Computation of the Compression of Frequency Modulated Pulses in Linear Dispersive Media Using a Time-Domain Method: Examples and Guidelines

Abstract

For the case of radio and microwave pulses propagating in the earth’s ionosphere, which motivated this work and is the source of our examples, the dispersion process is nearly timereversible, since collisional losses are very small. As a result, a long transmitted pulse of suitable amplitude and frequency modulation can be received as a compressed pulse of much higher amplitude at a chosen location in the ionosphere. The compression factor will be limited by source bandwidths and modulation control, distortion of broadband signals by antennas, and uncertainties in the propagating medium. In this paper, we present examples of compressed pulses for a variety of source waveforms. Our computer similations were performed by convolving the source signal with the dispersed waveform of a triangular element. We have obained a time-domain expansion, in which terms due to moments of the electron density, an earth’s axial magnetic field, and refraction of oblique trajectories, can be identified.

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Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.