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An original and simple approach to design electronically reconfigurable-bandwidth microwave planar bandpass filters is reported. It exploits the use of transversal signal-interference filtering sections shaped by a branch-line directional coupler arranged in reflection mode with switchable stubs. Thus, by switching on/off these stubs through p-i-n diodes, sharp-rejection filtering functions with different bandwidths and multiple out-of-band transmission zeros are produced. Analytical equations and guidelines for the synthesis of the basic switchable filter building block are detailed. Moreover, a 2 GHz two-stage proof-of-concept microstrip prototype, with five distinct bandwidth states ranging from ~10 to 50%, is built and characterised for experimental verification.