ANTHROPIC VLF
Many VLF transmission sources are man-made; some intentionally, others collaterally.
The most ubiquitous is the 60Hz hum of the power grid. Other collateral transmissions emanate from electrical devices, including power tools, hair dryers, induction cookers, headlamps, televisions, dimmer switches, computers, cell phones and security systems.
Intentional VLF signals are those generated by entities requiring global coverage, subsurface and/or seawater-penetrating properties. They include time signals, industrial mining operations, submerged submarine communications and transmissions used by national command authorities in wartime. In the US, coded submarine broadcasts operate on VLF frequencies from 14 to 60 kHz, and are transmitted from Minimum Shift Keyed high-powered sites including stations NLK (Jim Creek) in Seattle, Washington (24.8 kHz); NML in LaMoure, North Dakota (on 25.2 kHz); NPM at Lualualei, Hawaii (on 21.4 kHz and 23.4 kHz); NAU at Aguada, Puerto Rico (on 40.75 kHz), NPG at Dixon, California (on 55.5 kHz), and NAA at Cutler, Maine (on 17.8 kHz and 24 kHz).
