This paper will investigate differential space frequency coding and its applicability to multipath fading High Frequency (HF) radio channels. Orthogonal Frequency Division Multiplexing (OFDM) will be combined with differential Alamouti space frequency codes to measure performance on the Watterson HF channel model. Differential coding facilitates non-coherent reception and can thus also reduce receiver complexity. Numerical results will be shown for the CCIR poor (2ms, 1Hz) and extra poor (2ms, 2Hz) channel conditions for a system comprised of 2 transmit antennas and a single receive antenna. For comparison, performance will also be shown vs. a new single transmit and receive antenna HF OFDM Code Division Multiple Access (CDMA) scheme.