[en] High brightness electron sources for modern linac-based Free-Electron Lasers (FELs) have been characterized and optimized at the Photo Injector Test facility at DESY, Zeuthen site (PITZ). Since the time structure of the electron bunches at PITZ is identical to that at the European XFEL, the PITZ accelerator is being considered as a proper machine for the development of an IR/THz source prototype for pump and probe experiments planned at the European XFEL. Furthermore the development of such a THz radiation source at PITZ will allow also to test at this location new IR/THz radiation diagnostics devices and to develop an experimental setup for the characterization of the THz radiation, which can be used in parallel for the longitudinal diagnostic of the electron beam properties. For this reason simulation studies concerning the radiation generation using high gain FELs and Coherent Transition Radiation (CTR) have been started. In this work, start-to-end simulations for a Self Amplified Spontaneous Emission (SASE) FEL at a wavelength of 100 μm have been performed. The code ASTRA has been used to track the electron beams. The SASE FEL radiation is calculated by using the GENESIS1.3 code. The results of these studies will be presented and discussed in this contribution.