Abstract: A novel broadband planar sum-and-difference network based on ring bridge race circuit structures is presented in this paper. A new microstrip-to-slotline transition structure with wideband characteristics is proposed firstly, and then a modified rat race circuit which adopts the transition structure considered here is designed. Four such modified ring bridge circuits are connected to form the two-dimensional broadband planar sum-and-difference network. The measured results have demonstrated that the operating band (VSWR≤2) of the proposed sum-and-difference network is from 4.05GHz to 7.425GHz. The isolations among input ports and among output ports are more than 20dB and 30dB, respectively. The insertion loss of the sum port is less than 0.5dB and the null depth is below -30dB. The proposed sum-and-difference network has been successfully applied in the antenna feed systems of broadband monopulse radars, which exhibits advantages of excellent characteristics, simple structure and low cost.