US2638504A

US2638504A - High-frequency electrical device having gastight envelopes

Info

Publication number: US2638504A
Application number: US701542A
Authority: US
Inventors: Pearce Albert Frederick; Barford Norman Charles
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1943-06-11
Filing date: 1946-10-05
Publication date: 1953-05-12
Anticipated expiration: 1970-05-12
Also published as: SE125124C1; FR944067A

Description

PEARCE ET AL HIGH-FREQUENCY ELECTRICAL DEVICE HAVING GASTIGHT ENVELOPES May 12, 1953 Filed Oct. 5, 1946 I I 1 1/ I INVENTORSI ALBERT FREDERICK PEARC E NORMAN CHARLES BARFORD WEW ATTORNEY- Patented May 12, 1953 HIGH-FREQUENCY ELECTRICAL DEVICE HAVING GASTIGHT ENVELOPES Albert Frederick Pearce, Hampton Hills, and Norman Charles Barford, South Bend, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great I Britain- Application October 5, 1946, Serial No. 701,542 In Great Britain June 11, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires June 11, 1963 2 Claims.

' This invention relates to high frequency electrical devices having gas-tight envelopes and particularly, but not exclusively, to electron discharge devices employing cavity resonators.

It is necessary in devices where an electron beam is required to pass through a cavity reso nator to ensure that the beam is contained within an evacuated space from which the atmosphere has to be excluded. It is sometimes the practice in these cases to construct the device so that the envelope thereof passes through the resonator, part of the resonator lying within the envelope and part outside the envelope. In such cases electromagnetic waves can be applied to or abstracted from the resonator by means of a suitable coupling connected to the part of the resonator lying outside the envelope. In other cases, however, it is desirable to avoid the use of glass or other insulator inside the resonator particularly in cases where the device is intended to operate at high powers or at very high frequencies. In such cases, in order to provide for the passage of the electromagnetic waves to or from the resonator two constructions are sometimes employed. In one of such constructions the glass envelope of the device is sealed to the outer walls of the resonator leaving part of the resonator exposed to the atmosphere. A short length of waveguide is connected to the exposed part of the resonator to enclose an aperture therein and a glass seal is then applied transversely of the waveguide so as to make the resonator gas-tight, the resonator in this construction thus forming part of the envelope of the device. In the other construction a concentric line having a coupling loop in the resonator is provided which projects from the resonator through a vacuum seal. In this case the resonator may be totally enclosed Within an envelope, the resonator thus not forming part of t-he envelope. 1

Owing to the necessity of making vacuum seals either in the length of waveguide in the first-mentioned construction or to-the necessity of providing a vacuum seal for the concentric line, both of these constructions are found to be inconvenient in practice. It has, however, been proposed in British patent specification 'No. 506,881'to arrange a cavity resonator in an evacuated envelope andto couple the resonator by a slot to a dielectric waveguide comprising a.

metal tube arranged in the envelope, the metal tube being continued outside the envelope. In suclian arrangement diificulties may be experi enced owing to the escape of high frequency energy at the discontinuity in the waveguide due to the presence of the envelope. The object of the present invention is to provide an improved construction of device in which electromagnetic waves can be transmitted through the envelope without the necessity of making vacuum-tight seals in the path along which the waves are transmitted and in which the escape of waves such as might arise in said prior proposal is reduced or avoided.

According to one feature of the invention there is provided a high frequency electrical device comprising a gas-tight insulating envelope through which it is desired to transmit electromagnetic waves of given frequency wherein said waves are transmitted by a section of waveguide within the envelope and extending to the interior side thereof and by a co-operating section of waveguide on the exterior side of the envelope and wherein, in order to reduce substantially the escape of said waves at the discontinuity in the path of said waves due to the presence of the envelope between the adjacent ends of said sections, said adjacent ends are so dimensioned as to present a low impedance at the operatingfre quency at said discontinuity.

According to another feature of the invention there is provided a high frequency electrical device comprising a gas-tight insulating envelope and a hollow conductor within said envelope adapted to contain high frequency electromagnetic waves of given frequency, said hollow conductor having an aperture therein which lies adjacent to the interior side of said envelope and on'the exterior side of said envelope a section of waveguide is provided surrounding said aperture so that electromagnetic waves can pass from said aperture to the section of waveguide or vice versa and wherein, in order to reduce substantially the escape of said waves at the discontinuity in the path of said waves due to the presence of the envelope between the aperture and said section of waveguide the wall of said hollow conductor surrounding said aperture andv the adjacent end of said section of waveguide are, so dimensioned as to present a low impedance at the operating frequency at the discontinuity.

The present invention thus provides means whereby electromagnetic waves can be transmitted through an insulating envelope without the necessity of providing vacuum-tight seals in the path along which the waves are transmitted, and moreover, a construction in which the escape of said waves due to the presence of the envelope is substantially reduced. The invention is especially applicable to devices including within the gas-tight envelope a cavity resonator and in this case an aperture in the cavity resonator may be surrounded by the length of waveguide.

provided within the envelope or the resonator may be arranged to lie against the envelope the section of waveguide on the exterior side of the envelope surrounding the aperture in the resonator.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to. the accompanying drawings, in which:

Figure 1 is a cross-sectional view of the invention as applied to an electron discharge device embodying a hollow resonator,

Figure 2 is a section taken along the line 22 of Figure 1, and

Figure 3 illustrates a modification of the flanges employed in Figures 1 and 2 for reducing the escape of energy at the envelope of the device.

As shown in Figures 1 and 2. of the drawings, the. reference numeral 9' indicates a cavity resonator which. is arranged entirely within an envelope 2 of glass or. other suitable insulating material- It is necessary to maintain the space within the envelope gas or vacuum-tight and in order to apply to. or abstract high frequency electromagnetic waves from the resonator I without the use of elements which pass through the envelope 2- giving rise to the necessity of vacuum-tight seals, the resonator I is provided with a length of waveguide 30f any suitablefcrm in cross-section such as. of rectangular or circular form or of flared. formand which surrounds an aperture 4 inthe resonator IV to or from which. energy can flow. The length. of waveguide 3is such that the end thereof remote. from the resonator l abuts against the glass envelope 2. and onv the atmospheric side ct theenvelope 2 a co-operating length of. waveguide '5 is provided arranged coaxially with the section. 3.. These two sections are not sealed to the envelope and since the envelope passes between the sections the space within the envelope is maintained vacuum or gas-tight without the necessity of seals along. the length of the sections 3: and 5 It is of course not necessary that thesections 3 and Eabut against the envelope although. the ends of these sections are preferably shaped to conform to the configuration of the envelope 2- and are disposed as close thereto as possible. In order to prevent or reduce the tendency for energy to escape at the discontinuity in the waveguide sections 3. and 5 due to-the pres ence of the envelope 2 which. of course separates the adjacent-ends of said sections, these ends are dimensioned so as to present a low impedance to said energy at the operating frequency. As shown, these ends are provided with flanges 5 and. l respectively, the flanges each being of a width equal to where x is the wavelength of the electromagnetic waves in the material of which the envelope is made and n is any whole number including zero. Flanges having such dimensions constitute in effect a transmission line which is open-circuited at" itsen'd remote from the waveguide sections and appears therefore to be short-ci-rcuited at the waveguide sections, thus providing said low impedance. The thickness of the envelope should of course be kept as small as possible.

With the construction shown, electromagnetic waves can pass from the resonator through the section 3 through the envelope 2 and into the section 5 or vice versa withrvery little loss. In an alternative construction the waveguide section 3 can be omitted and the resonator l disposed close tothe inner surface of the envelope 2, electromagnetic waves then passing from the aperture 4 through the envelope 2 and into the waveguide section 5. The aperture will then be provided with a flange to co-operate with the flange I of the waveguide section 5 if the wall of the resonator is not (so-extensive with the flange 1.

If it is found that the provision of flanges 6 and 1 shown in Figures 1 and 2 do not sufiiciently prevent the? escape of energy, an arrangement as describedin British patent specification No. 562,674 may be employed as shown in Figure 3. In this arrangement the flange I on the waveguide section 5 is provided with an annular recess 8 the depth of this recess being effectively equal to a quarter of the wavelength, in air, of the energy transmitted through the guide. the recess 8 being, disposed from. the internal wall of theguide at a distance effectively equal to a quarter of a wavelength, in the material of which. the envelope is made, of the energy transmitted through the waveguide, whilst the portion of the flange 7 beyond the recess 8' is also arranged to be of the same radial length. The recess 8 provides in effect a transmission. line which is short-circuited at its end; remote from the envelope and. thereriore presents at the envelope a high impedance which is reflected asa low impedance at the inner surfaces of sections 3 andfi andso causesthe net impedance at that. point to appear lower than the low impedance obtained with the.

construction shown in Figures 1'. and 2. The. portion of the flange i beyond therecess. serves as a. quarter wave open transmission line presenting. a low impedance in effective series relation with said high impedance at the open end of recess 8. The flange 6' on the waveguide section. 3' is ex.- tended as indicated so. as to be co-extensive with. the periphery of the flange 1 Where standing Waves are present in the waveguide due for example to mismatching of. the debe understood that the invention is not limited in its application to such. devices as it can be applied to other devices employing cavity resonators, or to other devices where it is necessary to. transmit high. frequency electromagnetic waves through a vacuum or gas-filled envelope.

What we claim is:

1. A high frequency electrical. device adapted to operate at a given frequency comprising two aligned hollow conductors adapted to transmit. electromagnetic waves therethrough, dielectric material separating said conductors, and. means for presenting a low impedance at the; operating frequency between said conductors at: the inner surface thereof, said: means including parallel:

= flanges on the adjacentends of. said. conductors,

one of said flanges. being provided with a. recesshaving a depth equal to. a quarter wavelengthv in air at the operating frequency and having its. open end located at a distance from said inner surface 1 substantially equal to; a quarter wavelength at the operating frequency in said dielectric material, to provide a high impedance across said open end of said recess, the total Width of each of said flanges being equal to a half wavelength at the operating frequency in said dielectric material, whereby the quarter wave open transmission line section formed by the portions of said flanges outwardly from said recess provides, at said open end, a low impedance in series with said recess and the other of said flanges.

2. A device according to claim 1, wherein said dielectric material is a solid dielectric member in contact with said flanges.

ALBERT FREDERICK PEARCE. NORMAN CHARLES BARFORD.

References Cited in the file of this patent UNITED STATES PATENTS Number