An approximate procedure is proposed, to characterize the radiation from a class of open finite-length Wiener-Hopf (WH) structures, that is useful when a complete account of the direct feed radiation and of the open end effects is required, but the corresponding (exact) modified WH equation does not allow a closed-form evaluation. This procedure considers the finite-length problem as an appropriate superposition of two related semi-infinite WH problems, each one leading to an exact solution. It takes into consideration the discrete modes and the continuous spectrum waves launched at the feed end, and also at the structure open-end. The usefulness of this procedure is tested in the case of a finite-length multi-filar helix, fed by the open-end of a circular waveguide of the same diameter. Theoretical radiation patterns show a satisfactory agreement with experimental results.