Engineered porous structures are being used in many applications including aerospace, electronics, biomedical and others. The objective of this paper is to study the effect of 3D printed porous microstructure on the dielectric characteristics for RF antenna applications. In this study, a sandwich construction made of a porous acrylonitrile butadiene styrene (ABS) thermoplastic core between two solid face sheets has been investigated. The porosity of the core structure has been varied by changing the fill densities or percent solid volume fractions in the 3D printer. Three separate sets of samples with dimensions of 50 mm x 50 mm x 5 mm are created at three different machine preset fill-densities each using LulzBot and Stratasys Dimension 3D printers. The printed samples are examined using a 3D X-ray microscope to understand pore distribution within the core region and uniformity of solid volumes. The nondestructively acquired 3D microscopy images are then post-processed to measure actual solid volume fractions within the samples. This measurement is important specifically for Dimension printed samples as the printer cannot be set for any specific fill density. The experimentally measured solid volume fractions are found to be different from the factory preset values for samples prepared using LulzBot printer. It is also observed that the resonant frequency for samples created using both the printers decreases with an increase in solid volume fraction, which is intuitively correct. The results clearly demonstrate the ability to control the dielectric properties of 3D printed structures based on prescribed fill density.