Improved bandwidth four branch microstrip coupler using lumped and distributed elements

In this paper, the design of a four section branch line microstrip coupler is proposed using lumped and distributed elements. Compared to two- and three-branch couplers, four branch couplers have wider bandwidth but the branch line impedances become very high, which are difficult to realize using microstrip technique. A single branch line is first represented using two shunt capacitors and one series inductor. The shunt capacitors are easily realized using microstrip lines, but it is difficult to realize inductors beyond a certain value for high impedance microstrip lines. The design of a two terminal inductor using a circular spiral microstrip configuration is explained. The proposed four branch coupler makes use of such an inductor along with transmission line-based capacitors to realize a high impedance branch line. The coupler is designed using glass-epoxy FR4 substrate. The simulated and measured S-parameters of the designed four branch microstrip coupler are in good agreement with each other. The measured isolation of the coupler is better than 20 dB from 646 MHz to 1.261 GHz, with a peak of 43 dB at 900 MHz. All port return losses are better than 15 dB from 608 MHz to 1.275 GHz. Compared to a standard microstrip four branch coupler which achieves 41% bandwidth, the proposed coupler achieves 64.5% bandwidth for which isolation is 20 dB or more. High isolation over an improved bandwidth makes the coupler suitable for power monitoring and simultaneous transmit and receive (STAR) applications. Fabrication of the coupler on FR4 substrate makes it suitable for low cost realization.