A design of diplexer with high isolation and out-of-band rejection

In this paper, a diplexer with high isolation and out-of-band rejection is proposed. In the working band 1.5 GHz to 3.0 GHz, two transmission bands with bandwidth 30 and 22 MHz center at 1.95 and 2.12 GHz separately are realized. In each single path, a special section which not directly connects to “T” structure and the terminal port to let signals coupling twice before transmitting to the terminal. For each passband, to improve the roll-off rate, two transmission zeros are designed and set near the passband edges. Better than -30 dB out-of-band rejection is fulfilled, while the isolation between two bands is below -28 dB.