This paper presents a number of fundamental properties of full-duplex radio for secure wireless communication under some simple and practical conditions. In particular, we consider the fields of secrecy capacity of a wireless channel between two single-antenna radios (Alice and Bob) against an unknown number of single-antenna eavesdroppers (Eves) from unknown locations, where Alice and Bob have zero knowledge (except a model) of the large-scale-fading channel-state-information of Eves. These properties show how the secrecy capacity is distributed in terms of the location of any Eve, how the optimal jamming power applied by the full-duplex radio varies with various parameters, and how bad or good the worst cases are. In particular, these properties show how the quality of self-interference cancelation/suppression affects various aspects of the fields of secrecy capacity. The cases of colluding Eves and non-colluding Eves are treated separately and yet coherently. For non-colluding Eves, asymptotically constant fields of secrecy capacity are revealed. For each of the two cases, we also treat subcases with or without small-scale fading.