US2922127A - Output coupling - Google Patents

Description

Jan. 19, 1960' E. c. DENCH OUTPUT COUPLING I Filed Jan. 16, 1957 FIG.|

R o T A L L c S 0 T0 OSCILLATOR INVENTOR.

EDWARD G. DENCH new and improved construction for the output coupling States of America as represented by the Secretary of the Army Application January 16, 1957, Serial No. 634,595

1 Claim. (Cl. 333-34) The invention relates to an improved output coupling means for a magnetron, backward wave oscillator, or the like.

It is a principal object of the invention to provide a of a microwave oscillator which will allow for an effective vacuum seal between the oscillator and the output 1 waveguide and will permit broadband matching to the output waveguide.

This object and other objects and features of the invention and the manner of obtaining them will be best understood by reference to the following description of nited States Patent O F 2,922,127 Patented Jan. 19, 1960 dimensions of the recess are suitably chosen so that thermal efiects are compensated for.

One end of the waveguide is closed in the usual manner by a short circuit 9 placed approximately wavelength from the projecting portion of the inner conductor 3; the exact location is calculable by means well known in the art.

Broadband matching is effected experimentally by adjusting the depth of the conductor-ceramic cup combination into the waveguide, and by adjusting its lateral position.

As stated above, an important feature of the invention, in addition to the provision of the cup-like structure for the seal element, is the use of a ceramic material for the seal.

The use of ceramic instead of glass, which is usually used. fora vacuum seal of this kind, permits the achievement of a reproducible low-loss junction, and is clearly superior to the use of glass.

The particular type of ceramic used is manufactured by the Raytheon Manufacturing Co. and bears the designation Q.A.S.; it is characterized by a high alumina content. It is understood, however, that any type of ceramic may be used within the contemplation of the invention. Thisparticular ceramic has been found to possess the most advantageous properties. Some of these properties as compared with two glasses most frequently used previously as seals is given in the following table:

Comparison of high alumina ceramic with glass Property Glass 707 High M12312?) Ceramic Glass 704 Softening Pt. C.) 746 1,350 708. Thermal Expansion 2.5-100 0. 3 0):: 32 10 6 l0- 4.75X10- Compressive Strength (p.s.i.) 10,000. 250 0 10,000. Tensile Strength (p.s.i.) 10,000. 25,000 10,000. Dielectric Constant 4.0 (10K mc.20 C.) 8.6 (8600 Mc.20 C.) 4.6 (10K mc.-20 0.). LOSS F actor .0048 (l rue-20 O. .0089 (8,600 Mc.20 0.)--- .033 (10K mc.2C (1.). Porosity Vacuum Tight to .o/o" Vacuum Tight to .o/o Vacuum Tight to .o/o.

the invention taken in conjunction with the accompanying drawings, wherein:

Figure l is a sectional view of one embodiment of the invention showing the output coupling from a microwave oscillator to a waveguide, and Figure 2 is a sectional view of a coupling to a ridged waveguide.

The invention comprises a combination of structural features which together provide the desiderata set forth in the objects above. These features are: the use of a cup-like seal structure surrounding the inner coaxial conductor in the waveguide and the use of a ceramic material for the cup-like structure.

Referring now to Figure 1, there is shown a crosssectional view of one embodiment of the improved output coupling. This figure shows a waveguide 1 into which energy from the microwave device is to be coupled. The coupling unit itself is designated generally by the numeral 2 and includes an inner coaxial conductor 3 and an outer coaxial conductor 4. An output pipe 5 is shown and this pipe, through a flange 6, is connected to the output of the microwave device in any suitable manner, this connection forming no part of this invention. Outer conductor 4 is electrically connected to output pipe 5 by fitting into the recess 10 in the pipe.

As can be seen from the figure, the portion of the inner coaxial conductor which extends into the waveguide in order to couple energy thereto is surrounded by a ceramic cup 7 which also projects into the waveguide. This cup forms the seal between the vacuum of the microwave device and the atmospheric pressure present in the waveguide. As can be seen from the figure, the ceramic cup is recessed as shown at 8 and the center conductor is supported at its end by this recess and rides within it. The

The glasses shown in the table are standard glasses industrially available and manufactured by the Coming Glass Company. 7

It may first be noted that the ceramic has a high softening temperature, thus allowing tubes or couplings incorporating such seals to be processed at high bake-out temperatures. In addition, with ceramics one can depend on reproducible dimensions, i.e., ceramics can be moulded, fired and ground to the specific dimension. When glass is used the tolerance depends on the skill of the glass blower or the tolerance of the extruded glass. This would be a particular disadvantage in the present invention since 'it is desirable to have the cup-shaped seal and the recess therein conform with a high degree of tolerance to the other elements of the coupling. -It can also be seen from the table that the compressive strength of the ceramic is about 25 times that of glass and the tensile strength about 2.5 times that of glass.

In addition to these disadvantages, glass has a high dielectric loss at high temperatures, while the losses in the ceramic remain virtually unchanged over the temperature range. Therefore, any local heating of the glass would cause a progressive rise in dielectric loss and temperature until some melting would occur. If large amounts of RF. power are transmitted through the seal a small percentage loss could be a large actual loss and this would create spot heating and lead to wasted energy.

In addition it is noted that the insertion of the cup-like seal produces a discontinuity because its dielectric constant is dilferent from unity. The effect of such a discontinuity can be indicated in part by the voltage standing wave ratio or VSWR, which is the ratio of the largest instantaneous peak voltage occurring along a transmission When there is a mismatch or non-uniformitw the VSWR.

will in general be greater than-unity. Ahigh is an indication of'conditions which will produce such undesirable conditions as electrical 'stresses, local heating The use of the cupand power losses, among others. like structure composed of the Q.AIS. ceramic in. the instant invention yielded an impedance match'over 'a 25% band with a VSWR of less than. 1.4 to '1.

Figure 2 is an illustration of another embodiment of the invention wherein the microwaves device is coupled tothe ridge waveguide in order to provide better matching over a wider range of frequencies. 'The use of .a ridge waveguide generally for a broad band junction is shown, by way of example, in US. Patent 2,633,493, TissuedMarh 31, 1953, to S. B., Cohn. In 'Fig, '2, ceramiclse'al fifis shown attached to the outer conductor 14 6f thecoax cable and inner conductor 12 is's'hown asftr'fninated -in a capacitive load 15. V i

The output pipe is shown at 16, Element IS'is of conductive material and forms a portion of ridge 17 within the waveguide 11. The coupling is similar in an other respects to that shown in Figure 1. Q

The capacitive load 15 is used in order to increase the coupling from the center conductor to the ridge. The surrounding ceramic cup provides the vacuum seal and also increases the dielectric constant in the space'between element 15 and the ridge.

It is thus seen that the invention provides an improved output coupling for a microwave oscillator which allows for improved structural and mechanical performance 4 while concomitantly allowing maximum transfer of energy over a broadband of frequencies. @Obviously many modifications and variations of the. invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claim, the invention may be practiced otherwise than as specifically described.

What is claimed is: I I t A seal for a coaxial lineto ridge waveguide junction, said-coaxial linehaving an inner and'an outer conductor, said seal being coaxial with and fixedly attached to the inner side of the outer conductor, said outer conductor terminating at the waveguide. wall, said ridge having a cylindrical cavity therein, *theseal and'the inner conductor extending through the waveguide space and into the cylindrical cavity of said waveguide ridge, the seal composed of a ceramic material and terminating in a cup-like structure, and the inner conductor terminating in a con ductive'material inside the cup-like structure, thereby providing close coupling with the waveguide and vacuum sealing between the coaxial line and the waveguide.

References Cited in'the file of this patent UNITED STATES PATENTS OTHER REFERENCES Websters New International Dictionary, second edition, Unabridged, G&C Merriam Co, Publisher, Springfield, Mass.

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