Q.Define single hop distance.

A sky wave that is refracted back by the ionosphere returns to earth after a single bounce, and the distance it can cover in that one bounce is limited. The single-hop distance is the maximum distance, measured along the earth's surface, between the transmitting antenna and the first receiving antenna reached after one ionospheric reflection. It is set by the skip distance and by the curvature of the earth; for the F₂ layer this maximum is about 4000 km.

That is a long way, but it is not enough to reach right around the world. Since the average semi-circumference of the earth is roughly 20000 km, a single hop clearly cannot span the globe. To cover such distances, the signal must make several successive bounces — multiple-hop transmission. Here repeater stations receive the incoming ionospheric signal, then reflect and re-transmit it, so the wave hops repeatedly between the earth and the ionosphere, chaining several single hops together. …