Refer to figure.
The Sky Waves are the radiowaves of frequency between 2 MHz to 30 MHz. These radio waves can propagate through atmosphere and are reflected back by the ionosphere of earth’s atmosphere. Since these waves go from transmitter antenna to receiver antenna while traveling through sky, hence their propagation is known as sky wave propagation. Take a look at the annex , the path b represents the sky wave propagation. The sky waves are of practical importance at medium and high frequencies (i.e. at medium waves and short waves) for very long distance radio communication. The sky wave propagation is also known as ionosphere propagation, since the sky waves reach the receiver after reflection from the ionosphere. In a single reflection from the ionosphere, the radio waves cover a distance not more than 4000 km. With the help of sky wave propagation, a very long distance round the globe communication is possible.

The Space Waves are the radio waves of very high frequency, between 30 MHz to 300 MHz or more (VHF). The space waves can travel through atmosphere from transmitter antenna to receiver antenna either directly or after reflection from ground in the earth’s troposphere region. That is why the space wave propagation is also called as tropospherical propagation.Let’s have a look at the annex once again, path c represents space wave propagation. The space wave propagation is utilized in very high frequency (VHF) bands (between 30 MHz to 300 MHz), ultra high frequency (UHF) bands and microwaves. This is because, at such high frequencies, the sky wave and ground wave propagations, both fail. The space wave propagation is also called as line of sight propagation. This propagation is limited (i) to the line of sight distance and (ii) by the curvature of the earth.The line of sight distance is the distance between transmitting antenna and receiving antenna at which they can see each other, which is also called range of communication. Thus the range of communication can be increased by increasing the heights of transmitting and receiving antennas. The space wave propagation is utilized in television communication, radar communication etc.