Usage of Characteristic Modes With Specific Boundary Conditions for Wideband Antenna Design: Application to a Cage Loaded Monopole Antenna

In this paper, a technique to design a compact wideband antenna with the optimization of the input impedance is proposed. This technique is based on the use of two sets of characteristic modes. The first one is associated with an open circuit at the antenna input port while the second is associated with a short circuit at the antenna input port. The link of this analyze with a classical modal approach by a rational fraction approximation of the input impedance is demonstrated. This technique is first applied to a basic folded monopole antenna, then to a disc-loaded folded monopole antenna in the aim to optimize its bandwidth and radiation pattern while reducing its size. The disc-loaded folded monopole antenna is modeled and measured. Its voltage standing wave ratio performances are equivalent to those of the Goubau antenna for equivalent dimensions. The radiation pattern is monopolelike on the full bandwidth.