Fractal antennes and especially in High fréquences

Surrounding and ground condition plays a role in building the radiation pattern in space. Variation (could be weather effect, temp, changes in atmospheric layers, day/night, season etc) changes the scenario and it has different effect on different frequencies. The effect is more prominent in monsoon.

Apart from this, the material used as a substrate, may have characteristic variation in internal losses which will change the absorption and impedance char creates the effect