Quasi-static Design of Small Ultra-Wideband Antennas

Electrostatic methods calculate the radiation resistance, capacitance and Q-factor ratio design curves for ultra-wideband (UWB) monopoles. The lowest operating frequency is determined by the total electrostatic energy and the radiation resistance (Q-factor). Electrostatic methods with a line charge distribution were used to design the asymptotic conical dipole (ACD). This paper replaces the ACD line charge distribution with a charge distribution on a cone. The charge distribution on the cone is selected to give equipotential surfaces (monopole shapes) with conical feed points and top loads. Each charge distribution cone half-angle and cone length represents a unique monopole shape and also a point on the design curves. The 50 Omega and minimum Q-factor ratio monopole designs are selected from the Q-factor ratio design curve; then Maxwell's equations are numerically solved for the frequency range 30 MHz to 2.5 GHz. Minimizing the Q-factor ratio reduces the lowest operating frequency without increasing the voltage standing wave ratio of 1.2:1. This approach is a change from the "cut and try" method, where antenna shape is often arbitrarily selected and the Q-factor ratio is typically not used to select the antenna shape.