When a sky wave is launched by such an antenna, that phase shift is always significant unless the antenna is very close to the ground ( compared to the wavelength ).
12.
A skip zone is an annular region between the farthest points at which the ground wave can be received and the nearest point at which the refracted sky waves can be received.
13.
When using medium to high frequency radio telecommunication, there are radio waves which travel both parallel to the ground, and towards the ionosphere, referred to as a ground wave and sky wave, respectively.
14.
It can be mitigated to some extent by the use of a tall mast radiator of up to about 0.6 wavelengths height, which increases the ground wave field strength while reducing high-angle sky wave radiation.
15.
The sum of ground wave and first hop wave displays an interference pattern with interference minima if the difference between the ray paths of ground and first sky wave is half a wavelength ( or a phase difference of 180?).
16.
This led to the 1919 introduction of the Adcock antenna ( UK Patent 130, 490 ), which consisted of four separate monopole antennas instead of two loops, eliminating the horizontal components and thus filtering out the sky waves being reflected down from the ionosphere.
