Wave transmission lines and networks – Resonators – Cavity resonator
Utility Patent
1999-01-19
2001-01-02
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Resonators
Cavity resonator
C333S234000
Utility Patent
active
06169468
ABSTRACT:
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 '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.
REFERENCES:
patent: 4488132 (1984-12-01), Collins et al.
patent: 4677403 (1987-06-01), Kich
patent: 6002310 (1999-12-01), Kich et al.
patent: 6057748 (2000-05-01), Hsing et al.
patent: 23 27 362 A1 (1975-01-01), None
patent: 41 13 302 A1 (1992-10-01), None
patent: 2 598 853 A1 (1987-11-01), None
Gudmestad Terje
Hughes Electronics Corporation
Lee Benny
Summons Barbara
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