Fig 1  Principle of the proposed BPLF
Fig.2 shows the geometry of the proposed BPLF with a typical microstrip comb structure. Based on the theory of the microstrip coupled line, every two adjacent coupled lines are equivalent to a J inverter as shown in Fig.1. A 7-order filter is achieved with 7 microstrip resonant structures that are shorted with metal vias. PIN diodes are loaded between the other end of the resonant element and the ground. The proposed BPLF uses the tapped line at the input and output ports, which is equivalent to a transformer.
The dielectric substrate is made of alumina ceramic with a relative dielectric constant = 9.8 and a loss tangent of 0.001. The key parameters S 1 = 0.24mm, S 2= 0.63mm and S 3 = 0.75mm are determined by the coupling matrix. Other parameters are listed in table 1. These dimensions have been optimized to obtain resonance at 8.5GHz.