The present invention generally relates to the field of microwave antennas, and, more particularly, to a number of three-dimensional designs for the radiation element of an ultra-wideband monopole antenna with a symmetrical omni-directional radiation pattern operated in the frequency range between 3.1 GHz and 10.6 GHz. Said antenna is connected to the analog front-end circuitry of a wireless communication device used for transmitting and/or receiving microwave signals and meets the FCC requirements in terms of antenna gain, radiation pattern, polarization, frequency bandwidth, group delay, and size. It comprises a radiation element consisting of an air- and/or dielectric-filled cavity structure with a base plane and a radiator plane. A metallic ground plane having a relatively high surface impedance to electromagnetic waves within said frequency range, which is printed on a dielectric substrate, serves as a reflector. The monopole antenna further comprises an antenna feeding circuitry used for electronically steering the symmetrical omni-directional radiation pattern and a feeding line connecting the antenna feeding circuitry with the base plane of the radiation element. Thereby, parts of the analog front-end circuitry can optionally be placed within the air-filled part of the radiation element of the antenna. The proposed designs include a radiation element having the form of a truncated right circular cone, rotational-symmetric radiation elements with a convexly- or concavely-shaped 3D surface, respectively, a radiation element in the form of a truncated right regular pyramid with a square base plane, and radiation elements with a combined structure comprising a conical, pyramidal, convexly- or concavely-shaped first part and a closed cylindrical or cuboidal second part whose top plane is arranged on top of the congruent base plane of said first part. Further embodiments include radiation elements with the form of a radially notched cylinder or hemisphere as well as combined structures consisting of at least two convexly-shaped elements or two conical parts, respectively, stacked on top of each other.