General formulas are proposed for thoroughly evaluating the radiation efficiency of traveling-wave antennas operating in any radiating regime. Indeed, existing theoretical models only work in the scanned-beam region, where the radiating mode is considerably above cutoff and the perturbation method applies. The analysis provided here extends the application of the perturbation method close to the cutoff condition, thus providing a correct estimation of the losses even in the case of broadside radiation. This case is particularly important for Fabry-Perot cavity leaky-wave antennas (FPC-LWAs) based on partially reflecting screens (PRSs), which are commonly designed to generate a broadside pencil beam. However, the previous works on FPC-LWAs always assume either a lossless PRS, or a lossless dielectric cover layer thus neglecting metal and dielectric losses: a hypothesis that is no longer fulfilled at high …