The radiation field from a leaky mode on a semi-infinite printed-circuit line is investigated. Asymptotic formulas for the radiation field are presented for a mode that is either in the physical region (a fast wave with respect to the substrate mode into which leakage occurs), or in the spectral-gap region (a slow wave). These formulas, as well as a numerically exact representation of the leaky-mode radiation field, are used to examine the nature of the radiation field and to study how it changes as the leaky mode transitions from the physical region to the spectral-gap region. A comparison is made between the field radiated by the leaky-mode current and that radiated by a practical probe feed that launches the semi-infinite leaky mode current. Another comparison is made between the field radiated by the leaky-mode current and the exact field radiated by the total current that is excited by a practical gap voltage source on an infinite line.