Summary
An additional ionospheric layer, the F3 layer, has recently been identified in vertical incidence ionogram records from the equatorial station, Fortaleza in Brazil. This paper examines the HF propagation characteristics of this layer in the context of the signatures that would appear on backscatter and oblique ionograms. It is found that the relatively weak F3 layer reported at Fortaleza will not produce a pronounced signature on backscatter ionograms, because the F3 layer produces mostly relatively weak echoes at the same ranges as much stronger F2 propagated echoes. In contrast, on oblique ionograms, F3 layer signatures are likely to be observed emerging from the high angle F2 trace. Of course if the F3 layer is pronounced enough then distinct echo traces occur on both backscatter and oblique ionograms.