Macroporous silicon layers with randomly distributed vertical cylindrical pores of average diameter 120 nm were fabricated on selected areas on a p + (resistivity 5 mΩcm) silicon substrate by anodization in HF xethanol solution. The thickness of the layers was 50 μm and they are intended for use as RF isolation micro-plates for on-chip passive devices operated at microwave frequencies. In this respect, we present in this work results of their electrical characterization in the 40 MHz-7 GHz frequency range using a macroscopic platform, where the die containing the porous silicon layer on the highly doped Si substrate is inserted directly underneath a microstrip line. The highly doped Si substrate used acts as a lossy metal ground for the microstrip. We measured and simulated the scattering parameters of the system using a finite-element full-wave electromagnetic solver. We have chosen a broad frequency band extending from 40 MHz up to 7 GHz, beyond the first harmonic of the transmission line. By identifying measured and simulated S-parameters over the whole frequency band we extract both geometrical characteristics of the measurement set-up not accessible to direct measurements and the complex permittivity of porous silicon.