This paper presents a compact free-space measurement system for the characterization of dielectric materials within the X-band based on the line-network-network (LNN) calibration method. Using a pair of dielectric-filled horn antennas with enhanced directivity, the setup exhibits a total length of 250 mm between the antenna apertures and is suitable for measurements of planar samples with transverse dimensions equal to or greater than 250 mm×250 mm. Based on 3-D field simulations and measurements, the assumptions of plane wave incidence on the sample and negligible diffraction effects at the sample edges are investigated. The scattering parameters of the sample are obtained according to the LNN calibration method. Remaining multipath propagation effects caused by reflections at the antennas and their fixtures are filtered out via time gating. The complex permittivity is finally calculated from the transmission scattering parameter using the Newton-Raphson method. Results for exemplary dielectrics are in good accordance with reference values.