Reconsidering the Voltage-Gap Source Model Used in Moment Methods

To envision the delta-gap source as a physical gap of infinitesimal width may be confusing when the objective is to model an ideal voltage source of zero internal impedance. Also, rather than employing a spatially-extended impressed-source field to account for the actual feed geometry, we prefer to use a localized impressed-source field (ideal voltage source), and then explicitly model the feeding structure. A stationary formula for the input admittance is derived using the Lorentz reciprocity theorem. It is demonstrated that the current, voltage, and field relations are in accordance with the classical network theory. Despite this, we found a number of sign inconsistencies in the literature. We also prove that the input admittance as computed through a stationary formula (surface integration) equals the locally computed admittance at the feed point, provided that the electric-field integral equation (EFIE) has been discretized through Galerkin's scheme, in conjunction with a symmetric product for testing it.