Does anybody know technical applications of fractal structures?

3d graphics use fractals heavily.
fancy heat sinks use them also

The classic application of fractal theory is for measuring coastlines. The coastline of a land mass varies significantly depending on the degree of roughness used. As you mention, antennas often use fractals also, as it helps them receive a broader spectrum of frequencies with a more compact design.

Antenna design.
Estimating foliage transpiration in a forest.

Design of lightweight structures that are as strong as they can possibly be, both on a micro and macro scale.