Measurements on the wavefronts of ionospherically propagated HF radio waves made with a large aperture antenna array

The wavefronts of high-frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial nonlinearities and temporal variations which may limit the performance of large aperture antenna arrays. An experimental investigation has been undertaken in which the phase and amplitude of ionospherically propagated signals have been measured at several widely spaced antennas and the measurements have been interpreted in terms of the reflection process. Signals were received over a 122 km path for which pulsed soundings were available and from several European broadcast transmitters. The received signals exhibited widely differing behaviour and this is interpreted in terms of their modal content. For predominantly single moded signals, the observations indicate that for mid-latitude paths any random components normally contribute less than 10% of the received power.