Multimode and Dispersive Distortion in the Very-Low-Frequency Channel


  • Francis J. Kelly


The distortion of short, quasi-monochromatic VLF pulses by dispersion and multimode propagation in the earth-ionosphere waveguide is studied. The delays between received pulses on three waveguide modes are calculated with a β = 0.5 ionospheric electron-density profile having an ionospheric height of 70 km (day) and 90 km (night). The fractional spreading of short pulses caused by dispersion on individual modes is also calculated. Delays of up to 0.7 msec between pulses arriving on different waveguide modes were obtained. The dispersive spreading on the important modes is only a few percent for a 2-msec pulse.