US3032727A - Ultrahigh-frequency electro-magnetic wave transmission apparatus - Google Patents

Description

y 1952 R. LATHAM ET AL 3,032,727

ULTRAHIGH-FREQUENCY ELECTROMAGNETIC WAVE TRANSMISSION APPARATUS Filed May 2, 1960 Fig. 00 PRIOR ART P F W60 WWW:

70mm Maw/mm;

BY wwa 7 United States Patent Qfifice 3,032,727 ULTRAHIGH-FREQUENCY ELECTRO-MAGNETIC WAVE TRANSMISSION APPARATUS Robert Latham, London, and Frederick Charles Thompson, Danbury, England, Herbert Sixsmith, Boulder,

Colo., and Maurice Esterson, Risings, Great Baddow, Andrzej Folkierski, London, and Alan Hugh Pickering, Galleywood, Chelmsford, England, assignors to English Electric Valve Company Limited, London, England, a British company Filed May 2, 1960, Ser. No. 25,994 3 Claims. (Cl. 333-98) This invention relates to ultrahigh-frequency electromagnetic wave apparatus and is more particularly concerned with apparatus of the kind in which ultrahighfrequency electro-magnetic wave energy is required to be transmitted out of, or into, a hermetically sealed space.

This application is a continuation-in-part of our applica tion Serial No. 461,896, filed October 12, 1954, now abandoned.

In any apparatus in which ultrahigh-frequency electromagnetic wave energy is required to be fed through the enclosing wall of a hermetically sealed space, a dielectric window is provided in the wall to maintain the hermetic seal while allowing the waves to pass throughthe window. Thus, in apparatus in which centimeter waves are generated in the evacuated envelope of a magnetron or of a klystron, it is usual to take out the energy either by means of a coaxial line, the supporting dielectric of which acts as a window and is made part of the evacuated envelope of the tube, or by means of a wave guide with a vacuumtight window sealed in to form part of the tube envelope. The coaxial line type of arrangement has severe output limitations set by the danger of electric breakdown, especially at higher frequencies. The wave guide type of output is, therefore, much preferred at higher output powers, but this also has important limitations which will be described later herein, and which it is the object of the present invention to avoid.

The invention is illustrated in and further explained in connection with the accompanying schematic drawings in which:

FIG. 1 is a schematic showing of a known arrangement; and

FIGS. 2 and 3 are schematic representations of typical embodiments of the present invention.

Referring to FIG. 1, a wave guide shown therein consists of an input portion WGI in line with an output portion WGO. Wave power travels along the wave guide and its direction of propagation is indicated by the arrows marked P. Between the input and output portions. there is a dielectric window DW surrounded by choke ditches CD and hermetically sealed in vacuum-tight manner in a metal frame member FM which is shown broken away and which forms part of the wall of an evacuated chamber (e.g. the envelope of a discharge tube not otherwise shown), in which chamber the input portion WGI is positioned.

With this construction, since the plane of the window DW is at right angles to the run of the wave guide, the said window extending across an aperture through which the power is transmitted, the electric field is, as indicated by the arrows F, substantially parallel to the surface of the dielectric window DW. With this type of window the fraction of power lost in the window cannot be substantially reduced by varying the details of the construction within the general plan. Apart, therefore, 'from considerations of actual breakdown across the surface of the window, the power handling capabilities are limited by dielectric loss in the material of which the window is made. Accordingly, once this limitation is reached, the only way of increasing the power transmitted is to increase the size of the wave guide and the area of the window but this introduces other difficulties. For example, it becomes more difficult effectively to cool the window and the larger wave guide makes interfering modes possible.

One proposed solution of this problem consists in having at least the center portion of the dielectric window made of a much more refractory material than glass so that the window may withstand higher temperatures. The power loss in such a window, however, remains high and furthermore this arrangement is costly and critical to manufacture.

Another proposed solution consists in employing a hollow conical window mounted in a length of circular wave guide and having a length of at least half a wavelength at the operating frequency. This arrangement although effective to reduce the temperatures of the window, is costly to manufacture due to the special shape of the window and furthermore requires the provision of rectangular-to-circular transition wave guide sections where the device is to be used in conjunction with rectangular input and output wave guides as is commonly the case.

According to the present invention, a high power VHCl.l-

um-tight coupling comprises an input wave guide, an output wave guide having its axis parallel to and off-setfrom the axis of said input wave guide, said wave guides overlapping and terminating thereat and connecting section-means for coupling substantially all the wave energy in said input wave guide to said output wave guide comprising a section connected to the overlapping portions of said wave guides, a hermetically sealed window positioned between said wave guides in said section at their common junction, said window being so disposed that its effective surface is substantially perpendicular to the direction of the electric field of wave energy propagated in said wave guides in the dominant mode, and extending in a plane which is in spaced parallel relation to both said waveguides, said dielectric material providing the only wave energy coupling path between said wave guides. With regard to the modes of wave propagation in the arrangements in accordance with this invention, it is pointed out that in all cases the dominant mode of propagation is the important one. In the case of rectangular wave guides. this mode is the H and in circular wave guides It is the H11.

In FIG. 2, which shows one embodiment of the invention, two wave guide portions WGI and WGO are arranged to overlap, as shown. The adjacent guide walls at the region of the overlap may be, as shown, connected by a short section sealed to the waveguides. A wave energy path from WGI to WGO is provided by a vacuumti ht dielectric window DW encircled by choke ditches CD in the connecting section, the surface of the window DW being substantially perpendicular to the electric field direction of the energy propagated in the wave guide, which in this arrangement is in the H mode of propagation. This is indicated by the arrows F, the'arrows P representing the power flow. With this arrangement the fraction of power lost is considerably less than with the previous design described above, if the same dielectric material is used in each case. The choke ditches are not essential since direct sealing of the window without ditches may be employed. but the provision of the said ditches is preferred as making for easier assembly.

FIG. 3 shows a further modification in which the inlet wave guide WGI is superimposed over the outlet wave guide WGO with a plane dielectric window DW therebetween. This arrangement is suitable for rectangular wave guides in which the H mode of propagation takes place. the window being perpendicular to the electric field direction P and being surrounded by choke ditches CD. The outlet wave guide WGO, as shown, initially doubles Patented May 1, 1962 3 back on the inlet wave guide WGI. The arrows P in FIG. 3 again indicate power flow.

Arrangements in accordance with this invention are capable of handling much more power for a given size than known arrangements as typified by FIG. 1, and by providing suitable irises (not shown) as known per se, for impedance matching, can be arranged to fulfil the requirements for a satisfactory microwave window.

The ease of assembly of the structure will now be described in detail. Where it is required to braze metal, such as at the junction of waveguides WGI and WGO, it is an advantage to have the glass-to-metal seal spaced away from the heat which is normally generated at the brazing point. In the case of the embodiments shown in FIGS. 2 and}, there isshown between the glass window and the outer part of the waveguide a metal member which spaces the window and its metal-to-glass seal from the envelope itself. In order'that there may be an electrical short-circuit between the wave guides, the ditches are inserted. The particular advantage is, of course, that the metal between the envelope and the glass window can be of the requisite material for enabling an efiective metal-to-glass seal to be carried out at the point of contact with the glass.

While we have described our invention in certain preferred embodiments, we realize that modifications may be made and we desire that it be understood that no limitations upon ourinvention are intendedother than may be imposed by the scope of the appended claims.

We claim:

1. A high-power vacuum-tight coupling comprising an input wave guide, an output wave guide having its axis parallel to and spaced from said input wave guide, said wave guides overlapping and terminating thereat and connecting section means for coupling substantially all the wave energy in said input wave guide to said output wave guide comprising a section connected to the overlapping portions of said waveguides, a hermetically sealed window positioned between said wave guides at their common junction, said window being so disposed that its effective surface is substantially perpendicular to the direction of the electric field of wave energy propagated in said Wave guides in the dominant mode, and extending in a plane which is in spaced parallel relation to both said waveguides, said dielectric material providing the only wave energy coupling path between said wave guides, said window being off-set from the surfaces of both of said guides and a choke ditch encircling the window on each side thereof.

'2. A coupling as claimed in claim 1 wherein said input and output wave guides extend away from said connecting section means in opposite directions.

3. A coupling'as claimed in claim l wherein said output wave guide initially doubles back on said input wave guide and then curves away from said input wave guide, the arrangement being such that the electric field energy propagated therethrough is turned through substantially a right angle with respect to the direction of the electric field in the input channel.

References Cited in the file of this patent UNITED STATES PATENTS

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