On earth there are two methods of propagation i.e. ground wave and sky wave.

• In free space propagation occurs in the straight line or line of sight wave.

• Below let us compare the difference between ground wave and sky wave propagation.

Ground wave propagation:

• Ground waves exists between below 2MHz and covers ELF, SLF, ULF, VLF, LF and MF frequency bands.

• The signal exists in proximity to the earth surface, it follows contour of earth and hence it can propagate considerable distances.

• At lower frequencies, interference occurs due to atmospheric noise only, here trans it power is sufficient and hence maximum range of about 5000 miles can be achieved.

• Optimum antenna size is about $\lambda$/2 , As the distance from transmitter increases received signal strength decreases and follows exponential curve.

• They are also known as surface waves.

Sky wave propagation:

• Sky wave exists between 2MHz to 30MHz and covers ELF, SLF, ULF, LF, MF and HF frequency bands.

• It exists in the sky and depends on reflective characteristics of ionosphere layer, the signal reflected from ionized layer of the atmosphere.

• In this propagation mode, signal can travel number of hops, back and forth, the example of this type is SW radio.

• During right time, ionosphere layer is closer to the earth surface, it travels at a flatter angle, it travels larger distances there are more skip zones which results into no reception.

• During the daytime, ionosphere layer is away from earth surface, it travels with an angle it travels smaller distances.