On the Gap Source Model for Monopole Antennas

The gap source model extensively used in modeling monopole antennas is examined against the practical coaxial feed. Two approximate closed-form expressions for the input admittances of a monopole fed by a gap source and a coaxial line, respectively, are derived using the modal-expansion method. These two expressions are then set equal to estimate the gap height with respect to the monopole radius. Numerical results are presented to show the high accuracy of the gap source model using the estimated gap height.