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Electronically reconfigurable or tunable microwave filters are attracting more attention for research and development because of their increasing importance in improving the capability of current and future wireless systems. For instance, emerging ultrawideband (UWB) technology requires the use of a wide radio spectrum. However, the frequency spectrum as a resource is valuable and limited, so the spectrum is always being used for several purposes, which means it is full of unwanted signals when an operation such as a UWB wireless system is concerned. In this case, existing undesired narrowband radio signals, which vary from place to place and from time to time, may interfere with the UWB system's range. A solution for this is to introduce an electronically switchable or tunable narrow rejection band (notch) within the passband of a UWB bandpass filter. Such an electronically reconfigurable filter is also desired for wideband radar or electronic warfare systems. This paper discusses the development of electronically reconfigurable filter types.