136-137A theoretical analysis of the effect of the apparent height gradient dh'/df in the ionosphere on the shape of the reflected pulse has been made. It has been shown that when the gradient dh'/df is high, the pulse would normally be split into two. It has been further shown that the observed four split echoes may be the result of such a gradient in the electron density in the ionosphere, when the working frequency is near the critical frequency of the region
