This invention relates to waveguides for propagating electromagnetic energy and more particularly to energy-absorbing terminations for waveguide.
Modern communications systems require ever increasing bandwidths in order to accommodate the increasing amount of data being handled. As signal bandwidths increase, the frequencies of carriers upon which these signals are superimposed have become increasingly higher. At carrier frequencies below 100 Megahertz (MHz), transmission lines are for the most part in the form of coaxial cable or open-wire transmission lines. In the region between 100 MHz and about 8 Gigahertz (GHz), coaxial cables and waveguides both find use, with waveguides being used for those applications requiring lower loss or high power handling capability. At X-band (8.2 to 12.4 GHz) and beyond, the use of waveguides predominates over the use of coaxial cable. Waveguides are currently commercially manufactured for use at frequencies up to 140 GHz.
Complex waveguide assemblies are used for many purposes and find special use in communications satellites. For example, a microwave solid state transmitter for a satellite may be implemented by means of a large number of solid state power sources, each producing a few watts of signal energy which is either generated in or coupled into a waveguide. The signals from these power sources are combined by a "tree" type of waveguide power combiner, in which the power from the signal sources is applied in pairs, equal in amplitude and quadrature in phase, from the signal sources to inputs of waveguide directional couplers. Each directional coupler has two input ports and an output port, and also has a terminated waveguide port. Each directional coupler combines the signal powers which are equal and in quadrature, from the two input waveguides and sums their power in a single output waveguide. In this fashion, the signals from eight solid-state power sources applied by eight waveguides to four directional couplers may be added in pairs to produce twice the amount of power in each of four waveguides. The four waveguides are coupled in pairs to the inputs of two further directional couplers, which combine the signal powers into two waveguides which are coupled to a final directional coupler, which combines the last pair of signals onto a single waveguide. Such an arrangement requires N-1 directional couplers to combine the signal from N input waveguides. It will be understood that when the signal from large numbers of waveguides is to be combined, many directional couplers and a substantial number of waveguide connections are required. When such an assemblage is to be used on a satellite, it is imperative that the size and weight be minimized. It is clear that conventional "plumbing" assembly, in which each waveguide component has individual waveguide connection flanges interconnected by suitable waveguide lengths and elbows, has excessive weight due to the interconnection waveguides and flanges, and also has excess volume because of the space between waveguide components. A more satisfactory fabrication technique for an assemblage of waveguide components is to mill the entire waveguide assembly into a solid "monolithic" block of metal, and insert the requisite components. It is convenient when fabricating such as assemblage to fabricate two mating halves of a block which when mated together define both the waveguide components and their interconnection waveguides.
Among the waveguide elements required for use with a directional coupler is a terminating load. Such loads are intended to absorb all the energy flowing through a waveguide without reflection. When the two inputs are in phase quadrature, no energy flows into the terminating load. However, for any inequality in the amplitude of the inputs, or for any phase error, energy flows into the terminating load and must be absorb to reduce reflects back to the input. In the prior art, as described for example in U.S. Pat. No. 3,904,993 issued Sept. 9, 1975, to James, the waveguide load consists of a wedge of energy absorbing or lossy material located in and extending across the full width (larger cross-sectional dimension) of the rectangular waveguide. In the James arrangement, conductive septums are located within the lossy material to aid in carrying away heat. It has been found that such prior art terminations create difficulties in assembling the two mating portions of a waveguide assemblage such as that previously described. For example, an 8-way power combiner including seven directional couplers requires seven waveguide terminations or loads. As mentioned, the prior art energy absorbing material extends across the full width of the waveguide. The energy absorbing wedges, once cut to size, can be inserted into the proper location in one of the halves in the assembly. The wedges are retained in that location by use of adhesive. When so assembled, each wedge protrudes above the half of the waveguide into which it is inserted by a full half width of the waveguide. Thereafter, the mating half of the assemblage is placed on the half containing the absorbent wedges. It has been found to be extremely difficult to simultaneously fit all of the wedges into the mating halves of the waveguide. This difficulty exists even when the absorbent wedges are formed from a material which is resilient, such as an elastic foam. This problem is exacerbated if those portions of the absorbent material which are intended to be in contact with the walls of the mating waveguide are coated with adhesive. Even if assembly can be accomplished, the adhesive once cured prevents disassembly of the two halves of the waveguide assemblage and, if the assembly is forcibly disassembled, tears the absorbent material in a manner making reassembly difficult.
A waveguide termination arrangement is desired which is amenable to convenient assembly and disassembly of the mating halves of a waveguide assemblage.