Before the expected performance levels of Ultrawideband (UWB) communications can be reached, some challenges have to be overcome. It is therefore essential to study a suitable radiating structure especially for pulsed devices that need minimum signal distortion. For example, the antenna should faithfully replicate the transmitted pulse on reception. In practice, however, the change of radiation characteristics with frequency alters the antenna response. Aside from specifications like impedance matching, efficiency, and integration, the UWB antenna must maintain nearly constant radiation patterns and phase-center position over a large bandwidth. A triangular, coplanar-waveguide (CPW)-fed printed antenna with a specific groundplane has been presented by Nicolas Fortino from France's Institute of Technology and the Laboratoire d'Electronique Antennes et Tlcommunications (LEAT) CNRS together with LEAT's Jean-Yves Dauvignac, Georges Kossiavas, and Robert Staraj.
The researchers first optimized the triangular element to minimize return losses and the antenna's size in the 3.1-to-10.6-GHz frequency band. The antenna's shape was designed to provide 50-O impedance matching over that frequency band as well as an omnidirectional radiation pattern and a limited size at low cost. The proposed antenna is a triangular monopole with a printed groundplane fed by a coplanar waveguide. Taking a cue from fractal technology, an inverted triangular aperture was cut in the monopole.
The two upper triangles are not connected to the one that is fed. They can therefore be considered two parasitic elements that are coupled to an active triangular antenna. Although this aperture gives the "fractal" antenna the same resonant frequencies as the other two structures, it also benefits from reduced impedance variations. To enhance capacitive coupling effects between the active and parasitic triangles, slots were created between the triangular elements of the fractal antenna. Metallic extensions were added to the upper elements.
An optimized antenna with double folded arms was presented. Typically, an antenna with a standard groundplane will present a 3-dB aperture of 40 to 50 deg. in this frequency band. Yet the gain of the antenna with the double-folded groundplane varies between 3 and +2 dB for apertures of 90 to 120 deg. in this band. In addition, the measured diagrams remained almost constant from 3 to 8 GHz. See "Design Optimization of UWB Printed Antenna for Omnidirectional Pulse Radiation," IEEE Transactions on Antennas and Propagation, July 2008, p. 1875.