Wide Stopband Microstrip Diplexer Using a Novel Configuration for Frequency Division Duplex and GSM-4G Applications

This paper presents a high performance microstrip diplexer based on a novel geometrical structure. The realized diplexer consists of coupled lines and semicircular structures. It operates at 2.58 GHz and 2.72 GHz, for GSM-4G and wireless applications. The diplexer channels are close to each other, which fulfill the frequency division duplex (FDD) application needs. The wide stopband rejections for both S
₂₁
and S
₃₁
are provided up to 15 GHz. Despite of quite close channels, the simulated and measured isolation between the two channels (S
₃₂
) is better than -20 dB, from DC up to 20 GHz. The designing method is based on providing a theoretical approach to attenuate the harmonics and tuning the resonance frequency. Meanwhile, several transmission zeros are created after the passbands, which improve the stopband properties. The proposed diplexer provides sharp transition edges for both passbands. The high performance is obtained using a compact size of 0.047λ
²
. To verify the designing methodology, the proposed diplexer is fabricated and measured. Good agreement between simulation results and measurement results are observed.