk 1 and by alumina ring 2 do not afiect the resonant transmission characteristics of the window. Sincethe window must be hermetically sealed and in addition must also withstand atmospheric pressure, the circular construction results in a symmetrical strain pattern and hence a considerably greater freedom from cracking.

In this particular embodiment of my invention I have constructed a window for a waveguide; the narrow dimension of which is approximately 3% inches while the window flange diameter is approximately 4 inches. The relatively large'diameter of the window plus the extended portions of the dielectric in the form of flange 10=a'n'd ring 2 results in a very long creepage distance such that little or no failure has been experienced by arcing-over across the face on the high pressure side of this window or by erosion on the low pressureside of thiswinddw.

It may be readily appreciated from the foregoing description that there are numerous variations possible in the assembly of a window in accordance with the objects of my invention. However, it may be seen that the utilization of a welded joint 6 between rings 4 and 5 as well as a separate dielectric to metal seal permits each bond to be of the type to achieve an ideal supporting junction or an optimum metal to dielectric seal respectively and results in an improved window for hermetically sealing a section of a waveguide system. Also the projecting portions or flanges of dielectric protect the window and result in a window that is not subject to break down due to arc-over across the face on the high pressure side or erosion of the dielectric to metal seal on the low pressure side.

While I have shown and described my invention as applied to one particular embodiment thereof, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and therefore .I intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire 'to secure by Letters Patent of the United States is:

l. A Window for use in high power microwave apparatus comprising a rectangular waveguide, a metal ring, a substantiallyflat circular member of dielectric material hermetically sealed to said metal .ring at a circular region spaced inwardly from the edge of said circular member, said circular region having a larger diameter than ..the transverse dimension of said guide.

2. A window for use in high power microwave apparatus comprising a metal ring, a dielectricmaterial herme tically sealed to said metal ring with portions of said dielectric material extending beyond this seal, on both sides of and in planes parallel to and spacedtfrom opposite. sides-of the plane of saidlmetalring.

3. A window for use in high powerfmicrowave apparatus comprising a hollow waveguide, a metal ring-connected to said waveguide, a dielectric material hermetically sealed to said metal ring with portions of said dielec tric material extending beyondthe seal, on both sides of and in planes parallel to the plane of said metal ring, the extended dielectric portions being separate from said ring.

4. A window structure for transmitting high frequency, high energy power comprising a dielectric disk, a dielectric ring and an annular metal supporting ring hermetically sealed to said disk at a circular region spaced inwardly from the outer edge of said disk andsaid dielectric'ring being sealed to said annular metal ring at an area opposite said. region.

5. A window structure for transmitting high frequency, highenergy power comprising a dielectric disk, at dielectric ring and a metal supporting ring hermetically sealed between'said disk and dielectric ring, portions of said disk and said'dielectric ring extending beyond the sealin planes parallel to the plane'of the metal supporting ring.

v6. .A window structure for transmitting high frequency, high energy power comprising a dielectric disk, a dielectric ring and a metal supporting ring hermetically sealed betweensaid disk and dielectric ring, portions of said dielectric disk-and said dielectric ring extending beyond the seal, separate from and in'planes parallel to the plane of the metal supporting ring.

7. A window structure for transmitting high frequency, high energy power comprising a waveguide, a dielectric disk, a dielectriczrin'g and a metal supporting ring her metically sealed between the disk and dielectric ring, portions of said dielectric disk'and dielectric ringiextending beyond the seal, separate fromand in'planes parallel to the planes of the'metal supporting ring, and means for hermetically sealing said metal supportingring in-said waveguidestructure transverse to the direction of propagation of highfrequencyenergy so that the win'dowstrueture presents a gas tight separation between two portions of said waveguide.

8. A window assembly for use in a waveguide system which will transmit high frequency energy with minimum attenuation and which acts as a gas tight seal between separate portions of said system, comprising a waveguide with a flange portion, a solid disk of dielectric material, an annular dielectric ring, a flexible metal expansion ring sealed between the disk and dielectric ring and welded to a first supporting ring, and a second supporting ring Welded to said first supporting ring and secured to said waveguide flange portion, portions of said disk and dielectric ring extending beyond the seal, separate from and in planes parallel to the plane of said supporting rings.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS Great Britain Mar. 29, 1950

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