Abstract:
A multiplexer/demultiplexer structure is provided which multiplexes multiple channel signals through a common tee of a tee/manifold mulitplexer arrangement. This multiplexing significantly reduces the number of tees required for a given number of multiplexed channels. Accordingly, mulitplexer/demulitplexer design time is reduced and fabricated multiplexers/demultiplexers are lighter, smaller and less expensive. The tee multiplexing is facilitated with multiple access apertures that are isolated by a septum. The septum forms reduced-height waveguides which define a path length between apertures that is sufficient to significantly reduce higher-order modes and, therefore, apeture interactions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to waveguide structures and more particularly to waveguide multiplexers/demultiplexers. 
     2. Description of the Related Art 
     Frequency-division multiplexing is the process of transmitting a plurality of input signals over a common transmission path by assigning a different frequency channel for each signal. Thus, the combined signals can subsequently be separated by filtering and by providing separate transmission paths for the filtered signals. The filtering and providing processes are those of frequency-division demultiplexing. Because demultiplexing is the inverse function of multiplexing, the following discussion is restricted, for simplicity, to multiplexing. 
     Low-loss, high-power frequency-division multiplexing in the microwave region is facilitated by the use of waveguide multiplexers which typically form a plurality of input ports for the reception of microwave input channel signals and a single output port for delivery of the multiplexed signals onto a common transmission path. Generally, this path leads to common signal-processing structures, e.g., a microwave amplifier or a radiating antenna. 
     In conventional waveguide multiplexers, a plurality of input waveguides (typically referred to as tee&#39;s) are joined to a single output waveguide (typically referred to as a manifold) in a way that enhances electromagnetic signal transfer. For example, each tee is arranged to form an E-plane junction with the manifold in one exemplary multiplexer structure and an H-plane junction in another. In most multiplexer waveguide structures, the manifold has an open-circuited end for transmission of the multiplexed input signals. Opposite the open-circuited end, the manifold has a short-circuited end and the tees are spaced by selected distances from the shorted end. Each tee also terminates in a short-circuited end and forms an aperture in this short-circuited end for signal access to that tee. A waveguide filter is coupled to the aperture so that channel filtering is associated with signal transmission through the tee. 
     In practice, a number of problems complicate multiplexer design. First, each input signal travels down its respective tee and splits into two signals which propagate in opposite directions along the manifold. One signal propagates towards the manifold&#39;s open-circuited end and the other propagates to, and is reflected from, the manifold&#39;s short-circuited end. Tee and manifold distances must therefore be carefully chosen so that each reflected signal from the manifold&#39;s short-circuited end adds to signals entering the manifold from that reflected signal&#39;s respective tee, i.e., these signals must be substantially in phase when they meet. 
     Secondly, the reflected signals from the manifold&#39;s short-circuited end again split as they successively reach each tee, with one signal portion propagating down the manifold and the other portion propagating up that tee and being reflected from that tee&#39;s short-circuited end. Tee and manifold distances must also be chosen so that signals reflected from tee shorted ends arrive in phase with signals entering the manifold from that reflected signal&#39;s respective tee. 
     Because they lie in different frequency channels, each of the input signals propagates with a different guide wavelength λg. A successful multiplexer design must therefore take the different propagation wavelengths into account and realize a dimensional layout that enhances signal additions at each tee so as to enhance the transmitted channel energy at the manifold&#39;s open-circuited end. 
     Multiplexer design is further complicated by impedance mismatches at the junctions of the tees and the manifold which generate additional signal reflections. An acceptable multiplexer design must reduce these impedance mismatches as much as possible and yet accommodate the reflected signals from the remaining mismatches. 
     Impedance mismatches can result in an apparent electrical short circuit wall at a specific frequency. A manifold resonance can be created between this apparent electrical wall and the manifold&#39;s short-circuited end or any one of the tee short-circuited ends. These types of resonances further degrade multiplexer performance. 
     In addition, multiplexer transmission-line discontinuities (e.g., tee-manifold junctions) generate higher-order electromagnetic modes. Because multiplexers are generally associated with nonlinear processes (e.g., high-power amplification), the input signals typically include frequency harmonics and this combination of discontinuities and harmonics generates higher-order harmonic modes which propagate in the multiplexer with different guide wavelengths. At other discontinuities (e.g., downstream waveguide junctions), energy is exchanged between these propagating modes. A successful multiplexer design must also control the energy exchanges of propagating higher-order modes in order to enhance the transmitted channel energy. 
     These complications of multiplexer design generally increase exponentially with each additional frequency channel that is included in the multiplexer. It has been found, for example, that although a satisfactory design can be found relatively quickly for an eight channel waveguide multiplexer, a satisfactory design for a sixteen channel multiplexer is exceedingly difficult to obtain. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to multiplexer/demultiplexer structures and methods which facilitate simpler and less expensive design solutions than are conventionally available. In particular, structures and methods of the invention reduce the number of tees that are required for a given number of multiplexer channels. Multiplexer/demultiplexer designs which conventionally would have been complex and expensive are thus transformed into simpler, lighter, smaller and less expensive designs. Because the number of junctions are reduced, multiplexer/demultiplexer structures of the invention also exhibit improved performance. 
     These goals are realized with a primary waveguide and at least one secondary waveguide which is joined to the primary waveguide and which forms at least first and second apertures for signal access to the secondary waveguide. A plurality of waveguide filters are multiplexed to each secondary waveguide by coupling each through a respective one of the apertures. Signal isolation is obtained with a septum that is positioned between each adjacent pair of apertures. 
     The septum is preferably dimensioned to create an aperture-to-aperture transmission path that is sufficiently long (e.g., greater than (¼)λ g     avg   ) to significantly reduce higher-order modes and, therefore, aperture interactions. 
     Different embodiments of the invention can be formed with various waveguide configurations (e.g., circular, rectangular or dielectric) and with different tee-manifold junctions (e.g., E-plane and H-plane junctions). 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a communication spacecraft in an orbital plane about the Earth; 
     FIG. 2 is a block diagram of a transponder in the spacecraft of FIG. 2 wherein the transponder includes a demultiplexer and a multiplexer; 
     FIG. 3 is an enlarged, perspective view of multiplexer structure of the present invention that is included within the curved line  3  of FIG. 2; 
     FIG. 4 is view similar to FIG. 3 which illustrates another multiplexer structure of the present invention; 
     FIG. 5 is a perspective view of another multiplexer structure of the present invention; and 
     FIG. 6 is a graph of measured transmission and reflection characteristics in a prototype of the multiplexer structure of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 3,  4  and  5  illustrate multiplexer/demultiplexer embodiments of the present invention and FIGS. 1 and 2 illustrate an exemplary use of the invention. For descriptive simplicity, the multiplexer/demultiplexer embodiments will be described principally from a multiplexer perspective. 
     The multiplexer structures exemplified by FIGS. 3,  4  and  5  employ signal-isolating septums which reduce signal interactions and facilitate the multiplexing of multiple input signals through a single tee. Accordingly, the number of tee&#39;s required for a given number of multiplexer channels can be significantly reduced with consequent lowering of the complexity and cost of multiplexer designs and the size, weight and cost of fabricated multiplexers. In multiplexers with a large number of frequency channels (e.g., ≧16), tee reductions of the invention may even enable the realization of an otherwise unrealizable multiplexer. To enhance its clarity, a description of multiplexer structures of FIGS. 3,  4  and  5  is preceded by the following description of FIGS. 1 and 2. 
     As shown in FIG. 1, a spacecraft communication system is carried by a spacecraft  20  (e.g., a body-stabilized or spinner spacecraft) which orbits a celestial body such as the Earth  22  in an orbital plane  23 . The spacecraft  20  includes a body  24  which carries a pair of solar wings  25  and  26  to receive solar radiation and convert it into electrical energy for operation of the spacecraft&#39;s systems. The spacecraft body  24  also carries receive and transmit antennas  28  and  29  for communication with Earth-based communication stations. Typically, the spacecraft  20  also carries systems (e.g., thrusters  30  and  31 ) for maintaining the spacecraft&#39;s assigned orbital station and for maintaining a spacecraft attitude that enhances signal exchange between the spacecraft and the communication stations. 
     As shown in FIG. 2, a frequency converter/amplifier  42  is coupled between the receive and transmit antennas  28  and  29  to form a transponder system  40 . The converter/amplifier  42  has a plurality of amplifiers  43  arranged between a demultiplexer  44  and a multiplexer  46 . This structure is fed by a frequency conversion subsection  48  in which a mixer  50  and a local oscillator signal  51  are used to frequency convert the output of a low-noise amplifier  52 . The frequency conversion subsection  48  may also include pre-amplifiers  54  at the converted channel frequencies. The low-noise amplifier  52  is coupled to the receive antenna  28 . 
     Each of the amplifiers  43  is dedicated to a respective frequency channel of the transponder  40 . In the demultiplexer  44 , channel bandpass filters  56  are coupled through secondary waveguides in the form of tees  58  to a primary waveguide in the form of a manifold  60  which connects to the subsection  48 . Each of the channel filters  56  is connected to a respective one of the amplifiers  43 . Similarly, channel bandpass filters  62  are coupled through tees  64  to a manifold  66  of the multiplexer  46 . Each of the channel filters  62  is connected to a respective one of the amplifiers  43  and the manifold  66  couples to the transmit antenna  29  through output filters  68 . The output filters  68  are configured to reduce harmonics and higher-order electromagnetic modes which would otherwise degrade the radiating performance of the output antenna  29 . 
     In its operation, the transponder  40  receives input communication signals in a receive frequency band through the receive antenna  28 , converts the received signals to a transmit frequency band, amplifies the frequency-converted channel signals and retransmits the converted and amplified signals through the transmit antenna  29 . In an exemplary communications system, the transponder&#39;s receive antenna  28  might be configured and oriented to receive signals from a single Earth-based station and the transponder&#39;s transmit antenna  29  might be configured and oriented to transmit signals to an area of the Earth for reception by a plurality of Earth-based stations. 
     The manifold  66  of the multiplexer  46  has an open-circuited end  72  which couples the combined signal channels to the output filters  68  and output antenna  29 . Opposite the open-circuited end  72 , the manifold  66  has a short-circuited end  74 . The tees  64  are spaced from the short-circuited end  74  by distances which are selected to enhance signal addition between channel signals exiting the tees and channel signals which are generated by various reflection generators (e.g., tee and manifold short-circuited ends and waveguide impedance mismatches). 
     The microwave amplifiers  43  are typically high-power microwave amplifiers (e.g., traveling-wave tubes) which generate frequency harmonics because their amplification is a nonlinear process. In addition, signals passing through the transponder  40  typically encounter transmission-line discontinuities (e.g., waveguide bends and junctions) which generate higher-order electromagnetic modes. 
     This combination of frequency harmonics and transmission-line discontinuities gives rise to manifold resonances and propagating higher-order modes whose energy exchanges at other transmission-line discontinuities further complicate multiplexer design. As stated above, these complications cause conventional multiplexer designs for high numbers of channels to be exceedingly complex and expensive. Although these multiplexer problems have been described with reference to spacecraft, they occur in many other communcation structures (e.g., communication ground stations). 
     Accordingly, an embodiment  80  of the multiplexer  40  includes the structure of FIG. 3 which shows the manifold  66  and one of the tees  64 A forming an E-plane junction  81 . In contrast to conventional multiplexer structures, a pair  62 A and  62 B of the channel filters of FIG. 2 are coupled to a short-circuited end  82  of the tee  64 A. The shorted end forms first and second apertures  83 A and  83 B for signal access to the tee  64 A and the filters  62 A and  62 B are respectively coupled through the apertures  83 A and  83 B to the interior of the tee. A septum  84  extends away from the short-circuited end  82  and is positioned between the apertures  83 A and  83 B to provide signal isolation. The short-circuited end  82  can be configured in various ways that provide physical clearance between the filters  62 A and  62 B. In FIG. 3, for example, opposite corners of the shorted end  82  are chamfered to angle the filters away from each other. 
     In one multiplexer embodiment, the septum has a length  86  of (¼)λ g     avg    in which λ g     avg    is the average guide wavelength of channel signals that are processed through the tee  64 A. Thus, the septum  84  defines two subwaveguides in the form of reduced-height waveguides  88 A and  88 B which extend away from the short-circuited end  82 . Each of these waveguides forms a quarter-wave impedance transformer and, for signals having a guide wavelength substantially equal to λ g     avg   , these impedance transformers transform the short-circuited end  82  into an apparent open circuit (i.e., a very large impedance) at the opposite end of the septum  84 . 
     In operation of this multiplexer embodiment, a channel signal is filtered through the filter  62 A and coupled through the aperture  83 A to then propagate down the reduced-height waveguide  88 A. As this channel signal reaches the end of the reduced-height waveguide  88 A, it “sees” the signal open-circuited that is presented by the quarter-wave transformer action of the reduced-height waveguide  88 B. Thus, the channel signal is inhibited from propagating into the latter waveguide and, instead, propagates down the remainder of the tee  64 A and into the manifold  66  where one signal portion  90  propagates towards the manifold&#39;s open-circuited end and another signal portion  92  propagates towards the manifold&#39;s short-circuited end (this propagation mode may be, for example, a TE 10  mode). A different channel signal filtered through the filter  62 B propagates in a similar series of processes so that both signals are multiplexed through the same tee. 
     As stated above, a combination of frequency harmonics and transmission-line discontinuities gives rise to manifold resonances and propagating higher-order modes. The septum  84  is preferably dimensioned to create a transmission path from aperture to aperture (e.g., from aperture  83 A to aperture  83 B) that is sufficiently long that it significantly reduces the higher-order modes. Because the majority of higher order modes die out within (¼)λ g     avg   , a transmission path length which exceeds (¼)λ g     avg    (i.e., a septum length which exceeds (⅛)λ g     avg   ) will greatly reduce the higher-order modes and reduce apeture interactions. 
     The teachings of the invention can be practiced with various conventional configurations of microwave channel filters. For example, the filters  62 A and  62 B are shown to each form a cylindrical cavity in which one transverse end wall forms a signal-entrance aperture  100 . As shown for filter  62 A, this main cavity is divided into two cylindrical cavities  102  and  103  by a transverse septum  104  which forms two orthogonally-arranged apertures  106  and  107 . Filters of this type support the existence of two different modes (e.g., TE 11x  modes) which are coupled between the two cavities to realize a four resonator quasi-elliptic passband in a relatively small, lightweight filter. Other conventional microwave filters formed in various waveguides (e.g., rectangular or circular) to form various passband shapes (e.g., Chebyshev or quasi-elliptic) can be used to form equivalent multiplexer embodiments. 
     Other tees can be junctioned with the manifold  66  to each carry multiple channel signals in a manner similar to that of the tee  64 A. As indicated in FIG. 3, these tees may extend from the same broad wall of the manifold as the tee  64 A (e.g., the tee  64 B) or from an opposite broad wall (e.g., the tee  64 C). 
     Another multiplexer embodiment  110  is shown in FIG. 4 which is similar to FIG. 3 with like elements indicated by like reference numbers. In contrast to the embodiment  80  of FIG. 3, the tee  64 A and the manifold  66  are now arranged to form an H-plane junction  111  (other tees  64 B and  64 C are similarly arranged). Also the short-circuited end  82  need not be modified (i.e., chamfered as in FIG. 3) because the tee apertures  83 A and  83 B are now positioned in opposite broad walls of the reduced-height waveguides  84 A and  84 B. 
     The H-plane junction arrangement of FIG. 4 facilitates this different aperture arrangement in which electric field vectors can exit the apertures  83 A and  83 B to be arranged across the narrow dimension of the reduced-height waveguides  88 A and  88 B. In FIG. 3, the tee apertures  83 A and  83 B are positioned in opposite narrow walls of the reduced-height waveguides  88 A and  88 B so that electric field vectors can exit the apertures  83 A and  83 B and be arranged across the narrow dimension of the reduced-height waveguides  88 A and  88 B. 
     Yet another multiplexer embodiment  120  is shown in FIG. 5 which is similar to FIG. 3 with like elements indicated by like reference numbers. As in the embodiment  110  of FIG. 4, the tee  64 A and the manifold  66  are arranged to form an E-plane junction  81 . As in the embodiment  80  of FIG. 3, the tee&#39;s short-circuited end  82  is chamfered to facilitate multiple filter access to the tee  64 A but the chamfering is along the tee&#39;s broad wall. 
     In the embodiment  120 , two septums  84 A and  84 B divide the tee  64 A into three reduced-height waveguides  88 A,  88 B and  88 C. Three waveguide filters  62 A,  62 B and  62 C are coupled through the short-circuited end  82  for respective access to the reduced-height waveguides  88 A,  88 B and  88 C. To provide physical clearance between the filters, they are each coupled to the shorted end  82  through an evanescent aperture  122 . The evanescent aperture is essentially a thick-walled aperture which is formed by a short waveguide whose cutoff frequency for harmonic higher-order modes is above the operating frequency of the embodiment  120 . In contrast to the embodiments  80  and  110 , the filters are arranged so that their electromagnetic fields couple out of a filter side wall and into the evansescent aperture  122 . 
     Other embodiments of the invention may be configured with various waveguide members (e.g., rectangular or circular as shown by the broken line cross section  126  in FIG.  5 ), various junctions (e.g., E-plane or H-plane junctions) and various waveguide filter shapes (e.g., cylindrical, rectangular (as shown by the broken line  128  in FIG. 5) and spherical (as shown by the broken line  129  in FIG. 5) which realize various filter passbands (e.g., Chebyshev or quasi-elliptic). Although the filters  62  are coupled through narrow waveguide walls in FIGS. 3 and 5 and through broad waveguide walls in FIG. 4, other multiplexer embodiments can be formed that use a combination of these coupling arrangements. 
     In accordance with the invention, filters can be multiplexed through common tees to form efficient multiplexers/demultiplexers in a variety of microwave frequency bands (e.g., C band, Ku band or Ka band). For example, FIG. 6 shows a graph  130  of measured signal transmissions and reflections in a prototype of the multiplexer structure of FIG.  3 . Plot  132 FIG. 6 shows reflection at the open-circuited end of the tee  64 A (where it joins the manifold  66 ). Plots  134  and  136  of FIG. 6 show transmission respectively from inputs of the filters  62 A and  62 B to the open-circuited end of the tee  64 A. As indicated, reflected signals were below −23 dB in the passbands of the filters and transmission loss in each passband was extremely low. 
     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.