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:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on French Patent Application No. 01 04 255 filed Mar. 29, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a filter and to an antenna including the filter, which antenna can in particular be used in a mobile telephone network. 
     2. Description of the Prior Art 
     A telecommunication antenna sends and receives radio waves at frequencies specific to a telecommunication system using the antenna. Thus an antenna for the Global System for Mobile communications (GSM) sends and receives waves whose frequencies are in the 870-960 MHz band. 
     FIG. 1 shows an installation which includes a GSM base station  10  and a GSM antenna  14 . A base station is usually at ground level, for ease of maintenance, whereas an antenna is usually high up—on a pylon, water tower, etc.—to maximize its send and receive coverage area. For this reason the station  10  is connected to the antenna  14  by cables  16  transmitting radio waves between them. 
     Various forms of electromagnetic interference, due to waves sent by another antenna, for example, degrade the waves transmitted in this way. Also, the waves produced by the station  10  may include unwanted frequencies outside the GSM frequency band. A filter  12  is therefore placed between the base station  10  and the antenna  14 . The filter  12  processes the waves transmitted by the cables  16  to attenuate those whose frequency is outside the band used by the antenna  14 . The filter  12  is an air filter, for example, formed by a hollow enclosure with metal walls whose dimensions are such that waves at particular frequencies are attenuated by resonance as they propagate in the enclosure. 
     Locating filters outside the antennas has many drawbacks. The cables used in these installations are costly. The quantity of cable used is increased by locating the filters outside the antennas. Also, manual connection of the cables to the filters leads to additional costs and the risk of damage to the cables and the filters. Using cables between the filters and the antennas degrades the waves transmitted by the cables, because of transmission losses or external interference due in particular to signals radiated by other antennas. This is undesirable, especially for the waves sent to the antenna, because they are not filtered afterward. 
     U.S. Pat. No. 6,201,801 describes a single-band antenna in which a single send/receive filter is disposed inside the chassis or housing containing the radiating elements of the antenna. 
     Multiband antennas including radiating elements used for respective different telecommunication systems are known in the art. A multiband antenna of this kind requires filters, but producing filters incorporated into the same chassis or housing as the antenna is particularly difficult, because of the size of the filters. For example, in a multiband antenna including GSM radiating elements using the 870-960 MHz band and radiating elements for the Digital Cellular System (DCS) using the 1710-1880 MHz band, it is necessary to provide a GSM filter and a DCS filter respectively connected to the GSM radiating elements and to the DCS radiating elements. 
     The object of the invention is to propose a microwave filter that can easily be incorporated into a multiband antenna. 
     SUMMARY OF THE INVENTION 
     The invention provides a microwave filter including a transmission microstrip, at least one lateral microstrip connected to the transmission microstrip, and at least two dielectric resonators, and wherein said at least one lateral microstrip is coupled to said at least two dielectric resonators so that it can resonate with said at least two dielectric resonators. 
     The above filter enables filters to be incorporated into the chassis or housing of an antenna because the collaboration of at least two resonators with the same microstrip provides a filter which, for the same performance, is more compact than a combination of independent filters each including a dielectric resonator collaborating with a single lateral microstrip. 
     In a preferred embodiment, the lateral microstrips form a series of U-shapes, two successive U-shapes having a common branch. 
     In a particular embodiment, the center of each dielectric resonator is equidistant from two branches of a U-shape. 
     In a preferred embodiment, each dielectric resonator has a relative permittivity of not less than 10. 
     The filter advantageously further includes adjustment elements adapted to be moved arbitrarily relative to the dielectric resonators to modify respective resonant frequencies of the dielectric resonators. 
     In a preferred embodiment, each lateral microstrip has a length substantially equal to 3λ m /4 where λ m  represents a wavelength to be attenuated. 
     The invention also provides a microwave antenna including radiating elements and at least one filter as defined above in a common chassis or housing. 
     One embodiment of the antenna includes radio frequency protection for the filter. 
     Other features and advantages of the invention will become apparent from the description of embodiments of the invention given by way of non-limiting example and with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 already described, represents an antenna installation. 
     FIG. 2 shows a prior art filter with microstrip and dielectric resonators. 
     FIG. 3 is a partial view of the interior of one embodiment of an antenna incorporating two filters according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 shows a prior art filter with microstrip and dielectric resonator. The filter includes a transmission microstrip  20  constituting a transmission line for radio waves. A lateral microstrip  22  forms an orthogonal branch having a free end and an end connected to the microstrip  20  at a branching point  23 . The lateral microstrip  22  has a length of 3λ 22 /4, where λ 22  represents a propagation wavelength of certain waves transmitted by the microstrip  20 . The lateral microstrip  22  is disposed so that it can be coupled to a dielectric resonator  24 . 
     To guide radio waves, the microstrips  20  and  22  consist of a conductive material, such as a metal, deposited on an insulative material. The lateral microstrip  22  attenuates waves at a wavelength of λ 22  transmitted by the transmission microstrip  20  by dissipating their energy through a phenomenon of resonance at a frequency corresponding to said wavelength λ 22 . 
     Moreover, the center of the dielectric resonator  24  is placed at a distance of λ 22/24  from the connection point  23  of the microstrip  20  and the microstrip  22 . The resonator  24  attenuates waves at a wavelength of λ 22/24  transmitted by the transmission microstrip  20  by resonating with the lateral microstrip  22  at a frequency corresponding to a wavelength of λ 22/24 . 
     The wavelength λ 22/24  is close to λ 22 . For example, for wavelengths of the order of one millimeter, the differences (λ 22 −λ 22/24 ) are of the order of a few hundredths of a millimeter. This kind of 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 must be used. The size of the plurality of filters would then be too great compared to the available space within the chassis or housing of an antenna. 
     The invention provides a microstrip antenna including a transmission microstrip, at least one lateral microstrip constituting a branch, and at least two dielectric resonators coupled to the same lateral microstrip. It is then found that the range of wavelengths filtered by this single filter is expanded, at the cost of an increase in overall size that is smaller than if two or more than two independent filters were used each consisting of a dielectric resonator coupled to a single branch. 
     FIG. 3 is a partial view of the interior of a multiband GSM/DCS antenna  30  incorporating two filters  32 ,  34  according to the invention. The antenna  30  includes GSM radiating elements  40  for sending and receiving radio waves in the GSM band and DCS radiating elements  44  for sending and receiving radio waves in the DCS frequency band. FIG. 3 shows only one GSM radiating element  40  and one DCS radiating element  44 . The GSM radiating elements  40  and the DCS radiating elements  44  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  and the DCS base station is connected to inputs  46  and  52 . 
     The use of two feed inputs for the same radiating elements device is due to the nature of the radiating elements used. Each radiating element  40  or  44 , the operation of which is described in U.S. Pat. No. 6,025,798, for example, is equivalent to two independent dipoles at 90° to each other. Because of this 90° offset, the dipoles transmit signals correctly, regardless of the position of a sending or receiving antenna relative to the radiating elements. 
     The input  48  is connected to a filter  32  according to the invention to filter the waves transmitted between the GSM base station and the radiating elements  40 ; the input  50  is connected to a filter  34  according to the invention. The filters  32  and  34  are inside the chassis or housing  70  of the antenna  30 . 
     Only the filter  32  is described below, the filters  32  and  34  being identical. The filter  32  has an input  51  connected to the GSM input  48  of the antenna. The input  51  is a first end  54  of a transmission microstrip  56 . The other end  55  of the transmission microstrip  56  is connected by means that are not shown to one of the GSM radiating elements  40 . 
     The transmission microstrip  56  is made of a conductive material, for example a metal, disposed on an insulative material. It is connected to three lateral microstrips  58 ,  60  and  62  constituting branches disposed transversely relative to the microstrip  56  and having the same width and the same nature thereas. To be more precise, a first end of the lateral microstrip  58  is connected to the end  51  of the transmission microstrip  56 , a first end of the lateral microstrip  60  is connected to a central portion  61  of the transmission microstrip  56 , and a first end of the lateral microstrip  62  is connected to the other end  55  of the microstrip  56 . In this embodiment, the second ends of the microstrips  58 ,  60 ,  62  are not connected to anything. 
     The resonators  64  and  66  are of standard design. They are ceramic cylinders made of alloys containing magnesium, calcium, titanium, barium, zinc, zirconium or tin. These ceramic materials have high dielectric constants ε r , i.e. dielectric constants at least equal to 10. 
     The microstrips  58 ,  60 ,  62  and the dielectric resonators  64  and  66  have characteristics such that, and are disposed so that, some frequencies are attenuated by dissipation of energy due to resonance of the lateral micro strips  58 ,  60 ,  62  and the resonators  64  and  66  coupled to the lateral microstrips  58 ,  60 ,  62 . In particular, the lateral microstrip  60  is coupled both to the resonator  64  and to the resonator  68 . 
     In this 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 with the wavelength λ m  by resonating at the frequency corresponding to the wavelength λ m . 
     The resonator  64  is equidistant from the microstrips  58  and  60  and its center is at a distance of λ m /4 from the end  51  of the microstrip  56 , i.e. from the junction between the transmission microstrip  56  and the lateral microstrip  58 . The resonator  64  therefore resonates at a wavelength of λ m/64  with the microstrip  58 . This resonance dissipates the energy of the waves at wavelength λ m/64 , so attenuating them. 
     The lateral microstrip  60  also attenuates waves by resonance. However, it is found experimentally that this resonance occurs at a wavelength λ 60  offset from the wavelength λ m . Furthermore, the resonator  64  is also coupled to the lateral microstrip  60 . The resonator  64  then dissipates energy associated with a wavelength λ 60/64  by resonance, attenuating waves transmitted with that wavelength λ 60/64 . 
     The resonator  66  is equidistant from the lateral microstrips  60  and  62 . Its center is at a distance of λ m /4 from the branching point  61 , i.e. from the junction between the transmission microstrip  56  and the lateral microstrip  60 . Its characteristics are chosen so that the resonator  66  resonates with the microstrip  60  at a frequency corresponding to a wavelength λ 60/66 . The resonator  66  then dissipates energy associated with a wavelength of λ 60/66  by resonance, thereby attenuating waves transmitted with that wavelength λ 60/66 . 
     The waves transmitted by the transmission microstrip  56  are then filtered by the lateral microstrip  62 . The microstrip  62  attenuates waves transmitted at a wavelength λ 62  by dissipating energy by resonance at that wavelength. 
     Furthermore, the center of the resonator  66  is at a distance of λ m /4 from the branching 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 energy associated with the wavelength λ 62/64  by resonance, thereby attenuating waves transmitted at that wavelength λ 62/64 . 
     Thus waves transmitted by the transmission microstrip  56  are attenuated at a series of wavelengths covering a wide band. 
     It is found experimentally that a frequency band with a relative width from 1% to 5% of the center frequency is attenuated, the relative width of a band being defined as: 
     
       
         (λ max −λ min )/((λ max +λ min )/2)  
       
     
     where λ max  represents the greatest wavelength attenuated and λ min  the smallest wavelength attenuated, referred to an attenuation of 3 dB. 
     The filter is therefore equivalent to a plurality of prior art filters, i.e. filters associating a resonator with a single branch microstrip. However, thanks to a smaller number of dielectric resonators and branches, for equal performance the size of the filter is compatible with the restricted space available inside the chassis or housing of the antennas. 
     In a variant that is not shown, the lateral microstrips  58 ,  60  and  62  have a length of 3λ m /4 and their second ends are grounded. In this case, the centers of the resonators  64  and  66  are disposed at a distance of λ m /2 from the respective branching points between the transmission microstrip  56  and the lateral microstrips  58 ,  60 ,  62  so that they can resonate with the lateral microstrips  58 ,  60 ,  62 . 
     To tune it to different wavelengths, the filter  32  includes two adjustment elements  68  near the resonators  64  and  66 , respectively, which modify the wavelength attenuated by resonance. To be more precise, the elements  68  are grounded conductors which influence the capacitive effect of the resonator. The resonator can be modeled as a circuit including a resistor, an inductor and a capacitor in parallel with the inductor. Moving a conductive element  68  toward a resonator increases its capacitive effect and consequently modifies the resonant frequency. 
     In this embodiment a metal protective cap  31  covering all of the components of the filter  32  protects the filter from radio waves, and in particular from waves emitted by the GSM radiating elements  40  and the DCS radiating elements  44  of the antenna. 
     Because the filter  32  is near the GSM radiating elements  40  and the DCS radiating elements  44 , the degradation and the losses of the waves transmitted by the connections between these radiating elements and the filter are less than when the filter is outside the chassis or cap of the antenna. 
     Using resonators made of materials having high dielectric constants improves rejection, which can be better than −20 dB and is therefore significantly increased compared to that of microstrip filters with no dielectric resonator, which achieve a rejection of the order of −5 dB. 
     In terms of the quality factor Q, a microstrip filter coupled to dielectric resonators achieves values of 500 or 1000, whereas filters with no dielectric resonator achieve values of 50 to 200. 
     These high attenuations are particularly useful in telecommunication systems operating in closely spaced frequency bands. In this case, the radiating elements using a first frequency band degrade transmission in a second band close to the first band, and vice versa. This situation arises, for example, on simultaneous DCS transmission using the 1710-1880 MHz band and UMTS (Universal Mobile Telecommunication System) transmission using the 1910-2100 MHz band. 
     The present invention lends itself to many variants. Thus in one variant, not shown, the filters  32  and  34  are placed on the back of the antenna, i.e. behind a metal plate supporting the radiating elements on its front face.