Reconfigurable‐bandwidth bandpass filter within 10–50%

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.