A Novel Compact Balun Using a Weakly Coupled Line With Grounded Resonator

A compact balun is proposed using a very weakly coupled line loaded with a parallel resonator. Based on an equivalent circuit model, the balun behaves as a matching network for odd-mode operation and a bandstop circuit for even-mode operation. The proposed balun can exhibit a broad bandwidth from 2.4 to 3.6 GHz with return loss of S 11 below 10 dB. This balun provides only 0.1 dB amplitude difference and gives 180
^°
±5
^°
of phase difference between S 21 and S 31. The electrical size is only 0.088 λg ×0.078 λg and fractional bandwith is 40%. Different from previous broadband baluns, the proposed balun can be implemented in low coupling-coefficient structures, which means complex strongly coupled lines (more than two wires) aren't required. This idea is one of the main reasons to achieve a very compact balun.