The co-existence of millimeter-wave (mm-wave) and sub-6GHz antennas in a smartphone presents many performance-limiting aspects. When both antennas are attached to the metal frame, the feed lines of the mm-wave antennas might short-circuit the sub-6GHz antennas, and thus, may significantly affect their performance. This paper presents a method to design feed lines that function as transmission lines at mm-wave frequencies but correspond to open circuits at sub-6GHz. This study determines, in theory, the smallest achievable capacitive loading with different line types and experimentally validates the approach. The capacitive loading due to the feed line is small enough to maintain the sub-6GHz performance. At the mm-wave band, the insertion loss of the line is 1dB with a measured reflection coefficient below -10dB. The introduced common-mode capacitive load of the feed line on the sub-6GHz antennas corresponds to 0.19pF capacitance.
