Compact single‐ and dual‐band bandpass filters with controllable transmission zeros using dual‐layer dual‐mode loop resonators

An unbalanced microstrip bandpass filter (BPF) in dual‐extended doublet topology with source‐load (S‐L) coupling is first presented and researched based on dual‐layer capacitive‐loaded dual‐mode loop resonators. The input and output ports are located on the upper substrate layer. The lower dual‐mode loop resonator is coupled to the upper loop resonator using four coupling apertures in the middle metal layer. Single‐ and dual‐band responses with four controllable finite‐transmission zeros (FTZs) are analysed. Then, a balanced BPF in dual‐extended doublet topology without S‐L coupling on the basics of the proposed unbalanced filter is presented and researched. Single‐ and dual‐band differential mode responses with three controllable FTZs can be realised with good common mode suppression. Some design examples are provided to illustrate the performances of the realised single‐ and dual‐band unbalanced and balanced filters with controllable FTZs. The measured results agree well with simulated ones.