Double-sided parallel-strip line with an inserted conductor plane and its applications

In this paper, a novel multilayer structure using the double-sided parallel-strip line with an inserted conductor plane is investigated. A conductor plane is inserted into the middle of the substrate of the double-sided parallel-strip line in the proposed structure, which can function as virtual ground to isolate the nonidentical circuits on the top and bottom layers effectively. On the other hand, the conductor plane can be negligible for the identical circuits. Based on the new structure, a dual-band stub can be realized using the isolation characteristic of the conductor plane, where the stub can be treated as a combination of two back-to-back microstrip stubs operating at different frequencies. The common ground of the two microstrip stubs is provided by the inserted conductor plane. For demonstrating this idea, a bandpass filter with two transmission zeros is optimized, fabricated, and tested. The simulated and measured results are presented and show good agreement. The size of this filter is reduced approximately by half, as compared with that of the similar type of microstrip filters.