WO2003021711A1

WO2003021711A1 - Power microwave filter free of multipactor effects

Info

Publication number: WO2003021711A1
Application number: PCT/IB2002/003460
Authority: WO
Inventors: Salvador Marti Gombau; Ana Isabel Daganzo Eusebio
Original assignee: Alcatel
Priority date: 2001-09-01
Filing date: 2002-08-27
Publication date: 2003-03-13

Abstract

Power microwave filter free of corona and multipactor effects for use in space applications, as for example in satellites. The filter is housed in a hermetic structure in order to maintain the pressure level therein substantially constant. As a consequence of maintaining said internal pressure constant, the filter shall not suffer the so-called corona and multipactor effects.

Description

POWER- 'MICROWAVE FILTER FREE OF MULTIPACTOR EFFECTS

The present invention relates to a power microwave filter free of corona and multipactor effects. More specifically, the present invention relates to a power microwave filter for use in space applications, as for example in satellites, which has cavities in its interior, said filter being housed in a hermetic structure in order to maintain the pressure level substantially constant in the interior of the filter over a long period of time and preferably during the entire operational life of the satellite. As a consequence of maintaining constant said internal pressure, the filter does not suffer the so- called corona and multipactor effects. BACKGROUND OF THE INVENTION

The corona effect appears during the launch phase when the internal pressure passes through the critical pressure (a few Torr). This effect consists of a gas discharge produced by ionisation of the residual molecules during the evacuation of gases from the interior of the device.

The multipactor effect appears under conditions of high vacuum (<10^"5 Torr), when the free electrons, located between very close metallic surfaces, are accelerated by the electric field associated with the RF signal impacting against the inside walls of the device. The secondary electrons produced in this process impact again against the inside walls of the device provoking an avalanche phenomenon.

These effects become fairly critical at low microwave frequencies, as is the case of the S Band, although their inconvenience is not limited solely to said band.

For operating correctly, it is of interest to avoid that the filter works at very low pressures on its inside. Normally, it is complicated to seal the filter assembly such that the internal pressure is kept constant when it is subjected to vacuum surroundings, as it is the case when the satellite is in space. Under such circumstances leaks shall cause the pressure to drop gradually.

The behaviour of a filter having these characteristics has been described in the article titled: "Reduction of Gas-Discharge Breakdown Thresholds in the Ionosphere due to Multipacting" by G. August and J. B. Chown, the content of which is incorporated in this description by reference. The figure 1 shows a curve based on the content of the article mentioned, in which figure the changes in RF breakdown voltage in volts may be appreciated as a function of the filter pressure in mm Hg and it can be seen under what conditions the corona and multipactor effects arise. This curve corresponds to a given structure, in working conditions at the frequency of 30 Hz, with a separation (gap) between metallic surfaces of 4.8 cm and excited with brass electrodes.

The point C corresponds to the vacuum environment and it can be seen that as the pressure falls towards vacuum level, the maximum breakdown voltage stabilises at a determined value after which the multipactor effect is produced. This is the zone in which satellite equipment habitually works when being in vacuum surroundings.

Zone B contains the so-called "critical pressure" that corresponds to the point at which the corona effect is produced with the minimum breakdown voltage. After this point, comes the so-called transition zone in which the corona effect appears with breakdown voltages much higher than that of the critical pressure point, and where the multipactor effect is stabilised at a determined breakdown voltage, higher in any case, than the breakdown voltage of the critical pressure point. Finally, point A corresponds to the conditions at the earth's surface

(atmospheric pressure) and it can be seen on this part of the curve that the gas discharge, that is the corona effect, occurs at a much greater voltage, for which reason the filter is able to withstand a higher power before said effect takes place. In the current state of the art, it is difficult to achieve microwave bandpass filters capable of supporting power levels higher than 15 or 20 W and the possible solutions are fairly limited. Some of these solutions propose the use of lowpass filters and the so-called "notch" filters, based on coupled coaxial lines. The problem associated with the use of lowpass filters and notch filters is their limited electrical response insofar as in-band and out-of-band rejection is concerned.

Another known solution is the use of microwave filters based on coaxial technology, called "potting", in which the filters are completely filled with dielectric material. The problem associated with these filters is that in turn they have high insertion losses, and it is very difficult to guarantee, in practice, a complete absence of air in the filter and the final tuning. With a minimal presence of air in said filters it is possible to produce the corona effect. Thus, there is a need to furnish a microwave bandpass filter for space applications capable of withstanding power and which, in an effective manner, avoids the corona and multipactor effects.

DESCRIPTION OF THE INVENTION

This objective is attained if the filter operates during its useful lifetime in zone A instead of zone C in figure 1 , whereby as has been described above, its capability to handle power is enhanced and passing into zone B is avoided.

In order that the filter operates in a permanent way in the aforementioned zone A, the present invention proposes that the cavities of the filter be kept full of gas at atmospheric pressure, or any other pressure above that of the critical pressure, the structure of the filter being sealed in a hermetic fashion.

Thus an object of the present invention is that of providing a microwave bandpass filter, said filter comprising a base structure with at least one resonant cavity in the interior thereof, characterised in that a pressure in the interior of said filter is kept substantially constant at a determined value.

Another object of the present invention is that of providing a microwave filter, said filter comprising a base structure with at least one resonant cavity in the interior thereof, at least one electrical signal access element to the interior of the filter and at least one electrical signal access element to the exterior of the filter, characterised in that said electric signal access elements are hermetic, and in that said filter has additionally a lid suitable for being sealed on said base structure in such a manner that between said lid and said base structure a hermetically sealed space in the interior of the filter that contains at least one gas at a determined pressure is formed.

According to one aspect of the invention, said determined pressure in the interior of the filter is the atmospheric pressure. According to another aspect of the invention, said determined pressure in the interior of the filter is higher than a critical pressure, the latter being the pressure that corresponds to the point at which the corona effect is produced with a minimum breakdown voltage.

According to another aspect of the invention, the value of the critical pressure is less than 100 mm Hg and preferably between 3 and 50 mm Hg.

According to another aspect of the invention, said electrical signal access elements to the interior and to the exterior of the filter are of coaxial configuration via hermetic connector.

According to another additional aspect of the invention, said electrical signal access elements to the interior and to the exterior of the filter are of waveguide configuration via hermetic pressure windows.

According to another additional aspect of the invention, the at least one cavity of the filter is of coaxial configuration.

According to another additional aspect of the invention, the at least one cavity of the filter is of waveguide configuration.

According to yet another additional aspect of the invention, the lid that is sealed on the base structure to achieve a hermetic closure is a cover that supports tuning and assembly elements.

Another object of the present invention is that of providing a method for fabricating a microwave filter, said filter comprising a base structure with at least one resonant cavity in the interior thereof, at least one electric signal access element to the interior of the filter and at least one electric signal access element to the exterior of the filter, characterised by installing said electric signal access elements in a hermetic manner on the filter, and by sealing said base structure by means of a lid that is mounted on said base structure forming between said lid and said base structure a hermetically sealed space in the interior of the filter that contains at least one gas at a determined pressure.

Another object of the present invention is that of providing a satellite incorporating the microwave filter of the invention.

These and other beneficial characteristics of the invention are described in greater detail below with the assistance of the drawings attached.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a curve illustrating the behaviour of a typical structure, for example a microwave filter, of the type of those with which the invention is concerned and as it is known in the state of the art.

Figure 2 is a schematic representation of an embodiment of the microwave filter object of the invention with cavities and electrical signal access in coaxial configuration.

Figure 3 is a schematic representation of an alternative embodiment of the microwave filter object of the invention with cavities and electrical signal access in waveguide configuration.

DESCRIPTION OF PREFERRED EMBODIMENTS In figure 2 it is shown a schematic representation of a microwave filter cross section on a vertical plane according to a first embodiment of the invention. In said figure, the microwave filter comprises a base structure 6 that houses a plurality of resonant cavities 5, which in the case of the present example are coaxial cavities. Notwithstanding, other configurations of cavities are equally possible for the purposes of the present invention.

Coupling elements 3, as can be for example probes, provide coupling between some of said cavities 5 according to the design of the filter for its specific application. Likewise, the filter comprises tuning elements 2 of the types known, as for example tuning slugs. Said tuning elements are mounted on an assembly bracket 4, which may be a conventional cover of the filter, which is secured on the base structure 6 of the filter by means of assembly screws 4'.

The filter likewise has an electrical signal access element 7 to the interior and an electrical signal access element 7 to the exterior of the filter, both implemented by means of hermetic connectors, respectively. In the case of this embodiment, said access elements are of the coaxial type as may be appreciated in the figure.

It is to be pointed out that in the aforementioned figure 2, only those parts of the filter that are significant for a better understanding of the invention are shown. Other elements and parts of the filter that do not affect said understanding have not been shown for reasons of simplification, it being understood that in practice they are incorporated in the filter for its normal operation.

According to a solution proposed by the present invention, the filter has a lid 1 that is located on the base structure 6 covering in a complete and hermetic manner the top part of the latter in such a way that between the lid 1 and the base structure 6 a closed and hermetic space is formed with regard to the passage of air or other gases. Said closed space is shown in figure 2 as a dotted area. In the aforementioned closed space is to be found one or more gases, as may be simply air, at a determined pressure, like for example atmospheric pressure. Since the aforementioned lid produces a hermetic seal, the internal pressure of the filter is maintained substantially constant in spite of the filter being in the presence of a vacuum or surroundings at low pressure during the use thereof in a satellite or other means or support in space.

As has already been mentioned, each electrical signal access element 7 is also of the hermetic type, whereby it also contributes to maintaining the internal pressure of the filter at a constant level.

Optionally, the filter may comprise dielectric resonators inside the cavities 5. In figure 2 said resonators are indicated with reference number 8.

In figure 3 it is shown a schematic representation of a microwave filter cross section on a vertical plane according to a second embodiment of the invention. In said figure, the corresponding parts similar to those in figure 2 are identified with similar reference numbers. Thus, the microwave filter comprises a base structure 6 that houses a plurality of resonant cavities 5. In the case of the example of figure 3, the cavities take the form of a waveguide, it being possible to make use of other cavity configurations.

Likewise in the case of the present example, the coupling elements 3 for providing coupling between some of said cavities 5 take the form of windows, other coupling element configurations being possible. The tuning elements 2, being of known types, are mounted on the assembly bracket 4, which can be the conventional cover of the filter, which is secured on the base structure 6 of the filter by means of assembly screws 4'.

The electrical signal access elements 7 to the interior and to the exterior of the filter, in the case of the example of figure 3, are each implemented by the use of hermetic pressure windows, which are suitable for mounting in filters of the waveguide type.

In analogous fashion to that of figure 2, in the case of figure 3 also, for reasons of simplification, only those parts of the filter that are significant for a better understanding of the invention are shown. As may be appreciated in figure 3, the filter has a lid 1 that is located on the base structure 6 covering in a complete and hermetic manner the top part of the latter in such a way that between the lid 1 and the base structure

6 a closed and hermetic space is formed with regard to the passage of air or other gases. Said closed space is shown in figure 3 as a dotted area. In the aforementioned closed space is to be found one or more gases, as may be simply air, at a determined pressure, like for example atmospheric pressure. Since the aforementioned lid produces a hermetic seal, the interior pressure of the filter is maintained substantially constant in spite of the filter being in the presence of a vacuum or surroundings at low pressure during the use thereof in a satellite or other means or support in space.

As has already been mentioned, each electrical signal access element

7 is also of the hermetic type, whereby it also contributes to maintaining the internal pressure of the filter at a constant level. In both examples of figures 2 and 3, the form of hermetically closing the lid 1 on the base structure 6 and the hermetic fitting of the access elements can be carried out by means of known practices such as, for example, laser welding or vapour phase welding or conductive glues. Prior to implementing the hermetic seal, the filter undergoes tuning according to the requirements of the corresponding application.

Optionally, the pressure in the interior of the hermetically sealed space between the base structure 6 and the lid 1 can have values other than atmospheric pressure, provided that said values are greater than the previously described critical pressure. The value of the critical pressure is approximately p(mm Hg) = 1.66 * Freq (GHz), that is typically between 3 and 50 mm Hg, depending on the value of the frequency, for the bands currently employed in satellite communications (2 to 30 GHz) and always below 100 mm Hg.

Moreover, instead of making use of a lid 1 on the base structure 6, it may be chosen to employ the cover 4 for implementing the hermetic seal, in which case it would be between said cover and the base structure. In this case, it will be necessary to procure that the tuning elements 2 and the assembly screws 4' also seal hermetically.

The present technology for fabricating microwave bandpass filters with cavities for space applications only permits the manufacture of filters for power levels of around 1 W (free of corona effect) and 15 W (free of multipactor effect) in the S band. With the solution proposed by the invention, the fabrication of high-performance bandpass filters with transmission zeros, capable of handling around 100 W in the S band free from corona and multipactor effects, is made possible.

Claims

1. - Microwave filter for application in bandpass frequencies, said filter comprising a base structure (6) with at least one resonant cavity (5) in the interior thereof, characterised in that a pressure in the interior of said filter is maintained substantially constant at a determined value.

2. - Microwave filter for application in bandpass frequencies, said filter comprising a base structure (6) with at least one resonant cavity (5) in the interior thereof, at least one electrical signal access element (7) to the interior of the filter and at least one electrical signal element (7) to the exterior of the filter, characterised in that said electrical signal access elements (7) are hermetic, and in that said filter also has a lid (1) suitable for being sealed on said base structure (6) in such a manner that between said lid (1) and said base structure (6) a hermetically sealed space is formed in the interior of the filter that contains at least one gas at a determined pressure.

3. - Filter, according to claim 1 or 2, characterised in that said determined pressure in the interior of the filter is the atmospheric pressure.

4. - Filter, according to claim 1 or 2, characterised in that said determined pressure in the interior of the filter is greater than a critical pressure, this being the pressure that corresponds with a point at which the corona effect is produced with a minimum breakdown voltage.

5. - Filter, according to claim 4, characterised in that the value of the critical pressure is less than 100 mm Hg, and preferably between 3 and 50 mm Hg.

6. - Filter, according to claim 2, characterised in that the at least one electrical signal access element (7) to the interior and the at least one electrical signal access element (7) to the exterior of the filter are of a coaxial configuration with hermetic connectors.

7. - Filter, according to claim 2, characterised in that the at least one electrical signal access element (7) to the interior and the at least one electrical signal access element (7) to the exterior of the filter are of waveguide configuration by means of hermetic pressure windows.

8. - Filter, according to any of the previous claims, characterised in that the at least one cavity of the filter (5) is of coaxial configuration.

9. - Filter, according to any of claims 1 to 7, characterised in that the at least one cavity of the filter (5) is of waveguide configuration.

10. - Filter, according to any of the previous claims, characterised in that the lid that is sealed on the base structure (6) for implementing a hermetic seal is a cover (4) which supports tuning elements (2) and assembly elements (4').

11. - Method for fabricating a microwave filter, said filter comprising a base structure (6) with at least one resonant cavity (5) in the interior thereof, at least one electrical signal access element (7) to the interior of the filter and at least one electrical signal access element (7) to the exterior of the filter, characterised in that said electrical signal access elements (7) are fitted in a hermetic manner on the filter, and in that said base structure (6) is sealed by means of a lid (1) that is mounted on said base structure (6) forming between said lid (1) and said base structure (6) a hermetically sealed space in the interior of the filter that contains at least one gas at determined pressure.

12. - Satellite for use in space incorporating the microwave filter of claims 1 or 2.