Study of balun effects with electrically small antennas for a whitespace direction finding system

Direction finding is potentially useful to whitespace radio applications. Direction finding capability opens the possibility of direction aware communication for channel sharing. However, direction finding systems are typically large, and thus, not suitable for many packages. An electrically small, co-located direction finding system [1] creates the possibility for direction finding on size-constrained applications. This system works by calibrating to the distorted antenna patterns of the co-located elements. To ensure repeatability in the measurement of the radiation patterns of these elements, carefully designed baluns must be implemented. If the antenna current is unbalanced, or the balun allows too much current on the cable, the radiation patterns change dramatically and the performance suffers. This work discusses the limitations and practical effects of these baluns at and away from the designed frequency of operation. Two types of baluns are examined: the quarter wavelength bazooka balun, and the folded bazooka balun.