1. Ground waves. "Signals in the medium and shortwave bands travel by two basic means: ground waves and sky waves. (...) Instead of traveling in a straight line, radio signal tends to follow the
curvature of the Earth. This occurs because currents are induced in the surface of the Earth, which slows the wavefront near the ground. As a result, the wavefront tilts downward, enabling it to
follow the curvature of the Earth and travel beyond the horizon. With some exceptions, ground-wave propagation is generally used for signals below 2 or 3 MHz."
2a. Ionosphere: "Physicist Balfour Stewart suggested that the continuous but minor variations in Earth’s magnetic field might be caused by the presence of a layer of air capable of conducting
electricity in the upper atmosphere. Movement of this layer in the terrestrial field could produce electric currents by dynamo effect; these currents could, in turn, generate magnetic fields, which
would be superimposed upon the normal magnetic field observed at the Earth’s surface."
2b. Ionospheric bounce: The electron layers of the ionosphere reflect electromagnetic waves. Microwaves are not reflected, MF-HF are reflected and so do VLF which also undergo multiple reflections
and can travel around the globe. The earth and the ionosphere act as a wave guide.
Ionospheric heaters can control this process. Locations: https://youtu.be/lu0_lTXHICs
3. Ground dipole antenna - ELF communication with submarines
4. "Ambient backscatter techniques utilize the ever-present cloud of TV and cellular signals already in the air."
Use of "all available ambient radio sources from FM radio
stations, to digital TV stations, all the way up to cellular
networks for backscatter communication". https://www.facebook.com/elisabethsmithie/posts/1687898474846509
5. « Power line networking (PLN) uses existing electrical wiring, whether in a building or in the utility grid, as network cables, meaning they also carry data signals."
6. Millimeter-wave technology. "Facebook is building a new way to connect to the internet, employing millimeter-wave technology to connect buildings to the larger internet without fiber optic
7. Satellites: The altimetric satellite Jason 3 which measures sea surface level with an accuracy of 4cm sends beams of 13.6 GHz and 5.3 GHz and records the time for reflection.
8. Geodynamo: Fracking-like operations can influence magma chambers and movements and thereby the molten core of the earth which creates the magnetic field.
h creates the magnetic field.