The Phase Ionosonde is capable of giving more precise experimental data than the conventional sounder. This accuracy, which is an order of magnitude better than that of the conventional ionosonde, makes it necessary to seek improvement in the technique of true-height reduction. The method developed is such that the input data consist of the phases of echoes at specific frequencies, and when data from both magneto-ionic modes of propagation are used, it gives errors in height of the order of only a few metres. Furthermore, the precision of the data and the calculation is such that it eventually leads to the solution of the hitherto unsolved 'valley' problem, allowing useful estimates of the minimum plasma frequency and thickness of the valley to be made. The Phase Ionosonde has been used for the study of sporadic E and spread F phenomena. The instrument can be used to measure the thickness of a sporadic E layer by calculating the phase advance, caused by the presence of a sporadic E layer, on echoes from the F region. The method developed gives accurate results for the thickness of the sporadic E layer, as well as giving the shape of the nose of the layer. The sporadic E layer examined was found to have a sharp-peak nose, which is one of the few experimental results which agrees with the shape predicted by the wind shear theory. Ionograms obtained with the use of the Phase Ionosonde, when spread F conditions prevailed, provide evidence which tends to exclude scattering models or small scale TIDs as the cause for the somewhat fuzzy appearance of the F region reflected echo on an ionogram of the conventional sounder. It is concluded that, in this instance, spread F is caused by large scale TIDs with an approximate wavelength of 95 km, which is consistent with wavelengths predicted by the internal atmospheric gravity wave theory.