Characterization of Impedance Variations in Antennas for TETRA Terminals

Antenna input impedance is usually considered as a non-dynamical magnitude, which remains constant during the transmission or reception process. The main advantage is that it allows to simplify matching circuit design. However, in an actual situation, the impedance value is a dynamic quantity that can be very different compared to the static value. In this paper, we propose a histogram based method in order to identify the impedance variation of TETRA antennas. The statistical approach is carried out in the matching domain rather than in the impedance domain, because in this way the random vector has a compact support. The method is applied to model a practical situation where the terminal position changes from the user's pocket to the head, obtaining a bimodal distribution which cannot be identified directly by means of parametric methods. We prove experimentally that the dynamical values are different from the results obtained in an anechoic chamber, and it is required to perform a cluster analysis or a non parametric method to identify accuratelly the probability density function which characterizes the antenna impedance.