Studies of a focused-field filter for multiplexing microwave signals

The paper describes a novel form of microwave branching filter for multiplexing a wide-band signal into a number of separate channels. It may be realised in either waveguide or microstrip form. The principle of operation is derived from the operation of a focused frequency-scanning antenna located in a small TEM semicircular cavity. Different output-frequency channels are focused to emerge at different output ports located across the diameter of the cavity. The operation of the scheme is analysed and a number of filters are constructed in both microstrip and waveguide to demonstrate the concept. These designs operate in the 10GHz region and a typical microstrip 5-channel unit has an insertion loss of about 4dB and 17dB rejection of unwanted signals. Several methods to improve the performance are also discussed.