本文提出一種雙頻段 (Dual band) 電路設計方法，以串接耦合線 (Coupled line) 替換一段傳輸線，並利用耦合線之耦合效應改變三倍頻點，以獲得雙頻的效果。電路分析以耦合傳輸線理論推導出傳輸線與耦合線之等效關係及第二頻段之頻率點公式。此方法運用於雙頻段鼠圈式功率分配器 (Rat-race) 和分枝線耦合器 (Branch-line)，第一頻段設計在0.915 GHz，並將原三倍頻點移至2.45和2.58 GHz，形成雙頻特性。實際電路量測與模擬結果顯示，模擬與量測值相當吻合。本電路可以應用於雙頻段射頻標籤系統 (RFID)。

A novel dual band design using a coupled-line to replace a transmission
line section in order to shift the third harmonic frequency point to the designed position and form dual band characteristics is proposed. The structure is analyzed by transmission line theory to obtain the equivalent relation between coupled line and transmission line and the equation for determining the center frequency of the second frequency band. To verify this method, a 0.915/2.45 GHz dual band rat-race hybrid and a 0.915/2.58 GHz branch-line coupler were designed, fabricated, and measured. The measured results show very good agreement with the simulation data. These circuits could be used in dual band radio frequency identification (RFID) systems.