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Strange “stairs” received from a lot of european stations over 13kHz Some idea about this? #VLF #hamradio #hamr #swl https://t.co/6mOqNnJrRJ

Strange “stairs” received from a lot of european stations over 13kHz Some idea about this? #VLF #hamradio #hamr #swl https://t.co/6mOqNnJrRJ