Simultaneous comparisons of HF partial reflections from the ionospheric D region at 2 and 6 MHz have been made near Adelaide, Australia (35¿S, 138¿E). Echoes generally come from preferred heights, and these heights were the same on both frequencies. By accurately calibrating the receiving and transmitting systems used, it has been possible to compare effective voltage reflection coefficients at these two frequencies and hence provide further evidence that specular reflection processes are the primary means of scatter below about 75--80 km. At heights above 80 km, processes other than specular reflection appear to be important. Turbulent scatter may make a major contribution to the observed strengths, but this cannot yet be proved unambiguously. At 6 MHz, very little scatter was received from heights above 90 km, while strong echoes are usually observed at 2 MHz. This suggests that scales of 25 m are within the viscous range of turbulence at heights above 90 km.