High Frequency radio is used worldwide as an alternative to satellite for sky wave communications: ships at sea, aircraft out of range of line-of-sight radio networks, military operations, disaster areas with no communication infrastructure or distant regions lacking other communication means. It also avoids the costs, vulnerabilities and sovereignty concerns of satellite communications. In this dissertation the HF link between the Antarctic Spanish Station Juan Carlos I in Livingston Island, South Shetlands and Spain is studied. The aim of this study is to address the impairments that affect HF propagation (i.e., signal-to-noise plus interference ratio, multipath and Doppler shift and spread) and to design the physical layer of a low rate, low power and long haul HF link. Some proposals regarding this last issue are addressed, i.e., DSSS and OFDM. The challenge is to define the symbol characteristics that best fit the link to benefit from time and frequency diversity. This work may be useful to those communication engineers interested in the application of advanced modulation techniques in adverse environments.