At the lower shortwave frequencies e.g. 1.8 MHz, the antennas need to be physically large to enable good coupling to "space" and hence efficient radiation.
Half-wave horizontal dipoles are efficient radiators, if they are about half their length above ground or higher; if low to the ground relative to wavelength then horizontal dipoles suffer from large loss of signal in the earth and are inefficient radiators, despite their adequate length.
[a] Half wave dipoles are narrow band (only work over a very small frequency range) before serious impedance mismatch occurs.
This mismatch can be accommodated using an antenna tuner but these add to costs and modern shortwave communication often uses frequency hopping techniques.
Previous solutions include the Barker Williamson folded dipole, the Australian traveling-wave antenna and other designs by Guertler etc.