Microwave Multiplexer With Multimode Filter

This invention relates to multiplexers of microwave electromagnetic signals which differ in frequency and, more particularly, to a multiplexer having a plurality of channels tuned to specific frequencies, each channel including a filter for coupling both transverse-electric (TE) and transverse-magnetic (TM) waves to shape a bandpass characteristic with steeper skirts to allow for a closer spaceing of contiguous signal bands.

Microwave multiplexers are employed in a variety of communication systems ranging from radar to telemetry. For example, in the case of a satellite carrying two highly directive antennas for receiving two signals at different frequency bands, the two signals received from the respective antennas are advantageously combined via a microwave multiplexer. The multiplexer outputs the two signals in a common channel of broader bandwidth. Thereby, a single microwave channel receives both of the signals. Such a multiplexer may be reciprocal in its operation such that a plural-band signal traversing the multiplexer in the reverse direction can be split into two separate signals each having its own spectral transmission band. If desired, such multiplexers may be constructed to accommodate more than two spectral bands. It is advantageous if the various bands can be placed together as closely as possible so as to reduce the required bandwidth of the common output channel of the multiplexer.

A problem arises in that, in the past, the bandpass characteristic of the resonant structure in each of the channels of the multiplexer has had wider skirts than is desireable, the excess width of the skirts necessitating additional spacing between contiguous ones of the signal bands to ensure adequate channel separation. This reduces the number of separate signal channels that can be combined into a single output channel of prescribed bandwidth.

The aforementioned problem is overcome and other advantages are provided by a multiplexer having a set of individually tuned input channels, the tuning of each channel being provided by a resonant structure composed of a plurality of resonant chambers or cavities. In accordance with the invention, each of the chambers is provided with coupling structures which excite both TE and TM modes of electromagnetic wave propagation. The resultant resonant structure for each channel has a bandpass characteristic which is characterized by a reduction in the width of the skirts, that is, the skirts are steeper allowing for a closer placement of the contiguous signal channels while retaining adequate isolation between the signals of contiguous channels.

In a preferred embodiment of the invention, the launching of the TE and TM waves is accomplished by use of a 3 dB (decibels) coupler constructed with adjacent waveguides sharing a common wall, and wherein coupling probes are located in each of the waveguides. Thereby, a 90 degree phase shift is introduced between the two probes. The two probes penetrate a first chamber of the filter at an end wall thereof, there being a metallic disc located on the end wall alongside the two probes. In addition, two tuning posts are positioned on the opposite side of the disc and are arranged parallel to the two probes, the two turning posts and the two probes being uniformly positioned about the metallic disc. The probes excite TM waves in the chamber, and the disc interacts with the TM waves to excite a TE wave within the chamber.

Coupling of electromagnetic energy between successive ones of the chambers within a channel is accomplished by a composite coupling structure, a portion of which provides for the coupling of TM waves, and a portion of which provides for the coupling of TE waves. The composite coupling structure is placed in a common end wall between adjacent chambers. A set of four circular-segment slots provides for the coupling of TE waves, while a set of probes passing through the common end wall and extending into both of the chambers couples TM waves. The four probes are centered in respective ones of the four slots.

The 3 dB coupler structure is applied to the chambers at both ends of the resonant structure, one 3 dB coupler being at an input port and the other 3 dB coupler being appended to a side wall of a common output waveguide which connects the individual resonant structures of the respective channels. A feature of this structure is that a group of microwave signals of different frequencies propagating through the common output waveguide, and incident upon individual ones of the output couplers, react with the couplers in a manner dependent on the resonant frequencies of the respective channels. Signals having frequencies different from the resonant frequency of a specific channel are essentially unaffected by the presence of the channel and, accordingly, can propagate through the output waveguide without interference of the other channels. On the other hand, a microwave signal incident upon the coupler of a channel resonant at the frequency of the microwave signal is coupled into the resonant structure to propagate through that channel structure. Reciprocal propagation is attained in the multiplexer structure such that signals can propagate from input ports to a common output port for combination of a set of the signals, and can propagate from the common output port to the set of input ports for separation of the signals of a group of microwave signals.

The resonant structure in each of the channels may be regarded as a filter for passing the signal of a specific channel while rejecting signals of other channels. The individual chambers or cavities in each of the resonant structures may be regarded as filter sections, an increase in the number of filter sections providing for a sharper tuning of the passbands of the respective filters. Coefficients of coupling of microwave energy between the chambers of a resonant structure can be selected, in accordance with filter theory, to shape the bandpass characteristic. In view of the fact that the coupling structure between successive chambers is a composite structure for coupling both TE and TM waves, the slots thereof for coupling TE waves are positioned at a radial distance from the center of the common wall at which distance no transverse current from a TM wave is present. The probes located in the centers of the slots extend a sufficient distance away from the common wall so as to interact with the TM waves. Thereby, the composite coupling structure is able to process both TE and TM waves. In addition, by selecting a length to the probes and a length to the slots, coefficients of coupling are readily established for optimizing the shape of the bandpass characteristic in a signal channel. The structure of the filter of a single channel may be used for processing signals in microwave equipment other than multiplexers.

The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing wherein:

FIG. 1 is an perspective view of an embodiment of the multiplexer of the invention having two input ports and one output port;

FIG. 2 is a plan view of the multiplexer of FIG. 1, the view of FIG. 2 being partially sectioned along the line 2--2 in FIG. 1 to show the interior construction of an input waveguide assembly of a first signal channel and the interior construction of an output waveguide assembly of a second input signal channel;

FIG. 3 is an elevation view of the multiplexer of FIG. 1, the view in FIG. 3 being partially sectioned to show transverse-electric and transverse-magnetic coupling structures within a filter of a signal channel;

FIG. 4 is an isometric view, shown diagrammatically, of a filter of FIG. 3; and

FIG. 5 shows the bandpass characteristic of the filter of FIG. 4 operative with both transverse-electric and transverse-magnetic modes in accordance with the invention.

With reference to the figures, there is shown a microwave multiplexer 20 comprising a waveguide 22 having an output port 24. A plurality of input ports 26, two of which are shown in the figures, are formed within input waveguide assemblies 28 and 30 coupled via cylindrical filters 32 and 34, respectively to the waveguide 22. Input signals, in the form of electromagnetic waves, are inputted at respective ones of the input ports 26 to be combined by the multiplexer 20, whereby the sum of the input signals, (two input signals in FIG. 1) is outputted at the output port 24.

Each of the filters 32 and 34 comprises a plurality of resonating cavities or chambers 36 and 38. While only two of the chambers 36, 38 are shown in each of the filters 32 and 34, it is to be understood that three or more such resonating chambers may be employed if desired. As is well known, the resonant frequency of each of the resonating chambers 36 and 38 is dependent on the dimensions of the chambers 36 and 38. Each of the chambers 36 and 38 is formed as a right cylindrical section having a prescribed diameter and height, which diameter and height are selected to provide for a desired resonant frequency of electromagnetic waves induced in the chambers 36 and 38 in response to input signals applied to the input ports 26. Thereby, the filters 32 and 34 are tuned to their respective channel frequencies.

A useful characteristic of the filters 32 and 34 is manifested at the coupling of each of the filters 32 and 34 to the waveguide 22. A microwave signal propagating in the waveguide 22 will be coupled into a filter 32, 34 if the passband of the filter contains the frequency of the microwave signal. However, if the resonating frequency of the filter 32, 34 differs from the frequency of the microwave signal, then the microwave signal is rejected by the filter 32, 34 and continues to propagate through the waveguide 22 without significant interaction with the filter 32, 34. Similar comments apply to any other filters (not shown) which may be coupled to the waveguide 22. This characteristic is most useful in the combining of plural input signals because an input signal or a sum of input signals entered into the waveguide 22 can continue to propagate through the waveguide 22 without interference by the other filters. It is to be understood that, in the construction of the multiplexer 20, all of the filters are constructed to resonate at different frequencies, thereby to enable the multiplexing of signals of different frequencies to provide the sum signal at the output port 24.

It is also noted that the operation of the multiplexer 20 is reciprocal so that a signal comprised of the sum of a plurality of signals at different frequencies can be inputted at the output port 24 whereupon each of the microwave signals will exit respective ones of the ports 26 whereby each of the component microwave signals has been separated in accordance with the frequencies of the respective microwave signals.

Upon using the multiplexer 20 to multiplex a set of signals occupying different portions of the microwave spectrum, it is noted that a set of the input signals constitutes an input band of signals, in which each of the microwave signals occupies a portion of the band. While, ideally, each portion of the band allocated to a specific microwave signal is contiguous to the portion allocated to the next microwave signal, in practice, the band portions are separated by stop bands to allow space for the skirts of the bandpass characteristics of the respective filters as shown in FIG. 5. The amount of space designated for the skirts limits the efficiency of band utilization. Sharper skirts permit each of the useful portions of the band to be positioned more closely together so as to avoid a wasting of frequency space in the band. As is well known, the number of resonators in a chamber, and the number of chambers employed in each of the filters effects the bandpass characteristic portrayed in FIG. 5. While the skirts can be made more steep by increasing the number of chambers from the two chambers 36 and 38 in this embodiment of the invention, such additional chambers increases the complexity of the structure, and make the structure more difficult to tune than the relatively simple structure of the filters 32 and 34.

In accordance with the invention, the skirts of the bandpass characteristic of each of the filters are made more steep so as to permit a more close spacing of the adjacent signal portions of the spectrum by coupling a plurality of electromagnetic transmission modes through the filters 32 and 34. A single mode of electromagnetic wave is associated with broader skirts while the use of a coupling structure in the filters which provides for the propagation of plural modes, both transverse electric (TE) and transverse magnetic (TM), of electromagnetic waves provides the desired narrowing of the skirts of an individual filter pass band.

The invention provides for the coupling of both TE and TM within each of the filters 32 and 34. Both of these modes of waves carry power in the direction of the central axis in each of the filters 32 and 34. Since both of the filters 32 and 34 and both of the input waveguide assemblies 28 and 30 have the same form, except for their respective physical sizes which differ, only the filter 32 will be described in detail, it being understood that the same description applies to the other filter 34.

The TE and TM waves may be described in cylindrical coordinates of r (radius of a resonant chamber), .theta. (angle measured along the cylindrical surface about a central cylindrical axis), and z (the central cylindrical axis). In the foregoing cylindrical coordinates, the TE wave exists in a pair of TE.sub.112 modes, and the TM wave exists in a pair of TM.sub.110 modes. As will be understood from the ensuing description of the filters 32 and 34, there are two waveforms of the TE.sub.112 modes which are orthogonally polarized relative to each other and, also, two waveforms of the TM.sub.110 modes which are orthogonally polarized relative to each other. Resonance occurs in both waveforms of the TE and the TM modes at the same frequency because of the chamber configuration. There is no variation along the Z axis in the TM modes while, in each of the TE modes, there is one full guide wavelength of electromagnetic wave along the Z axis. The electromagnetic energy is coupled into and out of the filter 32, 34 by the TM modes, a part of the energy being converted into the TE modes within the filter 32, 34. The launching of the TM modes of electromagnetic radiation into the filters 32 and 34 from the input waveguide structures, the conversion between the TE and TM modes, the extraction of the TM modes of electromagnetic radiation from the filters 32 and 34 at the waveguide 22, and the coupling of the two modes of electromagnetic radiation between the chambers 36 and 38 of the filters 32 and 34 will now be described.

Each of the waveguide assemblies 28 and 30 has the same form of structure, the respective structures differing only with respect to the dimensions of the components thereof, which dimensions are selected in accordance with the frequency of waves to be coupled between the assemblies 28 and 30 and their respective filters 32 and 34. Accordingly, only the assembly 28 need be described in detail, the description thereof applying equally well to the assembly 30.

The waveguide assembly 28 is constructed in the form of a 3 dB (decibels) coupler 40 formed of two rectangular waveguides 42 and 44 sharing a common sidewall 46, which sidewall has an aperture 48 for coupling electromagnetic energy between the two waveguides 42 and 44. The waveguide assembly 28 has a top wall 50 and a bottom wall 52 which extend across the waveguides 42 and 44 to serve as top and bottom walls of the waveguides 42 and 44. The top wall 50 and the bottom wall 52 are joined by sidewalls 54 and 56 and the common sidewall 46 to form the structure of each of the waveguides 42 and 44. The cross section of each of the waveguides 42 and 44 has an aspect ratio of 2:1 wherein the width of the top wall of each of the waveguides 42, 44 is double the height of the sidewall 46. Also included are well-known tuning structures (not shown) located on the walls about the aperture 48. A front end of the waveguide 42 is extended to form an input port 26. The front end of the waveguide 44 is provided with a dummy load 58.

In order to excite the TM and TE modes in the filter 32, two coupling assemblies 60 and 62 are located in the common bottom wall 52 of the two waveguides 42 and 44, the coupling assembly 60 being positioned within the waveguide 42 and the coupling assembly 62 being positioned within the waveguide 44. Each of the coupling assemblies 60 and 62 is formed of a circular aperture 64 within the bottom wall 52 and a rod 66 of smaller diameter than the diameter of the aperture 64, the rod 66 being oriented perpendicularly to the bottom wall 52. The rods 66 extend from their respective waveguides 42 and 44 through the apertures 64 into the upper resonant chamber 36. Tuning posts 68 and 70 are located in the chamber 36 diametrically opposite the coupling assemblies 62 and 60, respectively, and extend in the chamber 36 from the wall 52.

Each of the coupling assemblies 60 and 62 is in the form of a coax-to-waveguide adapter or probe which may be dimensioned, in accordance with well known adapter and probe technology, to produce the desired coupling of the TM.sub.110 modes between the waveguides 42 and 44 and the upper chamber 36. The width and height of each of the tuning posts 68 and 70 is adjusted to cancel out any direct coupling of electromagnetic energy between the coupling assemblies 60 and 62.

In accordance with a feature of the invention, the coupler 40 divides the power of an input signal at an input port 26 equally between the waveguides 42 and 44. A characteristic of the coupler 40 is the fact that an electromagnetic wave coupled into the waveguide 44 experiences a phase shift of 90 degrees relative to the phase of the wave in the waveguide 42. As a result, electromagnetic waves coupled by the coupling assemblies 60 and 62 are out of a phase by 90 degrees. The two coupling assemblies 60 and 62 are spaced apart from the common sidewall 46 by approximately one-third of the width of the respective waveguides 42 and 44. The two coupling assemblies 60 and 62 excite the orthogonal TM.sub.110 modes in the chamber 36.

In accordance with the invention, an upper coupling disc 72 of a metal such as copper is placed at the top of a chamber 36 adjacent the two rods 66, the disc 72 being secured to the underside of the bottom wall 52. The disc 72 interacts with the TM.sub.110 modes to excite the TE.sub.112 modes of corresponding polarization. Thereby, both TE and TM modes are present in the chamber 36.

In the construction of the multiplexer 20, the assemblies 28 and 30, the filters 32 and 34, and the waveguide 22 are all constructed of metal, such as copper, as is common practice in the construction of waveguides and similar microwave components.

Similarly, the tuning posts 68 and 70 and the rods 66 are also constructed of a metal such as copper. In order to hold the rods 66 centered within their respective apertures 64, a plug 74 of electrically-insulating dielectric material, which may be a ceramic such as alumina, is disposed within each of the apertures 64. The plugs 74 are transparent to the electromagnetic radiation. The disc 72 may be secured by soldering to the underside of the wall 52.

The two chambers 36 and 38 are separated by a wall 76 which extends diametrically across the cylindrical space of the filter 32 bounded by an outer cylindrical wall 78. The wall 76 is supported by the cylindrical wall 78.

In accordance with a feature of the invention, four coupling assemblies 80, 82, 84, and 86 are disposed in the wall 76 and are positioned uniformly about a center of the wall 76. In the preferred embodiment of the invention, the cylinder formed by the wall 78 is a right circular cylinder, and the coupling assemblies 80, 82, 84, and 86 are positioned with ninety-degree spacing about the center of the wall 76. Each of the coupling assemblies 80-86 comprises a slot 88 having the form of a circular segment, and a rod 90 extending through the slot 88 perpendicularly to the wall 76. Each of the rods 90 is secured to the wall 76 by a bushing 92 of electrically-insulating dielectric material transparent to the electromagnetic radiation. Each of the slots 88 extends approximately 60 degrees in the circumferential direction, the exact amount being determined experimentally. The length and width of each of the slots 88, and the length of the rods 90 is adjusted to provide a desired coefficient of coupling between the corresponding modes in the chambers 36 and 38. The slots 88 are disposed on a common circle having a diameter such that, in the preferred embodiment of the invention, the four rods 90 are in alignment with respective ones of the two rods 66 and the two posts 68 and 70. The slots 88 provide for the coupling of only the TE.sub.112 modes, and the rods 90 provide for the coupling of only the TM.sub.110 modes in the chambers 36 and 38. The independence of coupling is determined by the radius of the slots 88 because there is no radial component of current in the wall 76 due to the TM.sub.110 modes at the locations of the slots 88. No axial current is present in the rods 90 due to the TE.sub.112 modes.

The waveguide 22 comprises a top wall 94 and a bottom wall 96 which are joined by sidewalls 98 and 100. As viewed in cross-section, the top and bottom walls 94 and 96 constitute broadwalls of the waveguide 22 and the sidewalls 98 and 100 constitute narrow walls of the waveguide 22.

Coupling of electromagnetic energy via the TM.sub.110 modes between the waveguide 22 and the filters 32 and 34 is accomplished by waveguide assemblies 102 and 104 extending from the sidewall 100. The two assemblies 102 and 104 connect respectively with the filters 32 and 34 for coupling electromagnetic power outputted by the filters 32 and 34 to the waveguide 22. While only two output waveguide assemblies 102 and 104 are shown in the figures, it is to be understood that additional ones of these assemblies are to be provided corresponding to the number of filters and input ports 26 employed in the construction of the multiplexer 20.

The construction of the output waveguide assemblies 102 and 104 follows that of the input waveguide assemblies 28 and 30. Each of the output waveguide assemblies 102 and 104 includes a 3 dB coupler 106 comprising two waveguides 108 and 110 of rectangular cross section, the two waveguides 108 and 110 sharing a common sidewall 112 having an aperture 114 for coupling power between the two waveguides 108 and 110. The top wall 94 and the bottom wall 96 extend over the waveguide assemblies 102 and 104 to form top and bottom walls of the waveguides 108 and 110. Sidewalls 116 and 118 and the common sidewall 112 in each of the assemblies 102 and 104 join the top and bottom walls of the assemblies 102 and 104 to form the waveguides 108 and 110. The dimensions of the aperture 114 and the inclusion of well-known tuning structures (not shown) disposed in the walls about the aperture 114 insure equal power division and a 90 degree phase shift between electromagnetic waves in the two waveguides 108 and 110. Coupling assemblies 120 and 122 are located in the top wall 94 of each of the waveguides 108 and 110 and extend through the top wall 94 for coupling electromagnetic energy between the lower chamber 38 and the waveguide 22. Each of the coupling assemblies 120 and 122 is formed of a section of coaxial transmission line having an inner conductor 124 and an outer conductor 126 which pass through the top wall 94 for coupling energy of the TM.sub.110 modes between the chamber 38 and the waveguide 22. The outer conductor 126 is formed simply of the walls of an aperture in the top wall 94. Torroidal dielectric plug 128 supports the inner conductor 124 within the outer conductor 128. Tuning posts 130 and 132 extend from the top wall 94 into the chamber 38, access to the tuning posts 130 and 132 for adjustment of their height being had via the waveguides 108 and 110, respectively. The tuning posts 130 and 132 may be formed as screws which may be advanced into the chamber 38 by rotation of the screws, thereby to tune the chamber 38 to the electromagnetic radiation. The posts 130 and 132 are positioned so as to be in alignment with the coupling assemblies 60 and 62 of an input waveguide assembly, and the coupling assemblies 120 and 122 are positioned so as to be in alignment with the tuning posts 68 and 70 of an input waveguide assembly. The multiplexer 20 is operable also upon interchanging the positions of the posts 130 and 132 with the coupling assemblies 120 and 122 because of symmetry in the generation of electromagnetic waves by the coupling assemblies 80-86 in the wall 76.

In the construction of the waveguide assemblies 102 and 104, the common wall 112 extends all the way, except for the aperture 114, from the sidewall 98 of the waveguide 22 to the opposite end of an output waveguide assembly 102, 104. It is also noted that the waveguide assemblies 102, and 104 do not contain a dummy load as do the input waveguide assemblies 28 and 30. The lack of the dummy load and the replacement thereof with a reflection end wall allows power propagating along the waveguide 22 to pass through the aperture 114 of a coupler 106 and to continue propagating along the waveguide 22 without attenuation to the output port 24.

A feature of the invention, as has been noted hereinabove, is the fact that individual ones of the filters 32 and 34 in cooperation with their respective coupling assemblies 120 and 122 provide for substantially no interaction with electromagnetic signals propagating along the waveguide 22 in frequency bands different from the passbands of the respective filters 32 and 34. Only in the case of an electromagnetic wave having the frequency to which a filter is tuned, does a filter, such as the filter 32, interact with the electromagnetic wave so as to provide for a path of propagation between the waveguide 22 and an input port 26.

To facilitate the tuning of the filters 32 and 34, the upper chamber 36 is provided with four tuning screws 134 (three of which are shown in FIG. 4) and the lower chamber 38 is provided with four tuning screws 136 (three of which are shown in FIG. 4). The tuning screws 134 and 136 are disposed in the cylindrical wall 78, and are directed inwardly along a diameter of the cylindrical wall 78. The four tuning screws 134 are positioned uniformly, 90 degrees apart, about a longitudinal cylindrical axis of the chamber 36 and, similarly, the four tuning screws 136 are positioned uniformly about a longitudinal cylindrical axis of the chamber 38. Each of the chambers 36 and 38 has an axial length of one guide wavelength of the TE.sub.112 mode along the central cylindrical axis. The four tuning screws 134 are positioned approximately one-quarter of the guide wavelength in the TE.sub.112 mode from the wall 76, and the four tuning screws 136 are positioned approximately one-quarter of the guide wavelength in the TE.sub.112 mode from the opposite side of the wall 76. Corresponding ones of the tuning screws 134 and 136 are disposed in common vertical planes containing the cylindrical axis. The tuning screws 134 and 136 are operative for tuning resonant frequencies of the TE.sub.112 waves. A turning of a screw 134, 136, adjusts the amount of penetration of the screw into the respective chambers 36 and 38 for tuning the TE mode of propagation within these chambers. It may also be desirable to provide tuning for the TM.sub.110 mode by use of insulated electrically-conductive pins (not shown) positioned inside each of the chambers 36 and 38 and oriented parallel to the cylindrical axis in each of the chambers 36 and 38. Signals inputted at the ports 26 and coupled via the filters 32 and 34 to the waveguide 22 are excited to propagate essentially in one direction, toward the output port 24, in the waveguide 22 due to the action of each output coupler 106 in summing together the waves in the waveguides 108 and 110 to form a resultant wave propagating toward the output port 24. A load 138 (FIG. 1) dissipates electromagnetic power flowing in a direction opposite the output port 24, thereby to prevent reflections of the signals from the back end of the waveguide 22. Electric field vectors for the TE.sub.112 and the TM.sub.110 modes are also shown in FIG. 4, the electric field vectors being identified by E(TE) and E(TM), respectively for the TE and TM modes.

The bottom of the chamber 38 and the top of the chamber 36 have the same configuration of microwave components to enable the conversion of a part of the electromagnetic energy between the TM and the TE modes, and the coupling of electromagnetic energy into and out of the filters 32 and 34 by the TM.sub.110 modes of electromagnetic waves. The disc 140 is placed at the bottom of the chamber 38 and secured to the top wall 94, the disc 140 having the same configuration as the disc 72 located at the top of the upper chamber 36. Both the discs 72 and 140 are centered on the cylindrical axis of the filter 32 and are centered between their respective coupling assemblies and tuning posts. Thus, the two coupling assemblies 60 and 62 and the two tuning posts 68 and 70 are positioned about the disc 72 at equal radial distances from the center of the disc 72. Similarly, the two coupling assemblies 120 and 122 and the two posts 130 and 132 are positioned at equal radial distances from the center of the disc 140.

In operation, the foregoing construction of the multiplexer 20 with the two filters 32 and 34 may be regarded as a filter with characteristics which are particularly suited for a contiguous channel microwave multiplexer. Each of the filters 32 and 34 comprises a linear set of cylindrical cavities (chambers 36 and 38) proportioned to support four modes of electromagnetic waves in each cavity, the cavities being resonated at the channel frequency. The modes include vertically polarized TM.sub.110 and TE.sub.112 which are coupled to each other, and the corresponding horizontally polarized TM and TE modes. The vertical and the horizontal polarization provide equal and independent paths through the filter (filters 32 and 34) capable of propagating a circularly polarized signal. Coupling between adjacent chambers 36 and 38 for TE.sub.112 type modes and for TM.sub.110 type modes serve as a bridge circuit for generating transmission nulls. The foregoing coupling assemblies and the coupling disc 72 and 140 introduce the characteristics of a complementary type directional bandpass filter appropriate for a contiguous channel multiplexer.

The above-described microwave construction of the multiplexer 20 provides the characteristics of a filter having two transmission poles per cavity for two polarizations, this being double the number of transmission poles obtainable heretofore. As a result, the filters 32 and 34 can be constructed with a reduced number of chambers, only the two chambers 36 and 38 being employed in the preferred embodiment, it being understood that additional chambers could be employed in other embodiments of the invention for further control of the bandpass characteristic in each of the filters. The transmission nulls can be adjusted by the bridge coupling at the coupling assemblies 80-86 in the wall 76 so as to provide for steeper skirts in the transmission characteristics portrayed in FIG. 5. The foregoing configuration provides an improved type of complementary-filter contiguous-channel multiplexer.

The reduction in size and weight is desirable for use in satellites having phased array antennas so as to obtain a more nearly optimum antenna and feed system. Details in the construction of filters and coupling devices is disclosed in the textbook "MICROWAVE IMPEDANCE MATCHING NETWORKS" by G. Mattaei, L. Young, and E. M. F. Jones, and also in the textbook "FIELDS AND WAVES IN MODERN RADIO" by S. Ramo and J. R. Whinnery. By way of example of the improvement offered by the invention, a filter disclosed in chapter 14 of Mattei et al has two polarizations with one transmission hole and no transmission nulls per cavity. The additional modes, poles, and nulls provided by the structure of the invention allows the attainment of a more useful bandpass characteristic with reduced weight and bulk of microwave components.

With respect to the operation of the multiplexer 20, in the upper chamber 36, the coupling assembly 60 and 62 in cooperation with the disc 72 and the tuning posts 68 and 70 introduce two independent TM.sub.110 modes which provide circularly polarized waves in the chamber 36. Equal reflection in the coaxial structures of the coupling assemblies 60 and 62 return power to the dummy load 58. The radii which locate the coupling assemblies 60 and 62 and the tuning posts 68 and 70 about the disc 72 are oriented 90 degrees apart from each other. The radial distance of each slot 88 is slightly less than half the radius of the chamber 36, namely, 0.480 times the chamber radius. At these points, the z component of the electric field is at a maximum and the circumferential component of the magnetic field is zero. The pair of posts 68 and 70, by virtue of their positions diametrically opposite the rods 66, balance out a direct coupling of electromagnetic energy between the coupling assemblies 60 and 62. Similar comments apply to the coupling assemblies 120 and 122 at the bottom of the lower chamber 38.

The discs 72 and 140 are relatively thin as compared to a guide wavelength, the thicknesses of the discs being less than approximately one-tenth of the guide wavelength. If desired, the disc can be replaced by a thin ring (not shown) along the outer periphery of the end wall of a chamber. Couplings of electromagnetic power are of opposite sense for the disc and the ring because the radial current in the end wall reverses at the foregoing value of radius (for location of the tuning posts 68 and 70) from the center for the TM.sub.110 mode, while there is no radial current reversal for the TE.sub.112 mode. In the event that convex or concave end walls were used in place of the disc or ring, the convex and concave walls would produce TM.sub.110 to TE.sub.112 couplings of opposite polarity, and resemble in a crude way the foregoing disc and ring.

The slots 88 permit the coupling of TE.sub.112 modes from one chamber 36 to the other chamber 38 without a coupling of TM.sub.110 modes. The rods 90 passing through the slots 88 provide for the coupling of TM.sub.110 modes between the chambers 36 and 38, such coupling of the TM.sub.110 mode being obtained independently of the coupling of TE.sub.112 modes. Probe coupling, by the rods 90, is the independent of the hole coupling, by the slots 88, in that the hole coupling applies only to TE modes while the probe coupling applies only to TM modes. The combination structure of the slots 88 and their rods 90 permit independent adjustment of the coupling coefficients of the TE and the TM modes.

Reduction of the various coupling coefficient results in a narrowed bandpass characteristic and, in addition, the time of propagation of a signal through the filter 32, 34 is increased. An enlargement of the coupling coefficient has the reverse effect. The foregoing structure is most versatile by allowing for independent control of the coupling or both TE and TM waves, both of which waves serve to carry the signal power. The result is a closer spacing of the contiguous signal spectra to allow for more signals in a given multiplexer bandwidth, while reducing the weight and bulk of the multiplexer.

It is to be understood that the above described embodiment of the invention is illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiment disclosed herein, but is to be limited only as defined by the appended claims.