Why are there dead zones near an antenna using the sky-wave propagation system? I have heard in my class that there are dead zones near the antennas which use the sky-wave propagation system. I have also been told that cell phones are unable to receive signal near the antennas. However I have been unable to find the reason behind these statements.
What determines the radius of these dead zones and what causes it?
 A: The propagation of radio waves in the earths atmosphere is affected by the ionosphere, the region 75 to 1000 km above the surface of the earth that is ionized by solar and cosmic radiation. Radio waves which are refracted or reflected off the ionosphere are called 'sky-waves'. The region between the earth and the ionosphere effectively forms a 'waveguide' which offers an important means of establishing long-distance radio communications around the globe.
The distance between the transmitted radio wave up to the ionosphere and its return to earth is called the 'skip distance' or 'hop distance'.
Some signals travel great distances around the globe by multiple reflections between the ionosphere and the earths surface ('multiple hops').
Sometimes, there are regions between the location where a signal is transmitted upward into the ionosphere and where it returns to the earths surface, in which there is no reflected or refracted signal. These are known as 'skip zones' or 'dead zones'. 
The 'radius' of these dead zones is largely determined by properties of the ionosphere itself and the frequency of the radio signal employed for the transmission.
Higher frequencies radio waves are unable to 'bend' as easily in the various layers of the ionosphere, resulting in the signals passing through some layers and reflecting off others. At times, these signals signals of certain frequencies pass through to outer space without being refracted or reflected. Typically, radio frequencies above 30MHz can not be transmitted using sky-waves. 
Since the lower altitude layers of the ionosphere largely disappear at night, the refractive later of the ionosphere is much higher above ground level at night, leading to increased 'skip distances' and larger 'skip zones'. 
Cell phones typically operate in the 850/900/1800/1900 MHz bands. Because the cell phone frequencies are subject to attenuation and reflection by buildings, mountains, trees, 'dead-zones' can occur due to destructive interference from multiple reflections in built-up areas.
