A method of determining an equivalent source associated with a device under test by an over-the-air test (OTA) test system is described. The OTA test system includes an analysis circuit and at least one measurement antenna. The method includes the steps of: conducting, by the at least one measurement antenna, at least two sets of measurements of electromagnetic waves emitted by the device under test, thereby generating measurement signals associated with the electromagnetic waves; determining, by the analysis circuit, at least one radiation parameter of the device under test based on the measurement signals, wherein the at least one radiation parameter is associated with the electromagnetic waves emitted by the device under test; and determining, by the analysis circuit, an equivalent source on a Huygens surface based on the at least one determined radiation parameter, wherein the equivalent source is associated with the device under test. The at least two sets of measurements are conducted on at least two measurement surfaces, wherein the at least two measurement surfaces are different from each other. Further, an OTA test system is described.