Analysis on rejection band for a practical broadband balun using an asymmetric coupled-line in free space

A practical broadband balun with a higher bandedge frequency for a differential-mode antenna is demonstrated. The practical balun consists of an asymmetric broadside coupled line in free space and via-pads to connect to the antenna or soldering pads on the same surface. The rejection band for the balun with different configurations of the pads is analyzed with mixed-mode S-parameters, and the upper band-edge frequency is shown to increase by using the proposed pad configuration. A balun consisting of a 4-mm-long broadside coupled line and the proposed pads is designed and fabricated. The measured rejection band frequency is 13.4 GHz, and is higher by 2.0 GHz than that with conventional pads. The measured insertion loss is less than 2 dB between 0.41 and 12.3 GHz.