HF radio focusing caused by the electron distribution between ionospheric layers

It is widely known that refraction by an ionospheric layer can cause radio waves to undergo skip distance focusing when ground-to-ground propagation occurs at radio frequencies above the layer critical frequency. Here it is shown by analysis and by experimental data that a similar type of spatial focusing is caused by the distribution of electrons between layers even in the absence of horizontal density gradients. The two types of focusing are conceptually similar, but they are opposite in the sense that the latter occurs at a maximum of range, whereas layer skip focusing occurs at a minimum. Associated with each type of spatial focusing is a time focusing and again, that caused by the layers themselves occurs at a minimum of time, whereas the time focusing caused by the distribution of electrons between layers occurs at a maximum. Experimental data indicate that the conditions between layers produce maximum range (or time) focusing during about half the daytime hours. The resulting signal strength on the ground is comparable to that which may be attributed to layer skip focusing.