Patent Publication Number: US-9899716-B1

Title: Waveguide E-plane filter

Description:
TECHNICAL FIELD 
     The present disclosure relates to a waveguide E-plane band-pass filter and to a transceiver comprising such a filter. The present disclosure also relates to a method of filtering a signal using a waveguide E-plane band-pass filter. 
     BACKGROUND 
     Abase station for a mobile communication system and microwave radio links used for data transport typically comprise one or more transceiver units connected to an antenna for transmitting and receiving microwave signals. These transceivers in turn comprise a diplexer/duplexer consisting of at least two band-pass filters. The filters of the diplexer may have different passbands so as to, e.g., prevent intermodulation between a transmission signal and a received signal. Herein, when referring to a passband of a filter, it is appreciated that a passband is defined by a center frequency and a bandwidth, the bandwidth being measured, e.g., when the return loss is lower than a certain level, such as −20 dB. 
     Microwave filters can be of the transmission line type, such as a microstrip arranged on a dielectric carrier. However, hollow metal waveguides are more often used as filters due to lower losses and a higher power capability compared to microstrip filters, even though a hollow waveguide filter will have a larger size than a microstrip filter. 
     The dimensions of a hollow waveguide filter are dependent on the frequency of the signal to be filtered, the selected filtering properties such as a certain passband, and on the type of filter used. Since the size of the waveguide must be on the same order as the wavelength of the frequency of the signal that is to be filtered, hollow waveguides are typically used for frequencies in the GHz range which have wavelengths in the mm range. 
     In some applications, such as in outdoor microwave radio or radio base station units, there are strict size limitations which must be adhered to. Thereby, the available space also dictates which type of filter can be used. It is therefore often desirable to reduce the size of a filter without degradation of the frequency properties of the filter. As an example, waveguide H-plane type filters are known to have advantageous frequency properties and they also can be made smaller than other comparable types of filters such as E-plane filters. However, H-plane filters require a large number of tuning positions making it costly and complicated to tune the filters. 
     A known alternative to H-plane filters are waveguide E-plane filters which do not need to be tuned. In an E-plane filter, a conductive foil or insert is arranged in the waveguide filter at or close to the location where the strength of the E-field (V/m) is the highest. The foil or insert comprises openings which act as resonators, thereby determining the poles of the filter, and consequently also contribute to determining the passband of the filter. However, an E-plane filter can not be made as small as an H-plane filter with the same filtering properties. 
     Accordingly, it is desirable to provide an improved waveguide filter which is both comparatively small in order to be used within a restricted space and also uncomplicated to manufacture, without degradation of the filtering properties. 
     SUMMARY 
     In view of above-mentioned and other desired properties of a microwave filter, it is an object of the present technique to provide an improved waveguide E-plane band-pass filter having a reduced size compared to prior art E-plane filters. 
     According to a first aspect, it is provided a waveguide E-plane band-pass filter comprising a tubular, electrically conductive waveguide body. An electrically conductive foil is arranged in the waveguide body and extending along a longitudinal direction of the waveguide body, the foil comprising a plurality of resonator openings. Furthermore, the waveguide body comprises at least one ridge protruding from an inner wall of the waveguide body and extending longitudinally along the longitudinal direction of the waveguide body. The foil is in mechanical contact with said at least one ridge and arranged to divide an inner volume of the waveguide body into two portions. 
     The technique disclosed herein is based on a realization that a waveguide E-plane band-pass filter can be provided which is reduced in size in comparison with known E-plane band-pass filters while maintaining, or in some cases even improving, the filter properties, by arranging at least one ridge within the waveguide body, and by arranging the electrically conductive foil in mechanical contact with the ridge. 
     According to some aspects, the foil is arranged to divide the inner volume of the waveguide body into two portions of equal dimension. 
     According to some further aspects, a cross section of a ridge has the same shape along the full length of the ridge. As an example, the ridge can have a rectangular cross section. 
     According to some aspects, the ridge comprises a plurality of protruding elements, where a distance between adjacent protruding elements does not exceed a quarter of a wavelength of a center frequency of the filter. 
     According to some aspects, the foil is in mechanical contact with a central portion of the ridge along a longitudinal length of the ridge. 
     According to some aspects, the size and shape of the ridge is selected such that a first harmonic frequency, and also higher mode frequencies, of the filter are higher than 1.5 times a center frequency of said filter. 
     According to some aspects, the foil is arranged along a symmetry line of the filter running along a longitudinal direction of the filter dividing the waveguide body into two symmetrical parts. 
     According to some aspects, the waveguide body comprises two body elements, where each body element comprises one half of a ridge and the foil being arranged at an interface between the two body elements. 
     According to some aspects, the waveguide body comprises at least two body elements, where one of the body elements comprises a ridge. 
     According to further aspects, the waveguide body has a rectangular cross section. 
     According to some aspects, the filter comprises two ridges protruding from opposing walls of the waveguide body. In a filter comprising two ridges, the foil is arranged extending between the two ridges according to some aspects. 
     According to some aspects, in a filter comprising two ridges, a cross section of the two ridges have the same shape along the longitudinal length of the two ridges. In some aspects, the two ridges are arranged opposing each other. 
     The object stated above is also obtained by a diplexer unit comprising a first filter according to any one of the above discussed filters. The filter is configured to be operatively connected to a radio transmitter and having a first passband and a second filter according to any one of the above discussed filters, the filter being configured to be operatively connected to a receiver and having a second passband. 
     The object stated above is further obtained by a radio transceiver comprising a radio transmitter, a radio receiver, a diplexer unit as discussed above. The diplexer is operatively connected to the radio transmitter and to the radio receiver and to an antenna. 
     The object stated above is also obtained by a method for filtering a microwave signal in a waveguide E-plane band-pass filter. The method comprises providing a microwave signal to the filter, band-pass filtering the signal using the waveguide E-plane band-pass filter forming a filtered signal. The waveguide E-plane band-pass filter comprises at least one internal ridge protruding from an inner wall of the waveguide body and extending longitudinally along the longitudinal direction of the waveguide body. 
     The object stated above is also obtained by a method for filtering a microwave signal in a radio transceiver comprising a waveguide E-plane band-pass filter. The method comprises acquiring a signal from an antenna, band-pass filtering the signal using the waveguide E-plane band-pass filter forming a filtered signal and providing the filtered signal to a receiver module of the radio transceiver. The waveguide E-plane band-pass filter comprises at least one internal ridge protruding from an inner wall of the waveguide body and extending longitudinally along the longitudinal direction of the waveguide body. 
     The object stated above is also obtained by a method for filtering a microwave signal in a radio transceiver comprising a waveguide E-plane band-pass filter. The method comprises generating a signal by a radio transmitter module of said transceiver, band-pass filtering the signal using the waveguide E-plane band-pass filter forming a filtered signal and providing the filtered signal to an antenna. The waveguide E-plane band-pass filter comprises at least one internal ridge protruding from an inner wall of the waveguide body and extending longitudinally along the longitudinal direction of the waveguide body 
     According to some aspects, in the methods discussed above, a cross-section of the at least one ridge of the waveguide E-plane band-pass filter has the same shape along the full length of the at least one ridge. 
     According to some aspects, in the methods discussed above, a waveguide E-plane band-pass filter comprises two ridges protruding from opposing inner walls of the waveguide. 
     The object stated above is also obtained by a radio transceiver module for filtering a microwave signal. The transceiver comprises an antenna module for transmitting and receiving a microwave signal a first waveguide E-plane band-pass filter module for band-pass filtering a transmission signal to form a filtered transmission signal. The filter module comprises at least one internal ridge protruding from an inner wall of a waveguide body and extending longitudinally along the longitudinal direction of the waveguide body. The transceiver further comprises a second waveguide E-plane band-pass filter module for band-pass filtering an acquired signal to form a filtered acquired signal. The second filter module comprises at least one internal ridge protruding from an inner wall of a waveguide body and extending longitudinally along the longitudinal direction of said waveguide body. The transceiver further comprises a radio transmitter module for providing the filtered transmission signal to an antenna, and a receiver module for receiving the filtered acquired signal from said filter. 
     Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present technique will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present technique may be combined to create embodiments other than those described in the following, without departing from the scope of the present technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present technique is now described, by way of example, with reference to the accompanying drawings, in which: 
         FIGS. 1A-B  are schematic illustrations of a prior art filter; 
         FIGS. 2A-B  are schematic illustrations of a filter according to an embodiment of the present technique; 
         FIGS. 3A-B  are schematic illustrations of a filter according to an embodiment of the present technique; 
         FIG. 4  is a schematic illustration of a filter according to an embodiment of the present technique; 
         FIG. 5A  is a diagram illustrating properties of a prior art filter; 
         FIG. 5B  is a diagram illustrating properties of a filter according to an embodiment of the present technique; 
         FIG. 6A  is a diagram illustrating properties of a prior art filter; 
         FIG. 6B  is a diagram illustrating properties of a filter according to an embodiment of the present technique; 
         FIG. 7  is a schematic illustration of a transceiver according to an embodiment of the present technique; and 
         FIGS. 8A-C  are flow charts outlining general method steps of methods according to embodiments of the present technique. 
     
    
    
     DETAILED DESCRIPTION 
     The present technique will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the present technique are shown. The present technique may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the technique to those skilled in the art. Like numbers refer to like elements throughout the description. 
     In the following detailed description, various embodiments of the waveguide E-plane filter according to the present technique are mainly described with reference to a filter having a rectangular cross section and to a ridge having a rectangular cross-section. 
       FIG. 1  schematically illustrates a prior art waveguide E-plane band-pass filter  100 . The filter  100  of  FIG. 1  is used as a comparative example and to outline the general properties of a waveguide E-plane filter  100 . The filter  100  comprises a hollow waveguide body  102  and an electrically conductive foil  104 . The inner dimensions of the waveguide body  102 , i.e. the width  108  and height  110 , generally determine the cutoff frequency of a waveguide. In an E-plane filter, an electrically conductive foil  104  is arranged within the waveguide body  102 , typically at or close to the center of the waveguide body  102  where the E-field has its maximum value. The foil  104  may also be referred to as a conductive insert or a filter insert. The foil  104  comprises one or more resonator openings  106  which determine the passband of the filter, where each opening corresponds to a pole of the filter. Therefore, the foil  104  is sometimes also referred to as a frequency determining foil  104 . The passband is defined as the band around a center frequency where the return loss is lower than a certain level, such as −20 dB. However, the passband may also be defined at other levels of return loss, such as −16 dB, depending on the requirements of the particular application in which the filter is to be used. 
     As an illustrating example, filter dimensions  108 ,  110  are given for a filter  100  having a passband with a center frequency at 8 GHz and a bandwidth of approximately 200 MHz. Such a filter  100  has a width  108  of 12.6 mm and a height of 28.5 mm. 
       FIGS. 2A-B  schematically illustrate a filter  200  according to an example embodiment of the present technique. The filter  200  comprises a tubular, electrically conductive, waveguide body  202  having a rectangular cross-section. A tubular waveguide body  202  should herein be understood as a waveguide body being hollow and elongated. The waveguide body  202  is here being illustrated as an open-ended waveguide. However, the present technique is equally applicable for a closed waveguide. The filter  200  further comprises an electrically conductive foil  204  arranged in the waveguide body  202  and extending along a longitudinal direction of the waveguide body  202 . The electrically conductive foil  204  can for example be made from a metallic material such as copper. As an alternative to copper, other materials having equivalent electrical properties can also be used. The foil comprises a plurality of resonator openings  206 , where each resonator opening  206  correspond to a pole of the filter. Thus, the filter  200  illustrated in  FIGS. 2A-B  is a five-pole filter as a result of the five resonator openings  206 . However, the present technique is equally applicable to E-plane filters having any practical number of poles, where the number and dimension of the resonator openings is selected based on the requirements of the particular application for which the filter is to be used. 
     The filter of  FIGS. 2A-B  further comprises a ridge  208  protruding from an inner wall of the waveguide body  202  and extending longitudinally along the longitudinal direction of the waveguide body  202 . The foil  204  is arranged in mechanical contact with the ridge  208 , at the center of the ridge  208  and along the longitudinal length of the ridge  208 , and arranged extending in a substantially perpendicular direction from the ridge  208  reaching an opposing wall of the waveguide body  202  to divide an inner volume of the waveguide body  202  into two portions  222   a - b . Since the foil  204 , the ridge  208  and the waveguide body  208  are electrically conductive, the foil  204  is in electrical contact with the waveguide body  202 . Even though the foil is illustrated as dividing the inner volume of the waveguide body  202  into two substantially equal portions  222   a - b , the foil  104  may also be arranged at a position offset from the center of the ridge  207  while still being in mechanical and electrical contact with the ridge  208 , the filter still maintaining its filtering properties. Furthermore, the ridge  208  has a rectangular cross-section which has the same shape along the length of the ridge  208 , and the ridge extends along the full length of the waveguide body  202 . Even though the ridge  208  herein is illustrated as having a rectangular cross-section, the ridge can in principle have an arbitrarily shaped cross-section, such as a triangular cross section or a free form cross-section. Since it is the surface area of the ridge which determines the influence of the ridge on the filter properties, the cross-section shape of the ridge can be selected based on the desired mechanical configuration of the filter and based on manufacturing considerations. In practice, a rectangular cross-section can for example be selected due to the ease of manufacturing. Furthermore, it is not strictly required that a ridge extends along the full length of the waveguide body. However, it should be noted that other configurations where the ridge is shorter than the waveguide body may lead to specific matching requirements for connecting to the filter. 
     The waveguide body  202  of  FIGS. 2A-B  is also illustrated as being divided into two substantially similar body elements  218 ,  220  of equal dimension along an imaginary symmetry line of the waveguide body  202 . The foil  204  is arranged between the two body elements  218 ,  220 . However, this is merely one of many different possible configurations of the waveguide body  202  and hence of the filter  200 . The waveguide body  202  may for example comprise three or more separate body elements being assembled to form a waveguide body and a ridge. In practice, the specific configuration of waveguide body elements and ridges may be determined based on manufacturing considerations. 
     Through the use of a ridge  208  in a waveguide E-plane band-pass filter, the dimensions of the filter can be significantly reduced while maintaining similar frequency filtering properties. Taking an 8 Ghz five-pole filter as an illustrative example, as outlined in relation to  FIG. 1 , a filter configured according to  FIGS. 2A-B , having the same passband as the prior art filter of  FIG. 1 , would have a width  210  of 9.5 mm and a height  212  of 19 mm. Thus, the filter  202  comprising a ridge  208  has a height which is reduced by more than 30% and a width which is reduced by about 25%, giving an overall reduction in cross section area of approximately 50%. Moreover, the length of the conventional filter  100  is about 155 mm, whereas the length of the filter  200  comprising a ridge is about 125 mm, a reduction of 8%. Taken together, this leads to a volume reduction of about 60% which provides a significant advantage for applications where the filter is to be used where the volume is restricted. Moreover, a filter  200  having a reduced size also leads to a reduction in the amount of material needed to manufacture the filter, and thereby to an overall reduction in manufacturing cost. 
     In  FIG. 2A , the ridge  208  has a height  214 , defined as the perpendicular protrusion from the inner wall of the waveguide body  202 , of 5.8 mm and a width  216  of 4.0 mm. The length of the ridge  208  is the same as the length of the waveguide body  202 . As a general principle, the size of the ridge is proportional to the size reduction of filter. However, the size reduction of the filter is in practice limited by the required size of the resonator openings in the foil. It is also possible to manipulate first harmonic and higher order mode suppression of the filter by tuning the geometry of the ridge, and in particular by tuning the surface area of the ridge. Accordingly, the precise dimensions of the ridge are based on design considerations with respect to particular filter requirements. Moreover, as discussed above, the cross-section shape of the ridge can in principle be arbitrarily selected, for example to suit a particular foil having specific dimensions for achieving a desired passband. 
     It should be noted that the above discussed dimensions are derived from computer simulations, and that a physical filter may have slightly different dimensions and properties, for example due to manufacturing tolerances and trade-offs between size and desired filter characteristics. As an example, manufacturing tolerances for the foil are in the range of +/−5 μm and manufacturing tolerances for the waveguide body and ridge is in the range of +/−30 μm. 
       FIGS. 3A-B  are schematic illustrations of an embodiment of a waveguide E-plane band-pass filter  300  comprising two opposing ridges  308 ,  310  extending from opposing sidewalls of the waveguide body  302 . The principles of the filter  200  discussed above in relation to  FIGS. 2A-B  applies also to the filter  300  of  FIGS. 3A-B . One consequence of using a filter  300  with two ridges  308 ,  310  instead of a single ridge  208  is that the two ridges  308 ,  310  can be made smaller than the single ridge  208 . In the present example, the ridges  308 ,  310  have a height of 3 mm and a width of 4 mm. The remaining dimensions of the waveguide body  302 , i.e. the width  312 , height  314  and length are the same as for the filter  200  of  FIGS. 2A-B . 
     Furthermore, the filter  300  comprises two waveguide body elements  320 ,  322 , where each element  320 ,  322  comprises a respective ridge  308 ,  310 . In other words, the waveguide body  302  can be said to be split along the height direction of the body. The skilled person readily realizes that the waveguide body  302  can also be divided in the same manner as the waveguide body  202  in  FIGS. 2 a - b   , and that the division shown in  FIGS. 3A-B  is equally applicable also to the filter  200  of  FIGS. 2A-B . Moreover, the two ridges  308 ,  310  are illustrated as being arranged directly opposite each other. Even though it is desirable to arrange the foil  304  in the region where the E-field is highest, the filter would still function even if one or both of the ridges and/or the foil would be somewhat offset from the center position. 
       FIG. 4  is a schematic illustration of a filter  400  according to an embodiment of the present technique where the waveguide body  402  comprises a ridge  408  made up of individual elements  410  protruding from an inner wall of the waveguide body  402 . As long as the gap between adjacent protruding elements  410  is smaller than approximately a quarter of a wavelength of a center frequency of the filter, the gaps will not interfere with the filter properties. The same requirement also applies to the distance between the outermost protruding elements and the respective edge of the waveguide body  402 . However, gaps which are larger than a quarter of a wavelength may case unwanted resonances in the filter. An advantage of using a ridge comprising individual elements is that the material consumption and thereby the weight and cost of the filter can be reduced. Assuming a center frequency of 8 GHz, the wavelength would be 44 mm, and a quarter wavelength would thus be approximately 11 mm. 
     The cross-section of the filter  400  in  FIG. 4  will be the same as the cross-section of the filter  200  in  FIG. 2 a    and both of the ridges  208 ,  408  will have the same cross-section shape and size. 
     In the same manner as discussed above in relation to the filter  200  of  FIGS. 2A-B , the filter  400  comprises a foil  404  having resonator openings  406 . Furthermore, the foil  404 , ridge  408  and the waveguide body  402  will have the same dimensions as the corresponding dimensions of the filter illustrated in  FIG. 2A-B  and discussed above given the example of an 8 GHz filter. 
       FIGS. 5A-B  are diagrams representing computer simulations of the performance of the prior art filter  100  and the filter  300  discussed above. In particular, curve  502  of  FIG. 5 a    illustrates the S 21  parameter and curve  504  illustrates the S 11  parameter of the filter  100 , where S 21  represents the transmitted signal and S 11  the reflected signal in a 2-port network. Likewise, in  FIG. 5B  the curves  506  and  508  illustrate the S 21  and S 11  parameters, respectively, of the filter  300  comprising two opposing ridges. 
     As can be seen when comparing  FIG. 5A  with  FIG. 5B , the passbands of the two filters  100 ,  300  are substantially the same, illustrating that the above discussed size reduction can be achieved without any noticeable change in passband properties. 
       FIGS. 6A-B  are diagrams representing computer simulations of the performance of the prior art filter  100  and the filter  300  comprising ridges as discussed above. Similarly to the curves in  FIGS. 5A-B , curves  602  and  604  represent the S 21  and S 11  parameters, respectively, of filter  100 . Curves  608  and  610  represent the S 21  and S 11  parameters, respectively, of the filter  300  comprising ridges. In  FIGS. 6A-B , resonant modes for the two filters are shown and by comparing the two diagrams it can be seen in  FIG. 6A  that the first harmonic is located at approximately 11 GHz and that a number of higher order modes  606  are visible. Comparing this to the filter  300  comprising opposing ridges, the first harmonic  612  in  FIG. 6B  is located at a higher frequency, namely at 12.5 GHz, compared to the first harmonic of the prior art filter  100 . This is an advantage since first harmonic and higher order resonant modes too close to the passband can lead to a higher noise level in the passband. Accordingly, the passband noise level is reduced through the use of a filter comprising a ridge, as a result of the first harmonic being located at a higher frequency compared to in a comparable filter without a ridge. 
     Furthermore, curve  608  of  FIG. 6B  show that higher order modes above the first harmonic  612  are suppressed by the filter  300 , meaning that the filter in practice also acts as a low-pass filter blocking frequencies above the first order resonant mode  612  This will provide a practical advantage when using the filter  300  in a system since a separate low-pass filter is often required in order to remove the higher order resonant modes  606  illustrated in  FIG. 6A . By using the filter  300  comprising a ridge, not only is the filter in itself smaller, it also reduces the overall number of components needed in a system, leading to a notable reduction in size and complexity, and thereby cost. It should also be noted that the same effects have been observed and the same reasoning applies for a filter comprising a single ridge, e.g. the filter  200  illustrated in  FIGS. 2A-B . 
       FIG. 7  is a schematic illustration of a radio transceiver  700  comprising a radio transmitter  702 , a radio receiver  704 , a diplexer unit  706  operatively connected to the radio transmitter  702  and to the radio receiver  704 , and an antenna  708  operatively connected to the diplexer. The diplexer unit  706  comprises a first filter f 1  and a second filter f 2 , where the filters f 1  and f 2  are waveguide E-plane band-pass filters comprising a ridge as discussed above. The first filter f 1  has a first a first passband and is operatively connected to a radio transmitter  702  (T x ), and the second filter f 2  has a second passband and is operatively connected to a receiver  704  (R x ). 
     In a diplexer, the passbands of the first and second filter f 1 , f 2 , are, in FDD (Frequency Duplex Distance), different and separated form each other in order to separate two different frequency bands in a receive and transmit path and to combine them in a antenna path. This is of importance for example in telecommunication systems where different frequency bands are handled by the same transceiver. 
     The passbands of the first and second filter f 1 , f 2 , can also be the same. The same T x  and R x  frequency can for example be used in a TDD (Time Duplex Distance) or with a OMT (Orthomode Transducer) based system, or in a full duplex system where cancellation is used to remove self-interference. 
       FIGS. 8A-C  are flow charts outlining general steps of methods according to various embodiments of the present technique. 
       FIG. 8A  illustrates the steps of a method for filtering a microwave signal in a waveguide E-plane band-pass filter. The method comprise providing  802  a microwave signal to the filter and band-pass filtering  804  the signal using the waveguide E-plane band-pass filter forming a filtered signal, the waveguide E-plane band-pass filter comprising at least one internal ridge protruding from an inner wall of the waveguide and extending longitudinally along the longitudinal direction of said waveguide. 
       FIG. 8B  illustrates the steps of a method for filtering a microwave signal in a radio transceiver, the transceiver comprising a waveguide E-plane band-pass filter. The method comprises acquiring  806  a signal from an antenna band-pass filtering  808  the signal using the waveguide E-plane band-pass filter forming a filtered signal, the waveguide E-plane band-pass filter comprising at least one internal ridge protruding from an inner wall of the waveguide and extending longitudinally along the longitudinal direction of the waveguide, and providing  810  the filtered signal to a receiver module of the radio transceiver. 
       FIG. 8C  illustrates the steps of a method for filtering a microwave signal in a radio transceiver, the transceiver comprising a waveguide E-plane band-pass filter. The method comprises generating  812  a signal by a radio transmitter module of the transceiver, band-pass filtering  814  the signal using the waveguide E-plane band-pass filter forming a filtered signal, the waveguide E-plane band-pass filter comprising at least one internal ridge protruding from an inner wall of the waveguide and extending longitudinally along the longitudinal direction of the waveguide body, and providing  816  the filtered signal to an antenna. 
     Even though the present technique has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art from a study of the drawings, the disclosure, and the appended claims. 
     Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the present technique.