A four branch microstrip coupler with improved bandwidth and isolation

This paper discusses the design of a four section branch line microstrip coupler at 900 MHz. The coupler exhibits return loss better than 20 dB for the frequency range 625 to 1180 MHz. A peak isolation of 44 dB is achieved, with the overall isolation above 20 dB for the range 680 to 1195 MHz. This design also addresses the problem of large branch impedances associated with four branch couplers. The coupler has been fabricated on a glass-epoxy FR4 substrate, thus making it a very low cost solution. The designed branch line coupler finds uses in narrow-band network analyzers for incident and reflected power measurements, and in the CDMA (824 - 894 MHz) and GSM900 (890 - 960 MHz) band applications.