Methodology for Efficiency Measurements of Electrically Small Monopoles for Animal Tracking

For tracking purposes, small migratory birds are outfitted with antennas and transmitters so that their daily movements and migration patterns can be observed. In order to improve the practice of tracking wildlife and to extend the ranges over which animals can be tracked, it is necessary to characterize and fully exploit the parameters of the antennas being attached to animals. These birds provide a challenging opportunity to study the antennas used in wildlife tracking because of their small size (~15 cm in length) and light weight (averaging 26 g). For the first time, the present study provides a measurement methodology that can be used to estimate the efficiency of an electrically small antenna mounted to an approximate bird model, utilizing the Wheeler cap method. Because the antenna is less than one-eighth of a wavelength long and does not have a well-defined ground plane, measurement techniques are carefully considered and carried out. The results indicate that the bird model acts as a lossy ground plane for the transmitting monopole antenna. The radiating efficiency of the antenna-bird model system is conditioned by the size of the antenna as well as the effective resistance of the bird model. New recommendations for antenna and tracking-system design are provided, based on this investigation.