High Isolation And Compact Microstrip Diplexer Design

Microwave diplexers are passive components with three ports to realize the function ofbidirectional communication through single channel by sharing a common antenna for the send andreceive(S/R) channel with desired isolation. It is of great value for the design of compact and highisolation microstrip diplexers as the communication systems develop towards miniaturization, andhigh performance. The author firstly designned several microstrip filters for WLAN and GPSapplications based on different resonators, then connected two BPFs for S/R channels at the commonports without junction matching. Transmission zeros can be created due to the load effect of twodifferent filters, which lead to high isolation between the S/R channels. Hence, compact and highisolation diplexers are achieved.The main work and contribution in this thesis can be summarized as follow:1. Microwave filter design theory is introduced firstly. Also the resonant characteristics ofcommon multiple-mode resonators are discussed in detail.2. A microstrip BPF is designned based on the quarter-wavelength resonator(QWR), realizing verydeep upper-stopband for the introduction of source-load coupling. Hence, a narrow-band diplexerapplied for3GHz and5.8GHz（WLAN）is formed by connecting two BPFs with different structuresand sizes at the common ports without the extra junction matching network.3. A compact and high isolation microstrip diplexer using short centred stepped impedanceresonator (SCSIR) and short stub loaded resonator is designed for GPS and WLAN application, withhigh selectivity, wide upper-stopband and a larger centre frequency ratio of two channels.4. A high isolation and compact microstrip diplexer for broadband and WLAN application isdesigned with high selectivity and wide upper-stopband. The isolation between two channels is betterthan37dB from0.1-8.5GHz.1dB fractional bandwidths (FBW) of the broadband channel is60%at3.5GHz, and the measured in-band group delays vary from0.25to0.55ns.5. A slotline diplexer with ultra widebands(UWB) for both send and receive channels is proposedby utilizing the half-wavelength U-shaped slotline reaonator. The center frequencies and3dB FBWsfor the send and receive channels are3.1GHz and67%,6.6Ghz and25%, respectively. The proposedUWB diplexer achieves miniaturization on the premise of high isolation between two channels, whichis better than37dB in the range of0.1-8.5GHz.