We propose an approach that enables increased range and throughput in over-the-horizon (OTH) High Frequency (HF) communications using a novel physical layer node cooperation scheme. By exploiting multiple (and sufficiently spaced apart) lowpowered man-portable radios in a collaborative fashion, it will be possible to obtain diversity gains similar to Multiple Input Multiple Output (MIMO) systems and, therefore, to maximize link reliability, to increase communication range for a given throughput and power or, to increase throughput for a given range and power. Our schemes are based on randomized space-time coding in the framework of distributed cooperative networks. In this paper, we compare single and multi-carrier transmission schemes, and we show that, by increasing the number of nodes while keeping the total transmit power fixed, we can approach the diversity obtainable by conventional MIMO schemes. We also observe that the Time-Reversal Space-Time Block Coding scheme (TR-STBC) performs the best among the three schemes we compare. The proposed architecture will provide power savings and diversity gains that will increase the reliability of OTHHF communications in the absence of terrestrial infrastructure.