RESUMO
This paper studies the air-to-ground ultra-wideband channel through propagation measurements between 3.1 GHz to 4.8 GHz using unmanned-aerial-vehicles (UAVs). Different line-of-sight (LOS) and obstructed-LOS scenarios and two antenna orientations were used in the experiments. Multipath channel statistics for different propagation scenarios were obtained, and the Saleh-Valenzuela model was found to provide a good fit for the statistical channel model. An analytical path loss model based on antenna gains in the elevation plane is provided for unobstructed UAV hovering and moving (in a circular path) propagation scenarios.
RESUMO
Excitation of electromagnetic fields in a cavity is studied in the time domain. A signal, which excites the fields, stands in Maxwell's equations as the electric current density given by an integrable function of coordinates and time. The problem is solved within the framework of the evolutionary approach to electromagnetics. The modal field expansions with time-dependent modal amplitudes are derived. Exact solutions for the amplitudes are obtained as the convolution integrals with time as a variable of integration, where the signal function stands as a parameter of the integrands. Two examples of the signal functions having a beginning in time are considered: (a) a surge modeled by the double-exponential function of time and (b) a sinusoid oscillating with an arbitrary frequency.
