Miniaturisation of wideband antennas by means of feed line topology alterations

It is a common practice to achieve reduction of wideband antenna size by incorporating appropriate geometrical modifications into conventional antenna topologies (e.g. monopoles or uniplanar structures). The authors investigate a particular type of modification, which is changing the geometry of the feed line. The two basic geometries include a taper and a stepped-impedance line. They carry out systematic investigations concerning the effect of these feed line geometries on obtainable antenna miniaturisation rates. For the sake of generality, feed line geometries of various complexities are considered (from two to six sections). An automated adjustment of antenna geometry parameters through numerical optimisation is performed to ensure design optimality for each combination of antenna/feed structure. The optimisation process is formulated as an explicit size reduction task with a constraint on the maximum in-band reflection level. The investigations are performed using two selected ultra-wideband monopoles. The results indicate clear advantage of the stepped-impedance feed line over the tapered one. On the other hand, considerable size reduction can be obtained by increasing the geometrical complexity of the feed line, although the design problem becomes more challenging due to the increased number of antenna parameters. Numerical results are validated experimentally for selected antenna prototypes.