The high frequency current is un-uniform in the wire due to external magnetic field. As a result, the conductive area of the wire is far smaller than its cross-section,and the excess resistance,namely the frequency-dependent resistance,is induced. The high frequency resistance is proportional to the square of the magnetic field the wire exposed to. The paper employees the Biot-Savart law to analyze the magnetic field distribution of the planar rectangular solenoid coils for evaluating the frequency-dependent resistance. The maximum quality factor of the coil and the corresponding optimal operation frequency are derivate based on the analyses of the field and resistance. A few coils are manufactured and their high frequency resistances are measured. The measurements show that the proposed method predicts the resistance under various frequencies and the optimal operation frequency precisely. Two coils,whose outer and inner dimensions are 460 mmí208 mm and 312 mmí64 mm,are employed to build a wireless power transfer system. Under the frequency slightly lower than the predicted optimal one,the system obtains the maximum DC-DC efficiency of 58. 7% and receives 50 watts power at the distance of 500 mm.%在外部磁场的影响下，导线中的高频电流呈不均匀分布，致使其导电面积远小于横截面积，从而引起额外的电阻，称之为高频电阻。高频电阻中的感生电阻与磁场的平方呈正比。应用毕奥-萨伐定律对Litz线平面矩形螺旋线圈中的磁场进行分析，以计算线圈中的高频电阻，进而分析其最大品质因数及最优运行频率。对几个线圈原型的测量表明，该分析方法较好地预测了线圈在不同频率下的电阻及最优运行频率。使用两个外边长460 mm×208 mm、内边长312 mm×64 mm的矩形线圈所制作的无线能量传输系统，距离500 mm时，在稍低于预测的最优频率时获得的最大DC-DC效率为58.7%，接收端功率为50 W。
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