158 Electronics Explained: The New Systems Approach to Learning Electronics As you can see, the radio wave actually returns to the Earth at some dis- tance from the transmitter. Receiving stations in the area of the refracted signal can receive it. In some cases, if the signal is strong enough and the character- istics of the Earth are right, the radio signal may actually be reflected by the Earth back up to the ionosphere where it is again bent and returned to Earth. Under the right conditions, the radio signals can make several hops, thereby greatly extending the distance of communications. With the optimum fre- quency, power level, and time of day, communications halfway around the world can easily be achieved. That is why it is easy for you to listen to world- wide radio broadcasts that occur in the shortwave frequency bands. VHF, UHF, and Microwaves Radio wave propagation in the frequency range above 30 MHz occurs essen- tially by direct antenna-to-antenna space waves. This is usually called line- of-sight (LOS) communications . Further, the higher the frequency of the radio wave, the less it is affected by the ionosphere. UHF and microwave signals penetrate the ionosphere, making satellite and long-distance spacecraft com- munication possible. LOS communications means that the transmitting antenna must be able to