X-axis: vertical incidence in MHz, Y-axis: virtual height in km The above clearly shows the effects of "blanketing" Es where any VI access to the F-layers are blocked. Notice also the multiple Es echoes caused by the ionosonde pulses "vibrating" back and forth between the earth and the layer. The foEs here is only about 5.5 MHz, giving a simple Secant Law MUF of just around 30 MHz.