Patent Publication Number: US-11646477-B2

Title: Waveguide cross-coupling filter with multiple parallel cavities

Description:
BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate a number of exemplary embodiments and are a part of the specification. Together with the following description, these drawings demonstrate and explain various principles of the present disclosure. 
       FIG.  1    is a block diagram of an exemplary remote radio unit (RRU) in which embodiments of an exemplary waveguide filter discussed below may be employed. 
       FIG.  2   ,  FIG.  3   , and  FIG.  4    are a perspective view, a side view, and an end view, respectively, of an exemplary waveguide filter configuration for operation as a bandpass filter. 
       FIG.  5    is a graph depicting a frequency response of a simulation of a downlink version of the exemplary waveguide filter configuration of  FIGS.  2 - 4   . 
       FIG.  6    is a graph depicting a frequency response of a simulation of an uplink version of the exemplary waveguide filter configuration of  FIGS.  2 - 4   . 
       FIG.  7    is a perspective cross-section of an exemplary waveguide filter created from a monolithic metallic structure defining a plurality of air cavities. 
       FIG.  8    is a side cross-section of another exemplary waveguide filter created from an assembly of metallic plates. 
       FIG.  9    is an exploded perspective view of the waveguide filter of  FIG.  8   . 
       FIGS.  10  and  11    are a side view and an end view, respectively, of an exemplary duplexer that is manufactured from a dielectric material and employs a plurality of waveguide filters. 
       FIG.  12    is a side view of an exemplary waveguide filter that is manufactured from a plurality of modular components of a dielectric material. 
    
    
     Throughout the drawings, identical reference characters and descriptions indicate similar, but not necessarily identical, elements. While the exemplary embodiments described herein are susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, the exemplary embodiments described herein are not intended to be limited to the particular forms disclosed. Rather, the present disclosure covers all modifications, equivalents, and alternatives falling within the scope of the appended claims. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Many remote radio units (RRUs), such as those employed as macrocell or microcell base stations for cellular communications (e.g., 4G and/or 5G LTE (Long-Term Evolution) communications), include one or more radio frequency (RF) bandpass filters (BPFs) that pass signals of a particular wavelength band for transmission from the RRU (e.g., via a downlink channel) or for reception by the RRU (e.g., via an uplink channel). In some circumstances, such a filter may be configured to provide low in-band insertion loss, supply significant out-of-band rejection, and support a significantly high transmission power. Due to these characteristics, these RF bandpass filters are typically bulky and heavy (e.g., to dissipate heat and to provide the desired signal transfer characteristics). 
     In some implementations, the RF bandpass filter may be implemented by a plurality of cross-coupled cylindrical resonance cavities to generate a number of filter “poles” to create a high level of out-of-band rejection. This particular type of bandpass filter often requires a significant amount of time to manufacture (e.g., due to assembly and soldering of components). As this manufacturing process normally introduces a significant level of variation in the size and/or shape of the resonance cavities that may adversely affect the transfer characteristics of the filter, the bypass filter often includes a number of tuning screws that facilitate adjustment of those characteristics as desired. This tuning process often consumes a significant amount of time (e.g., a half-hour or more) of a highly trained field technician for each filter employed in the RRU. 
     The present disclosure is generally directed to an RF waveguide-based bandpass filter that defines a series of cross-coupled cavities that are stacked in parallel, side-by-side. As will be explained in greater detail below, such a filter may provide excellent out-of-band rejection and low in-band insertion and return losses without the use of screws or other tuning mechanisms, thus enhancing the manufacturability of the filter while reducing the deployment time typically associated with an RF bandpass filter. 
     Features from any of the embodiments described herein may be used in combination with one another in accordance with the general principles described herein. These and other embodiments, features, and advantages will be more fully understood upon reading the following detailed description in conjunction with the accompanying drawings and claims. 
     The following will provide, with reference to  FIGS.  1 - 12   , detailed descriptions of exemplary waveguide filter configurations, associated waveguide bandpass filters, and related methods of manufacturing such filters. An example remote radio unit in which embodiments of an exemplary waveguide filter, as disclosed herein, may be employed is discussed in reference to  FIG.  1   . An exemplary waveguide filter configuration for use as a bandpass filter is described in connection with the various views of  FIGS.  2 - 4   , and an expected frequency response for separate uplink and downlink versions of the configuration are discussed in conjunction with  FIGS.  5  and  6   . With reference to  FIG.  7   , an exemplary waveguide filter created from a monolithic metallic structure is described, and another exemplary waveguide filter created from an assembly of metallic plates is explored in connection to  FIGS.  8  and  9   . An exemplary duplexer that employs a plurality of waveguide filters and that is manufactured from a dielectric material is described in conjunction with  FIGS.  10  and  11   . In associated with  FIG.  12   , an exemplary waveguide filter that is manufactured from a plurality of modular components of a dielectric material is described. 
       FIG.  1    is a block diagram of an exemplary RRU  100  in which embodiments of an exemplary RF bandpass filter, as described in greater detail below, may be implemented. As shown, RRU  100  may include an antenna  102 , a duplexer  104 , an RF amplifier module  106 , an RF modulator/demodulator (modem) module  108 , and a digital module  110 . In some examples, RRU  100  may exchange uplink data  120  (e.g., data received wireless by RRU  100  from a mobile device, such as a smartphone) and/or downlink data  122  (e.g., data to be transmitted wirelessly from RRU  100  to a mobile device) via digital module  110  with a baseband unit (BBU) that may be communicatively coupled to a backhaul network coupled to other BBUs and/or other communication systems. More specifically, in some embodiments, for downlink data  122 , digital module  110  may employ a digital encoder, a data serializer, and/or other circuitry to convert downlink data  122  into a usable form for RF modem module  108 . Further, digital module  110  may include a digital decoder, a data deserializer, and/or other circuitry to convert digital data received from RF modem module  108  to produce uplink data  120  that may be received and processed further at the BBU. 
     In some embodiments, RF modem module  108  may include a digital-to-analog converter (DAC) that converts digital data from digital module  110  derived from downlink data  122  to an analog signal that may then be modulated according to a wireless transmission protocol to produce an RF signal carrying downlink data  122  (e.g., an RF signal in a 4G or 5G LTE DCS (Digital Communication System) “B3” downlink wavelength band). The modulated analog RF signal may then be provided to RF amplifier module  106  that may amplify the RF signal for downlink data  122  prior to forwarding that signal to duplexer  104 . Further, RF amplifier module  106  may amplify an RF signal carrying uplink data  120  (e.g., an RF signal in a 4G or 5G LTE DCS “B3” uplink wavelength band) that is received from duplexer  104  and may forward that amplified signal to RF modem module  108 . Additionally, RF modem module  108  may include a demodulator that demodulates the RF signal, and then converts the resulting analog signal (e.g., using an analog-to-digital converter (ADC)) to produce corresponding digital data representing uplink data  120  to digital module  110 . 
     Duplexer  104 , in some embodiments, may include an RF bandpass filter  112  for uplink data  120  and a separate RF bandpass filter  114  for downlink data  122 . For example, RF bandpass filter  112  may filter RF signals received via antenna  102  outside of an uplink wavelength band (e.g., the LTE DCS “B3” uplink wavelength band), while RF bandpass filter  114  may filter RF signals received from RF amplifier module  106  outside of a downlink wavelength band (e.g., the LTE DCS “B3” downlink wavelength band). Further, duplexer  104  may operate as a three-port device that receives the RF signal carrying downlink data  122  via a first port and forwards a filtered version of that RF signal by way of a second port to antenna  102  while simultaneously receiving an RF signal carrying uplink data from antenna  102  at the second port and filtering that RF signal at RF bandpass filter  112  for output to RF amplifier module  106  via a third port. Consequently, duplexer  104  may allow the use of a single antenna  102  for full duplex communication by facilitating RF signal transmission and reception over separate, but associated, wavelength bands. 
     While transmission and reception bands for a single full duplex communication channel are discussed above in conjunction with RRU  100 , other embodiments of RRU  100  may service multiple such channels. Consequently, in some examples, RRU  100  may include multiple antennas  102 , duplexers  104 , and other modules described above to provide multichannel communication ability. 
       FIGS.  2 - 4    provide various views of an exemplary waveguide filter configuration (specifically a bandpass filter (BPF) configuration  200 ) that may result in a more easily manufactured and deployed RF signal filter relative to more conventional filters, such as those typically employed as BPFs  112  and  114  in RRU  100 . More specifically,  FIG.  2    is a perspective view,  FIG.  3    is a side view, and  FIG.  4    is an end view of BPF configuration  200 . As shown, BPF configuration  200  may include a series of RF “cavities” within which an RF signal propagates as the signal is filtered. In some examples, as described below, these cavities may be air-filled voids defined within one or more metallic structures. In yet other embodiments, the cavities may be a dielectric material (e.g., a dielectric material having a dielectric constant greater than the dielectric constant of air of approximately one) that may or may not be encased in, or otherwise supported by, a surrounding structure. 
     As illustrated in  FIGS.  2 - 4   , BPF configuration  200  may include a plurality of RF cavities  206  that are aligned in a series in parallel along a major axis (e.g., a y-axis of  FIG.  2   ), where each adjacent pair of cavities  206  are coupled by a corresponding inter-cavity slot  210  by which an RF signal may pass from one cavity  206  to another. Further, each RF cavity  206  may generally include a plurality of planar surfaces that define a first dimension aligned with the major axis (e.g., the y-axis of  FIG.  2   ), as well as a second dimension (e.g., aligned along an x-axis of  FIG.  2   ) and a third dimension (e.g., aligned along a z-axis of  FIG.  2   ) such that the second and third dimensions are aligned perpendicular to the major axis and each other. Further, in some embodiments, such as that depicted in  FIGS.  2 - 4   , the first dimension of each cavity  206  is shorter that the second dimension and the third dimension. Additionally, in some embodiments, as illustrated in  FIGS.  3  and  4   , the first dimension of each cavity  206  may be approximately one-twelfth of the wavelength λ (e.g., λ/12) of a wavelength of the RF signal to be passed by BPF configuration  200 , and the second and third dimensions may be approximately equal to wavelength A. Consequently, each cavity  206  may be characterized as approximating a narrow rectangular cuboid. Additionally, each two or more cavities  206  may possess slightly different first, second, and third dimensions based on different values of wavelength λ associated with the bandwidth to be passed by BPF configuration  200  (e.g., a resonance frequency associated with cavity  206 ). As shown in  FIG.  3   , for example, cavities  206  at opposing ends of BPF configuration  200  may be slightly larger along the second and third dimensions than cavities  206  positioned therebetween. In the example of  FIGS.  2 - 4   , BPF configuration  200  may include four cavities  206 , resulting in a four-pole filter structure. However, other numbers of cavities  206  (e.g., eight cavities  206 ,  16  cavities  206 , and the like) may be used in other examples. 
     Each cavity  206 , in some embodiments, may include at least one tuning “notch”  208  that essentially occupies, fills, or walls off a corner of cavity  206 . In the example of  FIG.  2   , each cavity  206  may include two tuning notches  208  representing cuboids located at diagonally opposing corner regions of cavity  206 . In some examples, in a plane defined by the second and third dimensions of cavity  206  (e.g., in the x-z plane of  FIG.  2   ), each tuning notch  208  may generally describe a square. Further, in some embodiments, each successive cavity  206  along the major axis may include tuning notches  208  at alternating opposing corners of each cavity  206 . For example, a first cavity  206  may include a tuning notch  208  at each of a first corner region and an opposing second corner region, while another cavity  206  adjacent first cavity  206  may include a tuning notch  208  at each of a third corner region and an opposing fourth corner region that do not align along the major axis with the first and second corner regions of first cavity  206 . In some examples, the corner locations of tuning notches  208  of each cavity  206  alternate in such a fashion along BFP configuration  200 . In some embodiments, tuning notches  208  may be sized along the second and third dimensions of corresponding cavity  206  to adjust an RF signal bandwidth associated with cavity  206 . 
     Inter-cavity slots  210  positioned between adjacent cavities  206 , as shown most prominently in  FIG.  4   , may be sized, shaped, and positioned relative to each other to create a zero transition between each pair of adjacent cavities  206 . As discussed in greater detail below, each zero transition may be associated with a particular frequency that defines the overall bandwidth of the signals to be passed by BPF configuration  200 . In some embodiments, as shown in  FIG.  4   , each inter-cavity slot  210 , as viewed along the major axis, may be shaped as a rectangle. Further, in some examples, each inter-cavity slot  210  may possess a length of a third of a wavelength (e.g., λ/3) and a width of a tenth of a wavelength (e.g., λ/10) to be passed by BFP configuration  200 . Additionally, when proceeding from one end of BPF configuration  200  to the other, each inter-cavity slot  210  encountered between consecutive cavities  206  may be oriented 90 degrees relative to the immediately preceding and/or subsequent inter-cavity slot  210 . 
     Further, in some embodiments, as viewed along the major axis, as depicted in  FIG.  4   , each inter-cavity slot  210  may overlap a portion of the immediately preceding and/or subsequent inter-cavity slot  210 , with each overlap creating an associated zero transition. In the particular example of  FIGS.  2 - 4   , three inter-cavity slots  210  are defined, where opposing ends of a second inter-cavity slot  210  positioned between a first inter-cavity slot  210  and a third inter-cavity slot  210  overlap a portion of the first and third inter-cavity slots  210  (e.g., extending halfway into the width of both the first and third inter-cavity slots  210 ). However, other overlap configurations of consecutive inter-cavity slots  210  (e.g., overlapping corners of consecutive inter-cavity slots  210 ) may be used in other embodiments. Each such overlap may be configured, in some examples, to tune a resonance frequency associated with a zero-transition corresponding with that overlap. 
     To direct an RF signal into one end of BPF configuration  200  and produce a resulting filtered RF signal from BPF configuration  200 , an RF inlet  202  may be provided to direct the incoming RF signal to a first cavity  206  by way of an inlet slot  212 . Further, the filtered RF signal may be directed from a last cavity  206  by way of an outlet slot  214  to an RF outlet  204 . In the particular example of  FIGS.  2 - 4   , inlet slot  212  and/or outlet slot  214 , as viewed along the major axis, may be rectangular in nature, with dimensions of one-half of a wavelength (e.g., λ/2) by one-twentieth of a wavelength (e.g., λ/20) associated with the bandwidth of the RF signal to be passed by BPF configuration  200 . Further, inlet slot  212  and/or outlet slot  214  may be oriented orthogonal to the nearest inter-cavity slot  210  of BPF configuration  200 . Additionally, as indicated in  FIG.  4   , inlet slot  212  and/or outlet slot  214  may be centrally located along one side of corresponding RF inlet  202  and/or RF outlet  204 . In some embodiments, RF inlet  202  and/or RF outlet  204  may be shaped as a rectangular cuboid, and/or may be configured to facilitate coupling with another waveguide component (e.g., an RF connector, such as an SMA (Sub-Miniature version A) connector, an SMP (Sub-Miniature Push-on) connector, an N-type connector, a DIN connector, and so on) for receiving and providing an RF signal. 
     In operation, BPFs employing BPF configuration  200  may receive an RF signal to be filters via RF inlet  202  and inlet slot  212 , through which the RF signal propagates into a first RF cavity  206  adjacent RF inlet  202 . In at least some examples, due to the size and orientation of cavity  206 , the RF signal may propagate within cavity  206  as a transverse electromagnetic mode (TEM) signal. As the RF signal passes through each cavity  206  by way of inlet slot  212 , inter-cavity slots  210  (e.g., numbering three in BPF configuration  200 ), and outlet slot  214 , with each slot oriented perpendicularly to an immediately preceding and subsequent slot, the zero transitions of BPF configuration  200  relating to the slots may impose the desired high out-of-band rejection on the RF signal. 
       FIG.  5    and  FIG.  6    are graphs depicting frequency responses of a simulation of two separate BPFs for two different frequency bands dimensioned and arranged according to BPF configuration  200 . More specifically,  FIG.  5    is a graph of the frequency response for a downlink BPF, such as downlink BPF  114  for the LTE B3 downlink wavelength band of 1805-1880 megahertz (MHz), and  FIG.  6    is a graph of the frequency response of a simulation of an uplink BPF (e.g., uplink BPF  112  for the LTE B3 uplink wavelength band of 1710-1785 MHz). As illustrated in  FIGS.  5  and  6   , the associated BPF patterned after BPF configuration  200  may provide S-parameter gain from RF inlet  202  to RF outlet  204  (e.g., denoted in  FIGS.  5  and  6    as S21, representing an insertion loss for BPFs  112  and  114 ) of only slightly greater than 0.2 decibels (dB), thus passing substantially all RF energy within the desired passband, while providing strong rejection outside the desired passband. In the case of  FIG.  5   , the zero transitions provided by inter-cavity slots  210 , as described above, may result in the low S-parameter gain “valleys” (e.g., as low as approximately −100 dB) at 1720 MHz, 1780 MHz, 1896 MHz, and 1926 MHz, resulting in a steep falloff in gain outside the desired passband (e.g., approximately 70 dB rejection in the corresponding uplink band). Similarly, in  FIG.  6   , low S-parameter gain levels are indicated at 1630 MHz, 1690 MHz, 1788 MHz, 1836 MHz, and 1910 MHz (e.g., resulting in approximately 40 dB rejection in the associated downlink band). While such performance is attainable using a four-pole filter design, as depicted in  FIGS.  2 - 4   , steeper out-of-band rejection may be attained in some embodiments by increasing the number of zero transitions and associated cavities  206 , such as by way of coupling two BPFs arranged according to BPF configuration  200  end-to-end, resulting in two four-pole filters cascaded. In yet other embodiments, additional poles may be generated by directly adding four RF cavities  206  and associated inter-cavity slots  210  to BPF configuration  200  to create a single eight-pole filter. 
     While particular reference is made herein to embodiments of BPF configuration  200  directed to LTE B3 uplink and downlink applications, BPF configuration  200  may be applied to other frequencies and frequency bands. In some examples, BPF configuration  200  may be configured to pass any frequency below 8 GHz and may provide a passband having a bandwidth of less than 30% of the frequency to be passed. 
     As discussed above, BPF configuration  200  may be implemented in various ways.  FIG.  7    is a perspective cross-section of an exemplary waveguide BPF  700  created from a monolithic metallic structure defining a plurality of air cavities. More specifically, a monolithic aluminum housing  702  (e.g., a 6061-type precipitation-hardened aluminum alloy) may be processed (e.g., machined, cast, or the like) to form RF cavities  206 , inter-cavity slots  210 , RF inlet  202  with inlet slot  212 , and RF outlet  204  with outlet slot  214 , as described above, to produce BPF  700 . Further, an exterior of aluminum housing  702  may be coated with a silver coating  704  (e.g., to provide solderability to the external surface of BPF  700  for shielding purposes, to reduce insertion loss of BPF  700 , and/or the like). In an example in which BPF  700  is configured as an LTE B3 uplink BPF  112  or downlink BPF  114 , BPF  700  may be approximately 203-by-204-by-130 millimeters (mm) in size. While silver is explicitly indicated in BPF  700 , other types of conductor coatings, such as palladium, copper, and so on, may be employed in other examples. 
       FIGS.  8  and  9    depict a BPF  800  employing a 16-pole design, in which four BPF configurations  200  may be employed end-to-end, with intermediate RF inlet  202  and RF outlet  204  omitted. More specifically,  FIG.  8    is a side cross-section of BPF  800 , and  FIG.  9    is an exploded perspective view of BPF  800 . Instead of employing a monolithic metallic structure, as discussed above in connection with  FIG.  7   , BPF  800 , as shown in  FIGS.  8  and  9   , may be created from an assembly of individual metallic plates coupled (e.g., bolted) side-by-side. Each plate may be machined, cast, and/or the like. In some embodiments, BPF  800  may include four substantially identical filter modules  801 , with each filter module  801  including a first cavity plate  810  defining a first RF cavity  206  and associated inter-cavity slot  210 , a second cavity plate  812  defining a second RF cavity  206  and associated inter-cavity slot  210 , a third cavity plate  814  defining a third RF cavity  206  and associated inter-cavity slot  210 , and a fourth cavity plate  816  defining a fourth RF cavity  206  and an outlet slot  214 , where each filter module  801  may be configured as an instance of BPF configuration  200 . Moreover, attached to a first of filter modules  801  may be an inlet plate  802  defining an RF inlet  202  and corresponding inlet slot  212 , and attached to a last of filter modules  801  may be an outlet plate  804  defining an RF outlet  204 . Such a design may facilitate a simple, cost-effective, and repeatable manufacturing and assembly process for BPF  800 . Also, in some examples, use of BPF  800  for one of the LTE B3 band filters (e.g., uplink BPF  112  or downlink BPF  114 ) may result in overall dimensions for BPF  800  of 203-by-204-by-330 mm. 
     As mentioned above, other waveguide media aside from air may be employed as RF inlet  202 , inlet slot  212 , cavities  206 , inter-cavity slots  210 , outlet slot  214 , and RF outlet  204  of BPF configuration  200 . For example, while air possesses a dielectric constant (or relative permittivity ε r ) of approximately one, use of another material (e.g., a ceramic) having a dielectric constant significantly greater than one results in a reduction in the physical wavelength of the RF signal having the same frequency (e.g., by the reciprocal of the square root of the dielectric constant), which may result in a corresponding reduction in the size of the resulting BPF incorporating that material in all three dimensions. Such reduction may not only be advantageous for installation as separate uplink BPF  112  and downlink BPF  114  in a communication system but may also facilitate a compact duplexer that combines uplink BPF  112  and downlink BPF  114 . 
       FIGS.  10  and  11    are a side view and an end view, respectively, of an exemplary duplexer  1000  that may be manufactured from a dielectric material (e.g., a ceramic) and may employ a plurality of waveguide filters. As shown, duplexer  1000  may include an uplink BPF  1012  and a downlink BPF  1014 , both of which may be coupled by way of a waveguide  1002  to an antenna (not shown in  FIGS.  10  and  11   ). In operation, RF downlink signals (e.g., from an RF amplifier module  106 ) may be provided (e.g., via a waveguide, cable, or other RF signal transmission medium) to downlink BPF  1014  for filtering prior to providing the RF signal via waveguide  1002  to the antenna for transmission. Simultaneously, the antenna may receive an RF uplink signal and direct that signal via waveguide  1002  to uplink BPF  1012  for filtering prior to amplification (e.g., via RF amplifier module  106 ). 
     Further, to impose a high level of out-of-band rejection in both uplink BPF  1012  and downlink BPF  1014 , each BPF may employ dual (and possibly identical) filter modules, each of which may be configured to its particular passband according to BPF configuration  200 : two filter modules  1022  for uplink BPF  1012  and two filter modules  1024  for downlink BPF  1014 . Consequently, presuming duplexer  1000  is to be deployed for the LTE B3 uplink and downlink bands, use of air-filled cavities for all four filter modules  1022  and  1024  and waveguide  1002  may result in a significantly large duplexer  1000  (e.g., several times larger than BPF  700  of  FIG.  7   ). However, in one example, by employing a ceramic for the various cavities with a dielectric constant of approximately 34 to construct duplexer  1000 , the overall size of duplexer  1000  may be limited to approximately 76-by-90-by-38 mm. 
     In some embodiments, the ceramic material constituting the cavities of duplexer  1000 , as shown in  FIGS.  10  and  11   , may be subsequently coated with silver (e.g., as mentioned above with respect to BPF  700  of  FIG.  7   ) or another metal to provide an RF boundary for the ceramic material, as well as to provide an environmental barrier and/or a solderable surface. Moreover, in some examples, portions of duplexer  1000  may incorporate one or more additional mechanical features (e.g., flanges, holes, etc.) for manufacturing and assembly of duplexer  1000 . 
     While in some embodiments duplexer  1000  can be machined from a single monolithic ceramic structure, duplexer  1000  may include a plurality of ceramic portions that are coupled together to form a BPF according to BPF configuration  200 .  FIG.  12   , for example, is a side view of an exemplary BPF  1200  manufactured from a plurality of modular components or portions of a dielectric material (e.g., a ceramic). In some examples, BPF  1200  may include four different shapes or portions of ceramic material: a first ceramic filter portion  1202 , a second ceramic filter portion  1204 , a third ceramic filter portion  1206 , and a fourth ceramic filter portion  1208 . 
     As organized in the embodiment of  FIG.  12   , as indicated by dashed lines therein, first ceramic filter portion  1202  may include an inlet/outlet and associated slot (e.g., a horizontal inlet/outlet slot), which may serve as RF inlet  202  in combination with inlet slot  212 , or RF outlet  204  in combination of outlet slot  214 ). Second ceramic filter portion  1204  may be shaped as a first RF cavity  206  in combination with an associated inter-cavity slot  210  (e.g., a vertical inter-cavity slot  210 ). Third ceramic filter portion  1206  may include a second RF cavity  206  (e.g., an RF cavity  206  that may be coupled to a previous cavity  206  by a vertical inter-cavity slot  210 ). Fourth ceramic filter portion  1208  may be another inter-cavity slot  210  (e.g., a horizontal inter-cavity slot  210 ). In some embodiments, one or more portions may be created using metallic discs (e.g., discs of copper, aluminum, or the like) filled with ceramic material to create the slots. 
     As depicted in  FIG.  12   , BPF  1200  is a four-pole filter, as provided in BPF configuration  200 , that includes two first ceramic filter portions  1202 , two second ceramic filter portions  1204 , two third ceramic filter portions  1206 , and a single fourth ceramic filter portion  1208 . Moreover, a midpoint of fourth ceramic filter portion  1208  may be aligned with a mirroring plane  1210  of BPF  1200 , and one of each of the two first ceramic filter portions  1202 , the two second ceramic filter portions  1204 , and the two third ceramic filter portions  1206  may be aligned on either side of mirroring plane  1210 . Moreover, in at least some examples, second ceramic filter portions  1204  may be rotated 180 degrees about a major axis of BPF  1200  relative to each other, as may be third ceramic filter portions  1206 . While BPF  1200  represents a four-pole filter, other BPFs may provide greater numbers of poles by employing different numbers of the same components or portions. 
     In some embodiments, each of first ceramic filter portions  1202 , second ceramic filter portions  1204 , third ceramic filter portions  1206 , and fourth ceramic filter portion  1208  may be bonded together (e.g., using an adhesive, such as epoxy, that may permit an RF wave to propagate therethrough with minimal signal loss). Further, in some examples, a conductive coating (e.g., a silver coating) may be applied to any or all exterior surfaces of BPF  1200  (e.g., after bonding the various components together). In some embodiments, a housing (not shown in  FIG.  12   ) may retain most or all of the components of BPF  1200  in a desired physical relationship to each other during the bonding process, and in some cases, that housing, or another housing, may be used during installation and operation of BPF  1200  (e.g., to provide structural integrity to BPF  1200 ). 
     As explained above in conjunction with  FIGS.  1 - 12   , the exemplary BPF configurations described herein may result in the production of smaller, lighter, more reliable, and better performing BPFs that may be more easily and quickly deployed in the field. Additionally, the associated methods of manufacture for such BPFs may facilitate a less expensive and more repeatable manufacturing process. Moreover, such benefits may greatly affect, in a positive manner, the cost, performance, and maintainability of associated duplexers and wireless communication systems (e.g., 4G and 5G wireless cellular communication systems) in which such BPFs are incorporated. 
     EXAMPLE EMBODIMENTS 
     Example 1: A radio frequency (RF) bandpass filter may include an RF transmission medium that defines (1) a plurality of cavities aligned parallel to each other along a major axis, where (a) each of the cavities includes a plurality of planar surfaces that define (i) a first dimension aligned with the major axis and (ii) a second dimension and a third dimension that are aligned perpendicular to the major axis and each other, where the first dimension is shorter than the second dimension and the third dimension and (b) each adjacent pair of cavities is coupled by an inter-cavity slot, (2) an RF inlet that couples an RF signal received at the RF bandpass filter to a first cavity of the plurality of cavities at a first end of the plurality of cavities, and (3) an RF outlet that couples a filtered RF signal from a second cavity of the plurality of cavities at a second end of the plurality of cavities opposite the first end externally to the RF bandpass filter. 
     Example 2: The RF bandpass filter of Example 1, where (1) the RF bandpass filter may further include a conductive housing and (2) the RF transmission medium may include air. 
     Example 3: The RF bandpass filter of Example 2, where the conductive housing may include aluminum. 
     Example 4: The RF bandpass filter of Example 2, where the filter may further include a conductive coating covering at least some portions of the conductive housing. 
     Example 5: The RF bandpass filter of Example 1, where the RF transmission medium may include a material having a dielectric constant greater than one. 
     Example 6: The RF bandpass filter of Example 5, where the material may include a ceramic. 
     Example 7: The RF bandpass filter of Example 5, where the filter may further include a conductive coating covering at least some portions of the RF transmission medium. 
     Example 8: The RF bandpass filter of any one of Examples 1-7, where the plurality of cavities may include the first cavity, the second cavity, a third cavity adjacent the first cavity, and a fourth cavity adjacent the third cavity. 
     Example 9: The RF bandpass filter of Example 8, where (1) each inter-cavity slot may include a rectangular cross-section when viewed along the major axis, (2) the rectangular cross-section of each inter-cavity slot may define a major dimension and a minor dimension less than the major dimension, (3) the major dimension of the rectangular cross-section of a first inter-cavity slot coupling the first cavity to the third cavity may be aligned with the second dimension, (4) the major dimension of the rectangular cross-section of a second inter-cavity slot coupling the third cavity to the fourth cavity may be aligned with the third dimension, and (5) the major dimension of the rectangular cross-section of a third inter-cavity slot coupling the fourth cavity to the second cavity may be aligned with the second dimension. 
     Example 10: The RF bandpass filter of Example 9, where, when viewed along the major axis, (1) a portion of the rectangular cross-section of the first inter-cavity slot may overlap a first end of the rectangular cross-section of the second inter-cavity slot and (2) a second end of the rectangular cross-section of the second inter-cavity slot may overlap a portion of the rectangular cross-section of the third inter-cavity slot. 
     Example 11: The RF bandpass filter of Example 8, where the plurality of cavities may further include a fifth cavity adjacent the third cavity, a sixth cavity adjacent the fourth cavity, a seventh cavity adjacent the fifth cavity, and an eighth cavity adjacent the sixth cavity. 
     Example 12: The RF bandpass filter of any one of Examples 1-7, where each cavity of the plurality of cavities may approximate a rectangular cuboid. 
     Example 13: The RF bandpass filter of Example 12, where the first cavity may further define (1) a first notch occupying a first corner region of the rectangular cuboid, (2) a second notch occupying a second corner region of the rectangular cuboid diagonally opposite the rectangular cuboid from the first corner region, (3) a third corner region between the first corner region and the second corner region, and (4) a fourth corner region diagonally opposite the rectangular cuboid from the third corner region. 
     Example 14: The RF bandpass filter of Example 13, where a subsequent cavity adjacent the first cavity may further define (1) a first corner region, a second corner region, a third corner region, and a fourth corner region aligning along the major axis with the first corner region, the second corner region, the third corner region, and the fourth corner region, respectively, of the first cavity, (2) a first notch occupying the third corner region of the subsequent cavity, and (3) a second notch occupying the fourth corner region of the subsequent cavity. 
     Example 15: The RF bandpass filter of any one of Examples 1-7 where at least one of the RF inlet and the RF outlet may be configured to be coupled with a waveguide. 
     Example 16: An RF duplexer may include (1) an antenna port, (2) a transmission port, (3) a reception port, (4) a first bandpass filter that couples the transmission port to the antenna port, and (5) a second bandpass filter that couples the reception port to the antenna port, (6) where each of the first bandpass filter and the second bandpass filter includes an RF transmission medium that defines a plurality of cavities aligned parallel to each other along a major axis, where (a) each of the cavities includes a plurality of planar surfaces that define (i) a first dimension aligned with the major axis and (ii) a second dimension and a third dimension that are aligned perpendicular to the major axis and each other, where the first dimension is shorter than the second dimension and the third dimension, and (b) each adjacent pair of cavities is coupled by an inter-cavity slot. 
     Example 17: A method of manufacturing a radio frequency (RF) bandpass filter may include (1) creating a set of conductive plates and (2) assembling the set of conductive plates side-by-side along a major axis to form the RF bandpass filter, where the RF bandpass filter includes an RF transmission medium that defines (1) a plurality of cavities aligned parallel to each other along the major axis, where (a) each of the cavities includes a plurality of planar surfaces that define (i) a first dimension aligned with the major axis and (ii) a second dimension and a third dimension that are aligned perpendicular to the major axis and each other, where the first dimension is shorter than the second dimension and the third dimension and (b) each adjacent pair of cavities is coupled by an inter-cavity slot. 
     Example 18: The method of Example 17, where the RF transmission medium may further include (1) an RF inlet that couples an RF signal received at the RF bandpass filter to a first cavity at a first end of the plurality of cavities and (2) an RF outlet that couples a filtered RF signal from a second cavity at a second end of the plurality of cavities opposite the first end externally to the RF bandpass filter. 
     Example 19: The method of either Example 17 or Example 18, where the set of conductive plates may include aluminum. 
     Example 20: The method of either Example 17 or Example 18, where the method may further include coating at least a portion of the set of conductive plates with a conductive layer. 
     The process parameters and sequence of the steps described and/or illustrated herein are given by way of example only and can be varied as desired. For example, while the steps illustrated and/or described herein may be shown or discussed in a particular order, these steps do not necessarily need to be performed in the order illustrated or discussed. The various exemplary methods described and/or illustrated herein may also omit one or more of the steps described or illustrated herein or include additional steps in addition to those disclosed. 
     The preceding description has been provided to enable others skilled in the art to best utilize various aspects of the exemplary embodiments disclosed herein. This exemplary description is not intended to be exhaustive or to be limited to any precise form disclosed. Many modifications and variations are possible without departing from the spirit and scope of the present disclosure. The embodiments disclosed herein should be considered in all respects illustrative and not restrictive. Reference should be made to the appended claims and their equivalents in determining the scope of the present disclosure. 
     Unless otherwise noted, the terms “connected to” and “coupled to” (and their derivatives), as used in the specification and claims, are to be construed as permitting both direct and indirect (i.e., via other elements or components) connection. In addition, the terms “a” or “an,” as used in the specification and claims, are to be construed as meaning “at least one of.” Finally, for ease of use, the terms “including” and “having” (and their derivatives), as used in the specification and claims, are interchangeable with and have the same meaning as the word “comprising.”