Anomalous transmission properties of a conducting fiber composite material at high frequency

Abstract

An anomalous resonant behavior is observed in the transmission through composite layers of randomly oriented conducting fibers at frequencies above the primary one-half wavelength dipole resonance. The echo of radiation from the encounter of waves propagating along fibers, which are oriented in the direction of the incident wave, with the discontinuity at the end, is found to be the cause of the anomaly. This end-fire echo dominates the dipole radiation pattern for electrically long conductors at oblique angles. Layers of randomly distributed 1 cm long fibers having a 0.8/cm3 number density are examined from 1 to 50 GHz through a series of numerical experiments using the finite difference time domain method. Composites made from the same fibers aligned with the incident electric field do not exhibit the anomalous resonant behavior. Transmission for these aligned fibers is well-explained using Beer’s law, given the total cross section for a single dipole.