Abstract:
A multiband filtering apparatus ( 40 ) for use in a communications system, said apparatus including a housing ( 21 ); a plurality of cavities ( 22.1, 22.2 ) disposed within said housing wherein each cavity includes a resonant structure, the resonant structure having at least one ceramic element ( 23.1, 23.2 ); at least one input port ( 27 ) coupled to a first resonator of said plurality of resonators; at least one output port ( 28 ) coupled to a second resonator of said plurality of resonators; and a closure member ( 25 ) adapted to engage said housing ( 21 ) and cap said cavities.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to communications filters. In particular although not exclusively the present invention relates to multiband cavity filters. 
     2. Discussion of the Background Art 
     Various forms of filters are employed in today&#39;s communications systems. Some of the more common types utilized are band pass, low pass, high pass and notch filters. A typical application of such filter types is within most household televisions and radios. Generally these devices employ band pass and low pass filters to select the desired station. Typically these tuning filters are constructed from conventional electronic components such as capacitors, inductors, resistors and operational amplifiers (in the case of active filtering). 
     While such filters are quite capable of handling transmissions in the AM, FM, VHF and selected UHF bands, they are not readily suitable for communications applications utilizing higher UHF frequency bands such as those used in microwave transmissions. At these higher frequency ranges some of the basic electrical characteristics of electronic components used in these filter constructions begin to degrade. This degradation alters the transfer characteristics of the filter causing distortion. 
     Accordingly, filtering in the higher UHF bands to EHF bands requires a different approach. One commonly used filter type for such higher bands, especially in high power communication systems is a cavity filter. Cavity filters are utilised in these high power systems due to their stability and their high Q factors. 
     One such use of a resonance cavity in a communication system is discussed in U.S. Pat. No. 2,337,184 entitled “Coupling Circuit”, which relates to a circuit for coupling a plurality of sources such as plurality of radio frequencies to a single load. A rectangular cavity resonator is coupled to a first transmitter, a second transmitter and a load, in this case an antenna. The cavity allows the two transmitters to utilize the antenna simultaneously without interference. The two transmitters excite two fundamental modes within the cavity,. the first mode being at the frequency of the first transmitter and the second being at the frequency of the second transmitter. The antenna is coupled to the resonator via a dipole p and is positioned in such a manner that it is excited equally by modes thereby allowing both modes to propagate through an antenna A. 
     U.S. Pat. No. 5,349,316 entitled “Dual Bandpass Microwave Filter” discloses a dual port bandpass filter. The filter consists of at least one resonance cavity having two independent modes of operation at displaced frequencies. This provides the filter with two independent passbands within the desired frequency band. In order to produce the two passbands the filter requires the incoming waveguide to be orientated at an angle to the filter such that both TE and TM modes are excited within the cavity, particularly the TE 1,1,1  and TM 0,1,0  modes. 
     Yet another form of dual mode cavity filter is discussed in U.S. Pat. No. 5,793,271. The filter in this instance is composed of one or more dual-mode resonant cavities. Each cavity produces two resonant modes at two different frequencies. The two modes have essentially the same field distribution but are orthogonal to each other. The cavity further includes a first set and a second set of tuning elements to tune the respective modes to the desired frequency. 
     One problem with the above discussed filter types is that they can be quite large and cumbersome. Furthermore the frequency tuning of such filters is relatively dependent upon the coupling tuning. This is the case with the filter of U.S. Pat. No. 5,349,316 which requires the signal coupling to be orientated at a certain angle in order to induce the required modes. This is not always possible and therefore the operation of the filter may be impaired. 
     Accordingly it would be advantageous to provide a multiband filter which is less obtrusive and provides for quasi-independent frequency and coupling tuning as well as providing an improved tuning arrangement. 
     SUMMARY OF THE INVENTION 
     Disclosure of the Invention 
     In one aspect of the present invention there is provided a multiband filtering apparatus for use in a communications system, the apparatus including: 
     a housing; 
     a cavity disposed within the housing, the cavity including a resonant structure positioned within the cavity, the resonant structure including at least one ceramic element; 
     an input port and an output port, each port coupled to the resonant structure; and 
     a closure member adapted to engage the housing and cap the cavity. 
     Preferably the resonant structure is positioned centrally within the cavity. Suitably the resonant structure is a multimode resonator, particularly where the filtering apparatus is for dual band filtering. 
     The ceramic element may be of annular, toroidal, cylindrical, elliptical or other suitable geometric configuration. Preferably the ceramic element is in the form of a puck. The puck may rest directly on the cavity floor. Alternatively the puck may be mounted on an appropriate support provided within the cavity. Preferably a TE01δ mode is used within the puck. 
     The resonant structure may also include at least one conductive element, suitably the conductive element is in the form of a post. The post may be positioned integral with or adjacent to the ceramic element. Preferably the post is aligned substantially co-axial with the ceramic element. Suitably the post extends upwardly from the floor of the cavity and terminates adjacent a rim of the cavity. Alternatively the post may terminate a predetermined height relative to the rim of the cavity. The post may also include a bore for receiving a tuning rod. 
     Preferably the cavity is dimensioned to produce at least one comb-line resonance mode. Most preferably the cavity is dimensioned to produce a comb-line resonance mode in the 900 MHz band and a TE01δ mode in the 1800 MHz band. 
     Suitably the input port and the output port are provided on opposing sides of the housing. The input and output ports may be a co-axial coupling, such as an F, N, SMA, 7/16 or other suitable type connector, or they may be a waveguide coupling such as a flange. 
     In another aspect of the present invention there is provided a multiband filtering apparatus for use in a communications system, the apparatus including: 
     a housing; 
     a plurality of cavities disposed within the housing wherein each cavity includes a resonant structure the resonant structure including at least one ceramic element; 
     at least one input port coupled to a first resonator of the plurality of resonators; 
     at least one output port coupled to a second resonator of the plurality of resonators; and 
     a closure member adapted to engage the housing and cap the cavities. 
     Preferably each of the resonant structures is positioned centrally within a respective cavity. At least one of the resonant structures may be a multimode resonator. 
     Each of the ceramic elements may be of annular, toroidal, cylindrical, elliptical or other suitable geometric configuration. Preferably each ceramic element is in the form of a puck. The pucks may rest directly on the floor of the respective cavities. Alternatively one or more of the pucks may be mounted on an appropriate support provided within the respective cavities. Preferably a TE01δ mode is used within the pucks. 
     Suitably each of the resonant structures may also include at least one conductive element, suitably the conductive element is in the form of a post. Each post may be positioned integral with or adjacent to a ceramic element. Preferably each of the posts extends upwardly from the floor of the respective cavity and terminates adjacent a rim of the respective cavity. Alternatively one or more of the posts may terminate a predetermined distance from the rim of the respective cavity. Each post may also include a bore for receiving a tuning rod. 
     The cavities are suitably dimensioned to allow for the propagation of TM01δ and TE01δ modes. 
     Suitably the input port and the output port are provided on opposing sides of the housing. The input and output ports may be a co-axial coupling, such as an F, N, SMA, 7/16 or other suitable type connector, or the may be a waveguide coupling such as a flange. 
     In yet another aspect of the present invention there is provided a multiband filtering apparatus having a first filtering path and second filtering path, the apparatus including: 
     a housing; 
     a first set of cavities of disposed within the housing; 
     a first set of resonant structures wherein each of the resonant structures of first set of resonant structures are disposed within a respective cavity from the first set of cavities, each of the resonant structures including at least one ceramic element; 
     a first input port coupled to a first resonator of the first set of resonators; 
     a first output port coupled to second resonator of the first set of resonators; 
     a second set of cavities disposed within the housing; 
     a second set of resonant structures wherein each of the resonant structures of the second set of resonant structures are disposed within a respective cavity form the second set of cavities; 
     a second input port coupled to a first resonator from the second set of resonators; and 
     a second output port coupled to a second resonator from the second set of resonators. 
     Suitably the first filtering path is provided through the first set of resonant structures, while the second filtering path is provided through the second set of resonant structures and at least one resonant structure from the first set. 
     At least one of the resonant structures from the first set of resonant structures may be multimode resonators. Each of the ceramic elements may be of annular, toroidal, cylindrical, elliptical or other suitable geometric configuration. Preferably each ceramic element is in the form of a puck. The pucks may rest directly on the floor of the respective cavities. Alternatively one or more of the pucks may be mounted on an appropriate support provided within the respective cavities. Preferably a TE01δ mode is used within the pucks. 
     Suitably each of the resonant structures from the first set of structures may also include at least one conductive element, suitably the conductive element is in the form of a post. Each post may be positioned integral with or adjacent to a ceramic element. Preferably each of the posts extends upwardly from the floor of the respective cavity and terminates adjacent a rim of the respective cavity. Alternatively one or more of the posts may terminate a predetermined distance from the rim of the respective cavity. Each post may also include a bore for receiving a tuning rod. 
     Preferably at least one of the resonant structures from the second set of resonant structures is a comb-line resonator. 
     Both the first and second sets of cavities are suitably dimensioned to allow for the propagation of TM01δ and TE01δ modes. Preferably the first set of cavities and second set of cavities are coupled together. 
     The input port and output port may be co-axial couplings, such as an F type connector, or the may be waveguide couplings such as a flange. 
     Suitably the housing, closure member and cavity or cavities (as the case may be) are formed from a conductive material, such as aluminium or other suitable metal. Alternatively the housing closure member and cavity may be formed from a suitable non-conductive material, such as plastics. Where the housing, closure member and cavity are formed from plastics material, the interior surfaces of the cavity are provided with a conductive coating. 
     The closure member may also include a frequency tuning arrangement, the tuning arrangement including at least one adjustable disk and at least one tuning rod. Suitably the adjustable disk is formed from a suitable metal such as aluminium and the tuning rod is a conductive threaded rod such as an M4 type screw. 
     Where the filter construction includes multiple cavities a coupling tuning arrangement may also be provided, the coupling arrangement including a floating disk and adjustment rod. Suitably the floating disk is formed from metal such as aluminium and the adjustment rod is a non-conductive threaded rod, resin screw, for example, an Ultem® resin screw. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings, which illustrate preferred embodiments of the invention, and in which like features are designated by the same reference numbers, and wherein: 
         FIG. 1  is a block diagram of the layout of a typical masthead amplifier (MHA); 
         FIG. 2  is a cross sectional view of the filter layout according to one embodiment of the present invention; 
         FIG. 3  is a top view of the filter of  FIG. 2  with the closure member removed; 
         FIG. 4  is a plot of the frequency response of the filter layout of  FIGS. 2 and 3 ; 
         FIG. 5  is a cross sectional view of a dual cavity filter of another embodiment of the present invention; 
         FIG. 6   a  is a top view of a four section filter of a further embodiment of the present invention with the closure member removed; 
         FIG. 6   b  is a top view of the filter of the further embodiment with the closure member fitted; 
         FIGS. 7   a ,  7   b  and  7   c  are plots of the frequency response of the filter of the further embodiment for the TM, TE and spurious modes, respectively; 
         FIG. 8   a  is a top view of still further embodiment of the present invention with the closure member removed; 
         FIG. 8   b  is a top view of the filter of  FIG. 8   a  with the closure member fitted; 
         FIGS. 9   a  and  9   b  are plots of the frequency response of the filter of the still further embodiment for the TM and TE modes respectively; and 
         FIGS. 10   a ,  10   b ,  10   c ,  10   d  and  10   e  are diagrammatic representations of the resonant structures for further embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1  there is illustrated the typical configuration for dual band masthead amplifier (MHA)  10  utilized in communications applications such as mobile telephony. The amplifier includes as antenna port (ANT Port)  15  and base station transceiver port (BTS port)  16 . The receiving arm of the MHA is composed of a set of dual band filter banks  11 ,  12 . The two banks are coupled together via a broadband low noise amplifier (LNA)  13 . The amplifiers transmitting arm includes a dual band filter  14 . A Bias-T  17  is coupled between the BTS port and the junction of the transmitting and receiving arms. The Bias-T may also be coupled via line  18  to the LNA  13 . The Bias-T extracts incoming DC from the BTS transmission line and inserts the signals from the alarm and monitor circuits. Where the Bias-T is coupled to the LNA the extracted DC is used to provide the reference voltage V cc  for the LNA. As previously mentioned, the size of such a MHA is very obtrusive and occupies a great deal of tower space which in turn adds to the cost of tower installation. The MHA is merely included by way of one example application of the filters of the present invention and other examples will be readily apparent to the skilled addressee. 
       FIG. 2  illustrates a cross sectional view of a multiband filter  20  according to one embodiment of the present invention. The multiband filter of  FIG. 2  is based on the concept multimode resonators. The design illustrated in  FIG. 2  is a comb-line TE filter layout. A cavity  22  is provided in housing  21 , the cavity includes a resonant structure composed of a conductive post  24  and resonator  23 . Post  24  extends upwardly from the cavity floor and terminates and terminates level with the cavity&#39;s upper rim. Post  24  may further include a bore  26  for receipt of a tuning screw  31  as discussed below. Resonator  23  is positioned within cavity  22  about the post  24  such that the resonator  23  and post  24  are substantially coaxial. In this particular example the resonator  23  is raised above the cavity floor via alumina support  19 . To complete the filter construction closure member in this instance lid  25  is then positioned on the housing  21  to capping cavity  22 . The lid  25  is secured in position on the housing by a series of screws. Lid  25  also provides a suitable mounting for the filters frequency tuning arrangement  30 . The arrangement includes adjustable metal disc  31  and tuning screw  32 . 
     A top view of the filter  20  of  FIG. 2  without lid  25  and tuning arrangement  30  attached is shown in  FIG. 3 . Resonator  23  is disposed with in cavity  22  about post  24  such that the resonator  23  is substantially coaxial with post  24 . Also shown in  FIG. 3  are the input port  27  and output port  28  for coupling the filter to the respective signal source and load. 
     In this particular example the resonator is a standard TE01δ puck. Positioning the puck within the cavity  22  substantially coaxial with the conductive post  24  lowers the comb-line mode below the TE01δ. As shown in  FIG. 2 , tuning arrangement  30  provides a further mechanism for adjusting the comb-line and TE filter modes in order to tune the filter to the desired frequencies. Lowering the metal disc  31  into the cavity tunes down the frequency of the comb-line mode and simultaneously tunes up the frequency of the TE01δ mode. While lowering the tuning screw  32  into the bore  26  tunes only the frequency of the comb-line mode and has no effect on the TE01δ mode. 
     In this instance the filter has been tuned as a dual band GSM900/GSM1800filter. The cavity is 40 mm deep and has a 38 mm diameter; sizing the cavity in this way produces a GSM900 filter with a bandwidth of 25 MHz filter and a GSM1800 filter with a bandwidth of 75 MHz. 
     The GSM900 band filter utilises a comb-line resonance mode, this mode offers the most compact construction for 900 MHz filter and a high spurious response. 
     For the GSM 1800 band filter the TE01δ mode is utilized. As the comb-line fields of the GSM900 filter are similar to the TM01δ mode accordingly the fields of the GSM900 filter are orthogonal to the TE01δ mode. Employing the TE01δ mode for the GSM 1800 filter gives the largest mode separation in frequency between the two filters and good spurious response. 
     The above discussed filter construction results in a 900 MHz filter with an estimated Q of 2800 and 1750 MHz filter with an estimated Q of 6000. The spurious modes only begin to appear at 2.05 GHz as shown in  FIG. 4 , which is a plot of the frequency response of the GSM900/GSM1800 filter. 
       FIG. 5  illustrates a cross sectional view of a dual cavity filter having an input port  27  and an output port  28  according to another embodiment of the present invention. Cavities  22 . 1  and  22 . 2  are disposed within housing  21 . Each cavity includes a resonant structure, the combination of conductive posts  24 . 1  and  24 . 2  and resonators  23 . 1  and  23 . 2 , the resonators being aligned substantially coaxial with the respective conductive post. Each of the posts may also include a bore (not shown) for receiving a tuning screw as discussed below. To complete the filter construction closure member  25  is positioned on the housing  21  thereby capping cavities  22 . 1  and  22 . 2 . The filter in this instance is capable of implementing TM01δ and TE01δ modes respectively. While it would seem that the modes implemented by this arrangement of the GSM 900/1800 filtering apparatus discussed above it would be appreciated by a person skilled in the art that the combined mode within the GSM 900/1800 filter may be a TM01δ mode. Accordingly in each instance the filtering apparatus of the present invention suitably employs orthogonal modes. 
     Frequency tuning arrangements  30 . 1  and  30 . 2  are also provided for the respective cavities  22 . 1  and  22 . 2 . Each tuning arrangement includes an adjustable disk  31 . 1  and  31 . 2  and tuning screws  32 . 1  and  32 . 2 . Varying the depth of metal disks  31 . 1  and  31 . 2  tunes the frequency of the TE01δ and TE01δ modes within their respective cavities  22 . 1  and  22 . 2  without affecting the modes of the neighboring cavity. While varying the depth of tuning screws  32 . 1  and  32 . 2  within post bores  26 . 1  and  26 . 2  tunes only the TE01δ mode of the respective cavities coupling between each cavity. 
     In order to control the mode coupling between each cavity of the filter a floating disk  33  is provided. The position of the floating disk within the filter is controlled via tuning rod  34 . Varying the depth of the floating disk  33  within the filter between the cavities varies the amount of TE01δ coupling between the respective cavities. The level of TM01δ coupling between the respective cavities is controlled via a further adjustable rod  34  varying the depth of the rod  34  varies the amount of TM01δ coupling between the respective cavities without effecting the TE01δ coupling. The advantage of this structure is that the frequency tuning and coupling tuning remain quasi independent. 
     With reference to  FIG. 6   a , there is shown a four section filter  50  having an input port  27  and an output port  28  according to yet another embodiment of the present invention. The filter construction in this case includes multiple cavities  22 . 1 ,  22 . 2 ,  22 . 3  and  22 . 4  provided within housing  21 . A common signal input  27  and output  28 ; thus the filter is a dual diplexed device. 
     Each of the four cavities includes a centrally disposed conductive post  24 . 1 ,  24 . 2 ,  24 . 3  and  24 . 4  and a resonator  23 . 1 ,  23 . 2 ,  23 . 3  and  23 . 4 , respectively. Each of the resonators  23 . 1 ,  23 . 2 ,  23 . 3  and  23 . 4  is positioned within its respective cavity  22 . 1 ,  22 . 2 ,  22 . 3  and  22 . 4  and aligned substantially coaxial with the corresponding post  24 . 1 ,  24 . 2 ,  24 . 3  and  24 . 4 . 
     To complete the filter construction closure member  25  is positioned on housing  21  thereby capping cavities  22 . 1 ,  22 . 2 ,  22 . 3  and  22 . 4  (not shown). Shown in  FIG. 6   b  are frequency tuning arrangements  30 . 1 ,  30 . 2 ,  30 . 3 , and  30 . 4  for the respective cavities  22 . 1 ,  22 . 2 ,  22 . 3  and  22 . 4  and output port  28 . The construction of the frequency tuning arrangements are the same as those discussed above, namely each includes an adjustable metal disk and tuning screw. Varying the depth of metal disk and screws within the respective cavities tunes the filter to the desired frequency ranges. 
     Coupling between each cavity of the filter is also implemented in a similar manner to that discussed above. Floating disks  33 . 1  to  33 . 3  (not shown) are provided between neighboring cavities. Varying the depth at which the floating disk is positioned within the filter  50  varies the level of TE01δ coupling between the respective cavities. While varying the depth of rods  35 . 1 ,  35 . 2  and  35 . 3  within the filter  50  varies the level of TM01δ coupling between the respective cavities. Adjustment of the floating disk is provided via rods  34 . 1 ,  34 . 2  and  34 . 3  as can be seen from  FIG. 6   b . The varying heights of the tuning rods  34 . 1 ,  34 . 2  and  34 . 3  indicate that the floating disks have been adjusted to various depths along the length of the filter to provide the desired level of TE01δ coupling. Similarly rods  35 . 1 ,  35 . 2  and  35 . 3  have been adjusted to various depths along the length of the filter to provide the desired level of TM01δ coupling. 
     In this particular example the TE01δ filter was tuned at 2190 MHz with a bandwidth of 15 MHz, and the TM01δ filter was tuned to a frequency 1845 MHz with a bandwidth of 20 MHz bandwidth, as shown in frequency response diagrams of  FIGS. 7   a  and  7   b , respectively. The filters spurious response is shown in  FIG. 7   c , with the spurious modes beginning to appear at 2.5 GHz. 
     With the four section filter of  FIGS. 6   a  and  6   b  it proved difficult to achieve a high input coupling bandwidth within the same cavity.  FIGS. 8   a  and  8   b  show one possible construction of a filter  60  employed to increase the input coupling bandwidth. Filter  60  is provided with two sets of cavities for the transmission of the TE01δ and TM01δ modes. Thus unlike the previous embodiments the filter is not diplexed. The diplexing function in this example is dealt with via the transmission lines. 
     As shown in  FIG. 8   a , the TE filter is a 3 section filter while the TM filter is a 4 section filter. The TE filter is of a similar construction to the  4  section filter discussed above. The TE filtering is provided through a first set of resonant structures the combination of resonator  23 . 1 ,  23 . 2  and  23 . 3  and conductive posts  24 . 1 ,  24 . 2  and  24 . 3 . Each resonator is positioned within a respective cavity from a set of cavities  23 . 1 ,  23 . 2  and  23 . 3  such that said resonator is substantially co-axial with the corresponding conductive post  24 . 1 ,  24 . 2  and  24 . 3 . 
     The TM coupling at input port  27 . 1  and output port  28 . 1  is provided via tapped resonators  61 . 1  and  61 . 2  centrally disposed within the second set of cavities  29 . 1  and  29 . 2 . The TE coupling is provided through horizontal posts  62 . 1  and  62 . 2  at input port  27 . 2  and output port  28 . 2 . 
     The structure of the present TM filter differs slightly from the examples discussed above. In this example the TM filter employs a second set of resonant structures in this case two standard comb-line resonators  61 . 1  and  61 . 2  centrally disposed with the respective cavities  29 . 1  and  29 . 2  of the second set of cavities. Resonators  61 . 1  and  61 . 2  are couple to input and output ports  27 . 1  and  28 . 1  via a direct tapping. 
     The TM filtering is then provided through the input resonator  61 . 1  through two sections of the TE filter resonator and post combinations  23 . 1 ,  24 . 1  and  23 . 2 ,  24 . 2  to output resonator  61 . 2 . 
       FIG. 8   b  shows the filter  60  with closure member  25  fitted to housing  21  for capping the first and second set of cavities. As with the above embodiments, both frequency tuning and coupling tuning arrangements are also provided for the respective cavities. The frequency tuning arrangements  30 . 1 ,  30 . 2 ,  30 . 3 ,  30 . 4  and  30 . 5  of similar construction to that discussed above. Each arrangement includes an adjustable tuning disk and tuning screw. Similarly the coupling tuning arrangement employed is the same as that discussed above. With floating disks provided between neighboring cavities the position of each disk within the filter being varied via the respective tuning rods  34 . 1 ,  34 . 2 ,  34 . 3 ,  34 . 4  and  34 . 5 . 
     In this instance the depths of the various elements of the frequency and coupling arrangements have been adjusted to provide a TM filter tuned to a frequency of 1750 MHz and having a bandwidth of 75 MHz, and TE filter tuned to a frequency of 2140 MHz with a bandwidth of 60 MHz. A plot each filter&#39;s frequency response is shown in  FIGS. 9   a  and  9   b  respectively. 
       FIG. 10   a  represents one embodiment of the resonant structure  70  for the present invention. In this particular example the body of the ceramic element  71  is of cruciform configuration with both the top surface  72  and bottom surface  73  of the each arm member being beveled. The body also includes a central void  74  with one or more curved surfaces  75 . Preferably the internal surfaces of the central void  74  are composed of two intersecting cylindrical bores. In this instance the resonant structure also includes a conductive post  76  positioned adjacent the ceramic element  71 . 
     A further embodiment of the resonant structure  80  for the present invention is depicted in  FIG. 10   b . As with the embodiment of  FIG. 10   a  the body ceramic element  81  is of cruciform configuration. The top surfaces  82  of the arm members are again beveled, however in this example the bottom surfaces  83  of the arm members are planar. The body also includes a central void  84  with one or more curved surfaces  85 . Preferably the central void includes hemispherical internal surfaces. 
     Yet another embodiment of the resonant structure  90  for the present invention is illustrated in  FIG. 10   c . The resonant structure  90  in this example includes pair of ceramic elements  91  and  92  and conductive post  93 . The body of each ceramic element in this instance is of annular configuration. All three elements of the resonant structure  90  are arranged concentrically, with the second ceramic element  92  being disposed within the central bore  94  of the first ceramic element  91  and post  93  being disposed within the central bore of the second ceramic element  92 . 
       FIG. 10   d  illustrates yet another possible embodiment of the resonant structure  100  for the present invention. In this instance the resonant structure  100  includes a single ceramic element  101 . The body of the ceramic element  101  is of cruciform configuration with a cubic central portion  102 . The upstanding edges of the cubic central portion are aligned with the axes of the arm members  103  of the cruciform. 
     A still further embodiment of the resonant structure  200  for the present invention is shown in  FIG. 10   e . In this example the resonant structure includes a ceramic element  201  and a post  202 . The body of the ceramic element  201  is of cylindrical configuration having first planar surface  203  and second planar surface  204  axially opposite to said first surface. A central bore  205  is also provide and extends from the first surface through the body of the ceramic element  201  to the second surface  203 . The ceramic also includes a series of recesses  206  disposed on the first surface about the central bore  205 . Post  202  is positioned within central bore  205  and extends outwardly from said second surface  203 . Unlike the above embodiments the post in this case is constructed from a non-conductive material. 
     In addition to the above filter types, the applicant has realized that there is a need more complicated filters employing the present invention to be produced and this is presently the focus of their ongoing research. At present an 8 section TM filter, and a 5 section TE filter with two TM low side poles and one TM high side pole is being investigated. 
     It is anticipated that the size reduction of a full masthead amplifier (MHA) employing the present invention, such as the single 1900 MHz and dual 1800/1900 MHz type MHAs, could be in the order 10% and 15% respectively. 
     The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia or any other country. 
     It is to be understood that the above embodiments have been provided only by way of exemplification of this invention, and that further modifications and improvements thereto, as would be apparent to persons skilled in the relevant art, are deemed to fall within the broad scope and ambit of the present invention described herein and defined in the following claims.