EP1249889A1 - Microwave filter with a dielectric resonator - Google Patents

Abstract

A microwave filter includes at least two dielectric resonators, a transmission microstrip, and at least one lateral microstrip constituting a branch connected to the transmission microstrip. Each lateral microstrip is coupled to at least one dielectric resonator to resonate therewith. The filter is compact and can therefore be incorporated into the housing of a microwave antenna, in particular a multiband antenna for mobile telephone networks.

Description

The present invention relates to a filter and an antenna comprising a such a filter, this antenna being usable in particular in a network for mobile phones.

Telecommunications antenna transmits and receives waves radio frequencies according to frequencies specific to a telecommunications operated by this antenna. Thus, an antenna intended for GSM system ("Global System for Mobile Communications") uses waves whose frequencies lie in the band 870-960 MHz.

FIG. 1 represents an installation which comprises a GSM base station, 10, and a GSM antenna, 14. A base station is usually placed on the ground, to facilitate its maintenance, while an antenna is generally placed in a high place - pylon, water tower, etc ... - so as to maximize its transmission and reception coverage. This is why the station 10 is connected to the antenna 14 by means of cables 16 transmitting the radio waves between these two bodies.

Various electromagnetic interference, for example due to waves emitted by another antenna, deteriorate the waves thus transmitted. In addition, the waves produced by station 10 may include spurious frequencies outside the GSM frequency band. A filter 12 is therefore placed between the base station 10 and the antenna 14. This filter 12 processes the waves transmitted by the cables 16 so as to attenuate the waves whose frequency is outside the band operated by the antenna 14. The filter 12 is, by example, of the so-called "air" type, that is to say formed of a hollow enclosure with walls metallic whose dimensions are such as frequency waves determined attenuate by resonance during their propagation in the enclosure.

The location of the filters outside the antennas presents many disadvantages. The cables used in these installations have a cost Student. However, their use is increased by the location of filters outside the antennas. In addition, the manual operations of connecting cables to filters entail additional costs and risk of deterioration of the these cables and filters. Use of cables between filters and antennas deteriorates the waves transmitted by these cables, following transmission losses and external disturbances due in particular to signals radiated by other antennas. These alterations are undesirable, especially for waves sent to the antenna, because these are no longer filtered by the after.

The document US 6,201,801 describes a single-band antenna in which a filtering device, for transmission or reception, is disposed inside the chassis or case comprising the radiating elements of the antenna.

Furthermore, there are known so-called multiband antennas, comprising radiant elements used respectively for different systems of telecommunications. For such a multiband antenna, it is necessary to provide filters, but making filters incorporated into the same chassis or case that the antenna is particularly delicate because of the size of the filters. For example, in a multiband antenna includes GSM radiating elements operating in the 870-960 MHz band, and radiant elements for the DCS ("Digital Cellular System") system, operating in the 1710-1880 MHz band, a GSM filter and a filter must be provided DCS connected respectively to GSM radiating elements and to elements radiant DCS.

The object of the invention is to propose a microwave filter which can be easily incorporated into a multiband antenna.

A first object of the invention is a microwave filter comprising: at least one dielectric resonator, a transmission microstrip, at least one lateral microstrip connected to the transmission microstrip, this lateral microstrip being coupled with the dielectric resonator so as to be able to resonate with this dielectric resonator;
characterized in that it comprises at least two dielectric resonators,
and in that at least one lateral microstrip is coupled with at least two dielectric resonators, so as to be able to resonate with these two dielectric resonators.

The filter thus characterized makes it possible to incorporate filters into the chassis or to the antenna housing because the collaboration of at least two resonators with the same microstrip makes it possible to obtain a more compact, with equal performance, compared to a combination of filters independent each with a dielectric resonator collaborating with a single lateral microstrip.

In a preferred embodiment, lateral microstrips form a series of U, two successive U having a common branch.

According to a particular embodiment, the center of a resonator dielectric is equidistant from two branches of a U.

In a preferred embodiment, the dielectric resonators used have a high relative permittivity, preferably at least equal to 10.

Advantageously, the filter may include adjustment elements can be arbitrarily moved relative to the resonators dielectric so as to modify the respective resonant frequencies of these dielectric resonators.

Furthermore, according to a preferred embodiment, the branches of a lateral microstrip have a length equal to 3λ _m / 4, where λ _m is a wavelength to be attenuated.

A second object of the invention is a microwave antenna comprising radiating elements and at least one filter located in the same chassis or housing, characterized in that it comprises a filter according to one of claims 1 to 7.

According to a particular embodiment, the antenna comprises a radio protection for the filter.

• la figure 1, déjà décrite, représente une installation d'antenne,
• la figure 2, représente un filtre de type connu, à microbande et résonateur diélectrique,
• la figure 3 représente une vue partielle de l'intérieur d'un exemple de réalisation d'antenne, comprenant deux filtres selon l'invention.

Other characteristics and advantages of the invention will appear with the description of some of its embodiments, this being carried out by way of description and without limitation, with reference to the attached drawings in which:

• FIG. 1, already described, represents an antenna installation,
• FIG. 2 represents a filter of known type, with microstrip and dielectric resonator,
• FIG. 3 represents a partial view of the interior of an exemplary embodiment of the antenna, comprising two filters according to the invention.

A known filter, with a microstrip and with a single dielectric resonator, is shown in FIG. 2. This filter comprises a so-called transmission microstrip 20 constituting a transmission line for radio waves. A lateral microstrip 22 forms an orthogonal leg having a free end and an end connected to the microstrip 20 at a branch point 23. The side strip 22 has a length of 3λ _22/4, where λ ₂₂ shows a wavelength propagation of certain waves transmitted by the microstrip 20. The lateral microstrip 22 is arranged so as to be coupled with a dielectric resonator 24.

To guide the radio waves, these microstrips 20 and 22 are made of a conductive material, such as a metal, deposited on an insulating material. The lateral microstrip 22 attenuates waves of wavelength λ ₂₂ transmitted by the transmission microstrip 20 by dissipating their energy by a resonance phenomenon at a frequency corresponding to this wavelength λ ₂₂ .

Furthermore, the center of the dielectric resonator 24 is placed at a distance λ _22/4 from the point of connection 23 of strip 20 and the strip 22. This resonator 24 reduces the wavelength of waves transmitted by λ _22/24 the transmission microstrip 20, by resonating with the lateral microstrip 22 at a frequency corresponding to a wavelength λ _22/24 .

This wavelength λ _22/24 is close to λ ₂₂ . For example, for wavelengths of the order of 1 millimeter, we observe differences (λ ₂₂ - λ _22/24 ) of the order of a few hundredths of a millimeter. Such a filter therefore attenuates a narrow range of wavelengths between the wavelengths λ ₂₂ and λ _22/24 . To attenuate a wider range of wavelengths, with this type of filter, a plurality of such filters should be used. The size of this plurality of filters would then be too large compared to the space available inside the chassis or housing of an antenna.

According to the invention, a microstrip filter is produced comprising a transmission microstrip, at least one lateral microstrip constituting a branch, and at least two dielectric resonators coupled to the same lateral microstrip. We can see that the range of wavelengths filtered by this one filter is extended, at the cost of an increase of the footprint which is smaller than if we used two or more than two independent filters each consisting of a dielectric resonator coupled to a single branch.

FIG. 3 represents a partial view of the interior of a multiband GSM / DCS antenna, 30, which comprises two filters 32, 34 according to the invention. This antenna 30 comprises radiating elements 40 GSM transmitting and receiving radio waves in the GSM band, and radiating elements 44 DCS performing these same functions in the frequency band DCS. In FIG. 3, a single radiating element 40 GSM and a single radiating element 44 DCS are shown. The radiating elements 44 DCS and 40 GSM are connected to base stations (not shown) external to the antenna 30. The GSM base station is connected to inputs 48 and 50 of the antenna 30, while the station DCS base is connected to inputs 46 and 52.

Using two power inputs for the same device of radiant elements is due to the nature of the radiant elements used. In effect, each radiating element 40 or 44, the operation of which is described for example in US patent 6,025,798, is equivalent to two dipoles placed at 90 degrees from each other, each of these dipoles being independent of the other. Thanks to such a 90 ° offset, these dipoles provide transmission suitable signals regardless of the position of a transmitting antenna or receiving with respect to these radiating elements.

To filter the waves transmitted between the GSM base station and radiating elements 40, the input 48 is connected to a filter according to the invention, 32; while the input 50 is connected to a filter according to the invention, 34. These filters 32 and 34 are inside the chassis or case 70 of the antenna 30.

Filters 32 and 34 being identical, only the description of filter 32 is performed below. This filter 32 has an input 51 connected to the GSM input, 48, of the antenna. This entry 51 is constituted by a first end 54 of a transmission microstrip 56. The other end, 55, of this transmission microstrip 56 is connected, by means not shown to one of the GSM radiating elements, 40.

The transmission microstrip 56 is made of a material conductor, for example a metal, placed on an insulating material. She is connected to three lateral microstrips 58, 60 and 62 constituting branches arranged transversely with respect to this microstrip 56, and having the same width and nature as the latter. More specifically, a first end of the lateral microstrip 58 is connected to the end 51 transmission microstrip 56; a first end of the lateral microstrip 60 is connected to a central part 61 of the transmission microstrip 56; and a first end of the microstrip side 62 is connected to the other end 55 of the microstrip 56. In this exemplary embodiment, the second ends of the microstrips 58, 60, 62 are not connected to anything.

The resonators 64 and 66 are produced in a conventional manner. These are ceramic cylinders made up of alloys containing magnesium, calcium, titanium, barium, zinc, zirconium, or tin. These ceramics have high dielectric constants ε _r , that is to say at least equal to 10.

The microstrips 58, 60, 62, and the dielectric resonators 64 and 66, have characteristics and are arranged in such a way that certain frequencies are attenuated by energy dissipation due to resonances lateral microstrips 58, 60, 62 and coupled resonators 64 and 66 with these lateral microstrips 58, 60, 62. In particular, the microstrip lateral 60 is coupled both with the resonator 64 and with the resonator 68.

In this exemplary embodiment, the microstrips 58, 60 and 62 have a length substantially equal to 3λ _m / 4, where λ _m represents a wavelength to be attenuated.

The microstrip 58 attenuates waves of wavelength λ _m by resonating at the frequency corresponding to this wavelength λ _m .

The resonator 64 is equidistant from the microstrips 58 and 60, and its center is distant from λ _m / 4 from the end 51 of the microstrip 56, that is to say from the junction between the transmission microstrip 56 and the microstrip lateral 58. This resonator 64 then resonates at a wavelength λ _{m / 64} with the microstrip 58. This resonance dissipates the energy of waves of wavelength λ _{m / 64} , thus attenuating the latter.

The lateral microstrip 60 also attenuates waves by resonance. However, it has been observed experimentally that this resonance takes place at a wavelength λ ₆₀ offset from the wavelength λ _m . In addition, the resonator 64 is also coupled with the lateral microstrip 60. The resonator 64 then dissipates an energy associated with a wavelength λ _60/64 by resonance, thus attenuating the waves transmitted with this wavelength λ _{60 / 64} .

The resonator 66 is equidistant from the lateral microstrips 60 and 62. Its center is distant from λ _m / 4 from branch 61, that is to say from the junction between the transmission microstrip 56 and the lateral microstrip 60. Its characteristics are chosen such that this resonator 66 resonates with the microstrip 60 at a frequency corresponding to a wavelength λ _60/66 . The resonator 66 then dissipates an energy associated with a wavelength λ _60/66 by resonance, thus attenuating the waves transmitted with this wavelength λ _60/66 .

The waves transmitted by the transmission microstrip 56 are then filtered by the lateral microstrip 62. In fact, this microstrip 62 attenuates waves transmitted with a wavelength λ ₆₂ by dissipating energy by resonance at this wavelength .

In addition, the center of the resonator 66 is distant by λ _m / 4 from the connection point 55 of the lateral microstrip 62. The resonator 66 resonates with the microstrip 62 at a frequency corresponding to another wavelength λ _62/64 . The resonator 66 then dissipates an energy associated with a wavelength λ _62/64 by resonance, thus attenuating the waves transmitted with this wavelength λ _62/64 .

Thus the waves transmitted by the transmission microstrip 56 are attenuated according to a series of wavelengths covering a band extended.

Experimentally, we observe that a frequency band with a relative width between 1 and 5% of the central frequency is attenuated, the relative width of a band being defined by: (λ max - λ min ) / ((Λ max + λ min ) / 2) where λ _max represents the longest attenuated wavelength and λ _min is the shortest attenuated wavelength, at -3 dB attenuation.

This filter therefore performs a function equivalent to several filters with known type, i.e. associating a resonator with a single microstrip of connection. But, thanks to a reduced number of dielectric resonators, and of branches, for equal performance, the size of the filter is compatible with the limited space available inside the chassis or boxes antennas.

In a variant not shown, the lateral microstrips 58, 60 and 62 have a length of 3λ _m / 4 and their second ends are connected to ground. In this case, the centers of the resonators 64 and 66 are arranged so as to be distant by λ _m / 2 from the respective connection points between the transmission microstrip 56 and the lateral microstrips 58, 60, 62, in order to be able to resonate with these microstrips side 58, 60, 62.

To allow the filtering of filter 32 to be adapted to different wavelengths, the filter 32 includes two adjustment elements 68 located respectively near resonators 64 and 66, and modifying a wavelength attenuated by resonance. More specifically, these elements 68 are conductors connected to ground, and which influence the capacitive effect of resonator. Indeed, the latter can be modeled as a circuit comprising a resistor, an inductor, and a capacitor in parallel on this inductance. Approach a conductive element 68 of a resonator causes an increase in its capacitive effect and, consequently, a modification of the resonance frequency.

In this exemplary embodiment, the filter 32 is protected from waves radioelectric, and in particular waves emitted by radiating elements 40 GSM and 44 DCS of the antenna, by a protective cover 31, metallic and covering all the constituent elements of filter 32.

The filter 32 being close to the radiating elements 40 GSM and 44 DCS, the degradation and losses of the waves transmitted by the links connecting these radiating elements to the filter are less than when the filter is located at outside the chassis or antenna cover.

The use of resonators made of materials having high dielectric constants results in improved rejection, rejection can be better than -20 dB, significantly increased compared to that of microstrip filters without dielectric resonator, providing a rejection on the order of -5 dB.

In terms of quality factor Q, a coupled microstrip filter with dielectric resonators reaches values of 500 or 1000, while filters without dielectric resonators provide values from 50 to 200.

These high attenuations are particularly useful in the case of telecommunications systems operating in bands of frequency close. Indeed, in this case, the radiating elements using a first frequency band deteriorate the following transmissions a second band adjacent to this first band, and vice versa. This situation arises, for example, when simultaneously planning DCS (operating in the 1710-1880 MHz) and UMTS ("Universal" transmissions) Mobile Telecommunication System "using the 1910-2100 frequency band MHz).

The present invention is susceptible of numerous variants. So, in a variant not shown, the filters 32 and 34 are placed on the back of the antenna, i.e. behind a metal plane supporting the elements radiant on its front face.

Claims (10)

Microwave filter (32) comprising: at least one dielectric resonator (64, 66), a transmission microstrip (56), at least one lateral microstrip (58, 60, 62) connected to the transmission microstrip, this lateral microstrip being coupled with the dielectric resonator so as to be able to resonate with this dielectric resonator;
characterized in that it comprises at least two dielectric resonators (64, 66),
and in that at least one lateral microstrip (60) is coupled with at least two dielectric resonators (64, 66), so as to be able to resonate with these two dielectric resonators. Filter according to claim 1, characterized in that lateral microstrips (58, 60, 62) form a series of U, two successive U having a common branch. Filter according to claim 2, characterized in that the center of each dielectric resonator is equidistant from two branches (58, 60, 62) of a U. Filter according to claim 1, characterized in that each dielectric resonator has a high relative permittivity, at least equal to 10. Filter according to claim 1, characterized in that it further comprises adjustment elements (68) which can be arbitrarily moved relative to the dielectric resonators (64, 66) so as to modify the respective resonant frequencies of these dielectric resonators. Filter according to claim 1, characterized in that each lateral microstrip (58, 60, 62) has a length substantially equal to 3λ _m / 4, where λ _m represents a wavelength to be attenuated. Filter according to claim 1, characterized in that it comprises: at least three lateral microstrips (58; 60; 62) connected to this transmission microstrip (56), and at least two dielectric resonators (64; 66) respectively placed between a first and a second lateral microstrip (58; 60) and between the second and a third lateral microstrip (60, 62). Microwave antenna (30) comprising radiating elements (40, 44) and at least one filter (55 to 66) located in the same chassis or housing (70), characterized in that it comprises a filter (32) according to the one of claims 1 to 7. Antenna according to claim 8, characterized in that it includes a radioelectric protection (31) for the filter. Antenna according to claim 8, characterized in that it comprises radiating elements (40; 44) working in different frequency bands.

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