Cost-efficiency, environmental sustainability, and dimension reduction are important aspects in wideband antenna design. Geopolymers could be an eco-friendly and cost-efficient solution for this application. The objective of this work is to develop new geopolymer-based composites with tailored dielectric properties for applications in radar antennas. For this, different formulations based on three metakaolin and two alkaline solutions were tested. The influence of magnetite was studied by insertion of 1, 5 or 10 ​wt % of Fe3O4 in different formulations. Furthermore, the influence of humidity was also emphasized. Dielectric investigations between 2 and 3.3 ​GHz were performed. The results showed that the metakaolin type had no effect on the dielectric characteristics, whereas the nature of activation alkaline solution had a significant influence. Indeed, an increase in permittivity from 3.5 to 5.9 is evidenced by the change in the alkaline solution. The addition of magnetite up to 10 ​wt % had a little influence on the polycondensation reaction and lead to a slight increase in permittivity and permeability values. Furthermore, the permittivity and the loss tangent increase at high relative humidity level, but the phenomenon is reversible. It was also demonstrated that time had no effect on the permittivity values.