The paper investigates the use of differential modulation techniques to achieve high rate data transmission by near vertical incidence sky wave high frequency propagation, using a 2.7 kHz voice channel bandwidth. Channel characteristics have been measured for a range of frequencies between 2.8 and 9.4 MHz, using a transmitter power of 100 W over a distance of 160 km. These measurements are used to characterise simulated channel conditions, and the simulator is then used to determine the performances of differential modulation methods. In particular, time-differential amplitude and phase shift keying and frequency-differential amplitude and phase shift keying modulation methods are investigated. Symbol error rates are presented for data rates ranging from 4.3 to 20.6 kbit/s, where the system performances are limited by time-varying channel dispersion rather than by additive noise.