Shortwave radio communication
Shortwave connections cannot guarantee a 100% connection due to the changing propagation conditions. Nevertheless, such devices are still in use, since large distances can be bridged with relatively simple equipment.
The lower shortwave range of 3-10 MHz is particularly suitable for so-called "NVIS connections". The signals are sent vertically upwards and reflected downwards by the ionosphere. No network structures such as satellite connections or relay stations are required for this.
Two autonomous NVIS stations can establish and maintain a fairly reliable connection without further assistance. Pure NVIS connections (without the influence of bumps) are relatively free from fluctuations in the reception level - both during the day and at night. The necessary antennas can be stretched without much effort. A simple dipole is sufficient and can easily be set up with little expenditure of time and personnel.
Low-lying areas in mountainous surroundings are no problem for a connection. The propagation path is short, direct and with little path attenuation. NVIS technologies can reduce interference and noise and thus improve the signal-to-noise ratio. The low path attenuation and the good signal-to-noise ratio enable you to work with low transmission power, which is why you can use battery-powered equipment. It is also supported by digital operating modes, which can decode even weak signals.
Because the 60m amateur radio band is in the middle of the NVIS spectrum, it works best. But the 80m and 40m bands can also be used.
Provided the propagation conditions allow it, you can reach stations at great distances with flat radiated signals in the upper shortwave range (10-30 MHz). However, it is possible that stations that are closer to you cannot be reached.
Mobile operation is possible on shortwave with correspondingly large antennas on the vehicle. The efficiency of shortened vertical radiators is of course not comparable with "full-blown" antennas for the same frequency.
Portable operation makes sense together with antennas that can be opened quickly. The range is very limited if the vertical radiators are fixed to the backpack.
In regions of the world where frequent thunderstorms are to be expected, reception can be severely impaired by frequent lightning discharges.
In the polar regions of the earth, the usability of shortwave devices can be restricted due to the interaction between the earth's magnetic field and the atmosphere during times of active polar lights.