Due to its effective pumping ability, nonevaporable getter (NEG) coating is considered for the vacuum chambers of the Compact Linear Collider (CLIC) electron damping rings (EDR). The aim is to suppress fast beam ion instabilities. The electromagnetic (EM) characterization of the NEG properties up to ultra-high frequencies is required for the correct impedance modeling of the damping ring (DR) components. The properties are determined using rectangular waveguides which are coated with NEG. The method is based on a combination of complex transmission coefficient S-21 measurements with a vector network analyzer (VNA) and 3D simulations using CST Microwave Studio (R) (CST MWS). The frequency ranges discussed in this paper are 220-330 and 500-750 GHz.