This invention relates to multiplexing UHF or microwave channels, and more particularly to an apparatus and method for multiplexing such channels using high quality filters directionally coupled to a manifold.
Multiplexing UHF or microwave channels is the process of combining signals of different frequencies. Demultiplexing UHF or microwave channels is the process of separating certain frequencies from a broad band signal. According to common practice, the term "multiplexing" will be used herein to describe both the process of combining and separating signals. Similarly, the term "multiplexer" as used herein shall refer to a device for combining signals, separating signals, or both.
The process of separating or combining signals is particularly desirable in UHF or microwave transmission. In UHF or microwave transmission, it is often desirable for a single microwave antenna to be shared by several transmitters or receivers operating on different frequency channels. The term "channel" as used herein shall refer to any particular frequency or band of frequencies such as, for example, a band of frequencies defined by the modulation or demodulation characteristics of a transmitting or receiving device. In moderate to high power applications, channel multiplexing has been performed by either directional filter multiplexers or multiplexers that utilize a waveguide T-junction manifold having direct connecting lines.
Directional filters are four port devices that comprise a rectangular input waveguide, one or more cylindrical resonator cavities, a rectangular output waveguide, and coupling apertures in both the input and output waveguides that couple the waveguides to either end of the resonator cavities. To form a multiplexer, several directional filters are connected together sharing a common input or output waveguide. The common waveguide forms a wide band waveguide capable of carrying or transmitting several different channels. Each directional filter serves either to separate a particular channel from wide band signals traveling along the common waveguide or combine a particular channel with other channels traveling along the common waveguide.
The coupling to both the input and output waveguide of a directional filter is directional. As a result of the directional coupling to the output waveguide, a circularly polarized signal having a given direction of rotation through the resonator cavities will translate through the coupling aperture of the output waveguide to a linearly polarized signal traveling in one direction through the output waveguide. Similarly, a linearly polarized signal of the appropriate frequency traveling through the input waveguide will translate through the coupling aperture of the input waveguide to a circularly polarized signal rotating in a single direction of rotation as it travels through the resonator cavities. Therefore, the coupling to each waveguide in a directional filter multiplexer is theoretically 100% and is not sensitive to phase or magnitude changes of any channel frequency present.
Directional filter multiplexers rely on the off-resonant reactance characteristics of the resonator cavity or cavities for frequency filtering. That is, directional filter multiplexers utilize the resonator cavity or cavities as a circularly polarized signal filter. However, circularly polarized filters provide relatively poor filter performance. This low quality filter response or performance has prevented directional filter multiplexers from being used in applications that require high performance characteristics and/or very close channel spacing.
Waveguide T-junction manifold multiplexers include a rectangular wide band waveguide or waveguide manifold capable of transmitting signals in several different channels, and several channel filters each joined to the manifold through a T-junction. Each filter produces a desired frequency filter response, thereby allowing only signals in a particular frequency range to travel through the particular channel filter. Because waveguide T-junction manifold multiplexers can utilize many different filter designs, such multiplexers have superior channel performance characteristics and may be used with very close channel spacing.
While waveguide T-junction manifold multiplexers have superior filter characteristics, impedance control required in such multiplexers is difficult to achieve. The difficulty in impedance control arises because each single channel filter is loosely coupled to the waveguide manifold. That is, signals in each channel travel in the waveguide manifold in both directions from the junction. As a result of the loosely coupled channels, the multiplexer designer must account for phase and magnitude of all channel filters. The solution for a multiplexer design becomes quite complex when even three channels are involved. When the number of channels becomes large, the solution becomes nearly impossible.