By now you would have realised that sky waves would return back to earth only beyond a certain distance. Skip distance is the distance from transmitter to first returning sky wave. Higher ionisation levels refract the radio waves more and therefore reduce skip distance. Higher Frequencies require higher ionisation levels hence increases skip distance. If the radio waves are refracted from higher refraction layers then its skip distance would be higher.
Dead Space of Sky waves
Dead Space is the distance from Surface wave range to the Skip distance. It is called dead space because we will not be able to receive both surface waves or sky wave signals in that space. In other words, the dead space is the place which is beyond the range of surface waves but lesser than the first point where the sky waves would be received. Obviously, dead space reduces with decrease in skip distance.
Effect of Frequency on Dead Space
We had learnt that as the frequency increases the critical angle will increase. Therefore, the skip distance and dead space will also increase. As frequency increases, the surface wave range will decrease. As a result, there is an increase in dead space, due to both the increase in skip distance and decrease in surface wave range. Conversely, a decrease in frequency will give a decrease in critical angle, skip distance and dead space. Hence lower frequencies are better to reduce the dead space.
Multi Hop Propagation
We are aware that sky waves return to the surface of earth due to refraction from the ionosphere. If the returning sky waves have sufficient power, they may get reflected by the earth’s surface back to the Ionosphere. Reflection of sky waves back to the ionosphere causes multi-hop propagation. Multi-hop propagation unexpectedly increase the range of sky waves. Multi-hop propagation is only possible in high power transmissions. Maximum sky wave range could be achieved if the radio wave path is tangential to the surface of earth. Ground based HF transmitters and receivers transmit waves tangential to earth to obtain maximum range through sky waves.
Sky Wave Propagation during Day
Sky wave propagation in LF and MF bands vary between day and night. During day time lot of ions are present in the D Layer, which is the lowest layer. Therefore, ionospheric attenuation is very high even at lower levels. As a result of this sky waves vanish due to absorption of waves by the D layer.
Sky Wave Propagation at Night
At night, we had learnt that D layer disappears and the E layer moves higher. As a result, sky waves refracted only by the E layer. Since the signal has to travel higher, the returning sky waves become unusable due to weak signal strength.Therefore, the range of sky waves increase, but interfere with the same surface wave propagation. The end result is that Non Directional beacons in LF and MF bands would give wrong indications at short range.