The paper describes the implementation of a robust 75 baud adaptive frequency hopping (FH) model, and its evaluation on an 800 km HF link in which its performance is compared with fixed frequency signal formats. The aim has been to achieve effective data transmission, in very severe channel conditions, comprising interference from other users, noise, and frequency selective fading. The modem uses sixth order frequency diversity, with automatic excision of interference at the receiver, but without the use of a control link from the receiver to the transmitter. The open loop system approach has been used to avoid the difficulty of establishing the control link under difficult transmission conditions. The error structure resulting from the FH modem is such that forward error correcting codes are particularly effective, and their application is investigated extensively. Also, an interleaving technique which is especially relevant to the FH signal is described and used to advantage.