We derive necessary conditions for unique geolocation of multiple radio-frequency sources. These conditions identify the maximum number of transmitters that can be located by multiple passive antenna arrays, often referred to as Resolution capacity (RC). Our derivations extend previously published results in the field of Angle-Of-Arrival (AOA). We show that if each array consists of M elements and there are L arrays, the number of narrowband sources that can be located is (M - 1)L. This number is L times higher than the number of narrowband signals that can be resolved by each array if AOA is used. This observation leads to the conclusion that gelocation by AOA is suboptimal and other methods should be developed that can exploit the information collected by all the arrays together. We also derive similar results for wideband signals. Numerical examples are used for demonstration of the mathematical results.