Abstract:
The thermal expansion of a microwave device such as a microwave resonator is partially or completely compensated by an externally mounted thermal expansion element. The microwave device includes a sidewall and an endwall affixed at its periphery to the sidewall. The thermal expansion compensation element is disposed external to the microwave device, between the endwall of the microwave device and a rigid external support. As the sidewall lengthens with increasing temperature, the thermal expansion compensation element expands to flex the endwall in the opposite direction to the growth in length of the sidewall, so that the central portion of the endwall remains in approximately the same position regardless of the temperature change.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to microwave devices and, more particularly, to the compensation of the thermal expansion of the length of a microwave resonator. 
     A microwave resonator is a device having a hollow tubular body through which electromagnetic waves of microwave frequency are transmitted. Although a variety of shapes may be used, in a typical case the microwave resonator is a hollow cylinder with a sidewall and endwalls that define a microwave cavity. By establishing resonances within the cavity, the resonator may be made to serve as a filter to select a particular microwave frequency for transmission. Such microwave resonators are discussed more fully in U.S. Pat. No. 4,677,403, whose disclosure is incorporated by reference. 
     When the microwave resonator acts as a filter, the transmitted wavelength is a function of the interior dimensions of the microwave cavity, particularly the distance between the endwalls. As the temperature changes, these dimensions change as well, thereby altering the resonant frequency of the microwave resonator. Temperature changes are experienced in applications such as spacecraft microwave systems, whose temperatures during service may vary by several hundred degrees or more. 
     To negate the effects of such temperature changes and maintain the resonant frequency more nearly, preferably exactly, constant, it has been known to provide thermal expansion compensation for the dimensions of the microwave resonator. In one approach, the endwall is mounted to (or is) the end of a sliding piston that stays stationary as the sidewall expands and contracts. This approach has the disadvantages of permitting microwave energy leakage through the space between the sidewall and the endwall, unless care is taken to seal the space between the sidewall and the endwall, and potential binding of the endwall to the sidewall at some temperatures. In another approach, described in the &#39;403 patent, the endwall is sealed at a fixed location to the sidewall, and a ring of a material of different coefficient of thermal expansion is affixed to the endwall and within the microwave cavity to compensate for the sidewall thermal expansion. This approach, while useful for many applications, has the disadvantage in others of altering the radial expansion of the endwall. Further, with this approach a hysteresis has been observed, so that the temperature compensation is not purely a function of temperature, but instead is a function of the history and direction of temperature change, as well as the temperature. 
     There is a need for an improved approach for the compensation of temperature changes in microwave devices. The present invention fulfills this need, and further provides related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a microwave device having temperature compensation for dimensional changes which otherwise alter the properties of the device. More specifically, the invention provides a microwave resonator or filter whose dimensional changes are compensated so as to control the resonant frequency of the device as its temperature changes. In most cases, the dimension of interest of the microwave device is adjusted so as to be constant or nearly constant with changing temperature, but other variations may be achieved if desired. 
     In the present approach, the endwall of the microwave device remains fixed and sealed to the sidewall, so that there is no leakage or potential binding of a piston to its walls, as in the case of the piston-type compensators. The radial expansion and contraction of the sidewall and the endwall are not hindered. There is no hysteresis in the temperature compensation. 
     In accordance with the invention, a microwave device comprises a sidewall having a sidewall axis, and an endwall lying substantially perpendicular to the sidewall axis. The endwall has an endwall periphery affixed to the sidewall and an outwardly facing surface. The sidewall and the endwall together define a microwave cavity. The microwave device further includes a rigid external support located outside the microwave cavity, and a thermal expansion compensation element outside of the microwave cavity and disposed between the outwardly facing surface of the endwall and the rigid external support. Preferably, the rigid external support is affixed to the sidewall and moves therewith, so that the forces generated by thermal expansion strains in the thermal expansion compensation element react axially between the endwall and the sidewall. 
     In the most preferred embodiment, the microwave device is a microwave resonator serving as a filter. The filter is cylindrically symmetrical. The thermal expansion compensation element is disposed coincident with the cylindrical axis with one end contacting the central portion of the outwardly facing surface of the endwall. 
     The material of construction and the axial dimensions of the temperature compensation element are chosen to achieve a desired change in microwave resonance properties with temperature changes. In most cases, it is desired that the dimensions, and thence the microwave resonance properties, are approximately constant as a function of temperature. To achieve this objective, the total length change of the thermal expansion element is selected to be the same or about the same as the total length change of the sidewall. That is, the product of the length of the thermal expansion element times its coefficient of thermal expansion is selected to be the same or about the same as the product of the length of the sidewall times its coefficient of thermal expansion, over the temperature ranges expected during service. Alternatively, the axial dimension of the microwave device, measured to the center of the endwall, may be allowed to increase or decrease by a controlled amount as the temperature changes. 
     The present invention thus provides a microwave device whose properties are compensated for temperature changes. The axial endwall dimension of the device may be controlled to change in any selected manner, from decreasing, to no change (the usual case), to increasing during temperature increases. When the temperature decreases, the length returns to its prior value for any temperature within the service range, without a hysteresis. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not limited to this preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a temperature-compensated microwave device; 
     FIG. 2 is an exploded perspective view of the end of the microwave device of FIG. 1; and 
     FIG. 3 is a schematic side elevational view illustrating the change in configuration of the microwave device of FIG. 1, at a higher temperature. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a temperature-compensated microwave device, in this case a microwave resonator or filter  20 . The microwave filter  20  includes a sidewall  22  and an endwall  24  affixed to the sidewall  22  along the outer periphery  26  of the endwall  24 . (Only one endwall  24  is illustrated, but typically the opposite end of the sidewall  22  is closed with a similar endwall and thermal-expansion compensation structure as will be described subsequently.) The endwall  24  is sealed to the sidewall  22  along the outer periphery  26 , and cannot slide or otherwise experience a gas or electromagnetic leak therebetween. The sidewall  26  may be any operable shape, such as cylindrical, rectangular, spherical, etc. Preferably, it is cylindrical as illustrated, with a cylindrical axis  28 . The sidewall  22  and the endwall  24  in cooperation define a hollow microwave cavity  30 . 
     The microwave filter  20  further includes an iris plate  32  having an opening therethrough, illustrated as a cross-shaped slot  34 . The iris plate  32  couples electromagnetic energy between the microwave cavity  30   a  and the microwave cavity  30   b . Couplers  36  and  38  provide the respective input and output of microwave signals into and out of the microwave filter  20 . 
     A thermal expansion compensation structure  40  is affixed to an end  42  of the sidewall  22 , external to the microwave cavity  30 . The thermal expansion compensation structure  40  includes a support  44  attached external to the sidewall  22  at its end  42 . The support  44  is preferably rigid in that it does not substantially flex during service. Equivalently for the present purposes, the support  44  may be attached to any relatively rigid external (relative to the microwave cavity  30 ) structure instead of to the sidewall  22 . Attachment to the sidewall  22  is preferred, however, because it is not necessary to consider the effect of thermal expansion dimensional changes in any other external structure. 
     A thermal expansion compensation element  46  is disposed between an outwardly facing surface  48  (relative to the microwave cavity  30 ) of the endwall  24  and the support  44 . The thermal expansion compensation element  46 , sometimes termed a “compensation button”, preferably lies along the cylindrical axis  28 , so that it contacts the outwardly facing surface  48  in its central region. 
     FIG. 2 illustrates an approach for the construction and attachment of the thermal expansion compensation structure  40 . The support  44  includes a cross-shaped base  50  and four standoffs  52 . Each standoff  52  is attached to an ear  54  projecting from the end  42  of the sidewall  22 , by means of a fastener  56 . Equivalently, the standoffs  52  may be attached to the end  42  by welding (as in FIG. 1) or any other operable joining process. One end of the thermal expansion compensation element  46  is attached to the center of the base  50  by a fastener  58 . The opposite end of the thermal expansion compensation element  46  presses against the outwardly facing surface  48  of the endwall  24 . 
     The length of the portion of the sidewall  22  whose thermal expansion is to be compensated is L s , and the length of the thermal expansion compensation element  46  is L b , both dimensions measured parallel to the cylindrical axis  28  and at a reference temperature that is conveniently chosen to be room temperature, 70° F. The portion of the sidewall  22  to be compensated may be any portion of the total length of the sidewall  22 . In FIG. 1, the length L s  of the portion of the sidewall  22  to be compensated is the length from the iris plate  32  to the endwall  24 , but the length could instead be the entire sidewall length or any other portion thereof. The sidewall  22  is made of a material having a linear coefficient of thermal expansion parallel to the cylindrical axis  28  of CTE s , and the thermal expansion compensation element  46  is made of a material having a linear coefficient of thermal expansion parallel to the cylindrical axis  28  of CTE b . The values of CTE s  and CTE b  are average values measured over the temperature range expected during service. The values of CTE s  and CTE b  may be the same or different, but typically the material of construction of the thermal expansion compensation element  46  is selected such that CTE b  is substantially larger than CTE s , for reasons to be discussed subsequently. 
     FIG. 3 schematically illustrates the length and configuration changes occurring when the microwave filter  20  is heated. These changes are exaggerated in FIG. 3 so that they are visible, but in practice the changes are typically on the order of a percent or less. 
     As the microwave filter  20  is heated by a temperature ΔT, the length of sidewall  22 , measured parallel to the axis  28 , increases by an amount ΔL s =L s ×CTE s ×ΔT. In the absence of temperature compensation, the endwall  24  would move relative to the iris plate  32  by ΔL s , changing the resonance length and thence the performance of the microwave filter  20 . 
     Over this same temperature change ΔT, the length of the thermal expansion compensation element  46  changes by an amount ΔL b =L b ×CTE b ×ΔT. The increase in length of the thermal expansion compensation element  46  tends to negate the change in position of the endwall  24  due to ΔL s , by causing the endwall  24  to bow into the microwave cavity  30 , as shown in FIG.  3 . To achieve temperature compensation of the length so that the central portion of the endwall  24  is at the same location even after the temperature change ΔT, ΔL s  is set equal to ΔL b  in the design process. Accordingly, 
     
       
         L s ×CTE s ×ΔT=L b ×CTE b ×ΔT 
       
     
     or 
     
       
         L b =L s ×(CTE s /CTE b ). 
       
     
     Thus, in one approach to the design process, the material of construction, having a characteristic CTE s , and length L s  of the sidewall  22  are selected. Then the material of construction of the thermal expansion compensation element  46 , having a characteristic CTE b , is selected. From these choices, the required length L b  of the thermal expansion compensation element  46  is calculated according to the above relationship. 
     This determination is based upon maintaining the central portion of the endwall  24  in the same position before and after the temperature change. For other applications, it may be desired that the position of the central portion of the endwall  24  may move in a specific manner so that the length of the microwave cavity is either increased or decreased by a desired amount. That is, the temperature compensation element is selected such that it does not totally negate the length change of the sidewall. This requirement may be accommodated by providing that (L s −L b ) be a specific value and utilizing a calculation like that set forth above. However, the above approach sets forth the preferred embodiment. In all of these calculations, the thermal expansion changes due to the changes in the lengths of the standoffs  52  may be introduced as desired, or the standoffs may be made of a material such as a ceramic or low-expansion metallic alloy with a very small coefficient of thermal expansion. 
     In the preferred case outlined above of a constant position for the midpoint of the endwall  24 , the length ratio L b /L s  of the thermal expansion compensation element  46  to the sidewall  22  is readily estimated as the ratio of the thermal expansion coefficients CTE s /CTE b . For example, a conventional microwave filter  22  for a Kμ band microwave system is 2.0 inches long and has a sidewall  22  made of a conventional alloy of iron-36 weight percent nickel (also known as INVAR™ alloy) having a coefficient of thermal expansion of 1.54×10 −6  inch/inch° C. A preferred embodiment of the thermal expansion element  46  is made of aluminum, having a coefficient of thermal expansion of 25×10 −6  inch/inch° C. The estimated length of the thermal expansion element  46  for this 2 inch long filter is 2×(1.54/25), or about 0.12 inch. 
     The present invention is operable with both metallic and nonmetallic sidewalls and thermal expansion compensation elements. Some preferred materials for use in the present invention are: sidewall: INVAR™ alloy, aluminum, and aluminum-beryllium alloys; and thermal expansion compensation element: aluminum, and ULTEM™ polyetherimide plastic. 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.