On Autonomous Phase Balancing of the Coplanar Stripline as a Feedline for a Quasi-Yagi Antenna

Abstract

This paper provides a clear demonstration that a coplanar stripline (CPS) as a balanced line can act to re-establish the phase balance in the presence of phase deviation from a 180° distance between two input signal lines connected to the CPS. A half-wave delay line splitter connected to a CPS works as a balun to feed a balanced antenna, such as a quasi-Yagi antenna. This type of balun structure has been widely used to feed balanced antennas with a frequency bandwidth of up to 60%. Nevertheless, no clear explanation has been given regarding how the broadband antennas could be implemented with this type of balun structure. In this paper, through 3D EM simulations and measurements, it is shown that the half-wave delay line splitter indeed only results in the 180° phase balance at one frequency, but the subsequently connected CPS acts to recover the phase balance between the signal lines. The CPS can recover the phase balance within a phase imbalance of ~π/3, which determines the usable bandwidth of this structure. For a demonstration, with a center frequency of 10 GHz, samples of the half-wave delay line splitter connected to a CPS are fabricated and measured.