The wavefronts of high frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial non-linearities and temporal variations which limit the performance of large aperture receiving arrays. The objective of this investigation was to measure the phase and amplitude of ionospherically propagated signals in order to relate these parameters to the reflection process. This thesis describes the design and construction of a large aperture multi-element array and its implementation for wavefrot investigations. The hardware and software developed to control the equipment and to record the measurements are described. The procedures required to verify the performance of the experimental system are discussed and results are presented which demonstrate the accuracy of the measurements. The array was utilised for studies of signals received from several transmitters situated throughout Western Europe. The results obtained demonstrate the widely different behaviour of signals received over the various propagation paths and these have been related to the modal content of the received signals. Limited periods existed during which a single ionospheric mode was received and data corresponding to this condition have been compared with those which would be expected if the signal consisted of both a specular component and a cone of diffracted rays. This model is unable to explain the experimental results. Numerical models of the received signal were therefore developed. Results of these and comparisons with experimental results suggest that the measured parameters can be explained by the existence of a specular component with a varying direction of arrival (DOA), plus some contribution from random components. The experimental results indicate that the random or diffracted components normally contribute less than 10% of the received power in a single moded signal.