Comparing vertical incidence ionograms during spread-F conditions at the equatorial station Huancayo and modified range time interrsity records of 50 MHz scatter echoes at Jicamarca, it has been shown that the range type of spread-F is very efficient for the back-scattering of VHF radio waves. On the otherhand, the frequency type of spread-F does not seem to produce strong echoes. It is suggested that the range type of spread-F ionogram is due to the reflection (ω = ωp) of radio waves from large scale irregularities with structure as small as 3 m below or near the base of the F-region The frequency type spread-F ionogram is suggested due to scattering from large scale irregularities with no 3 m counterpart situated around the region of peak ionisation density. © 1978.