Keywords: VLF 18.1 kHz spectrogram.svg en Spectrogram of a 18 1 kHz VLF signal picked up using a small loop antenna and a sound card The vertical stripes are distant lightnings 2014-07-10 13 31 56 own Olli Niemitalo 65 25 cc-zero Uploaded with UploadWizard Communication Spectrogram Submarines |