Miniaturized extended-stopband microstrip ring resonator bandpass filter using uniplanar compact photonic bandgap (UC-PBG) structures

This paper addresses a modified bandpass ring resonator filter providing compact size, low insertion-loss, wide bandwidth, sharp rejection and suppressed higher order modes. The filter enhancement follows a threefold approach. First, UC-PBG structures are used to suppress the higher order passbands yielding an upper stopband that reaches 4.7 GHz, beside offering a further advantage in terms of size reduction. Second, the use of internal folded stubs allows the exploitation of the internal area of the ring resonator, which translates into an overall size reduction of 84.5%. Finally, a solution employing via holes to replace the tuning stubs is explored in order to extend the lower stopband. EM simulation, equivalent circuit model, as well as measurement results validate the followed design approach and are in excellent agreement within each other.