Study on a Compact Cavity System for Measuring the Shielding Effectiveness of Reinforced Concrete in the Low-Frequency Band

Evaluating the electromagnetic shielding effectiveness (SE) of reinforced concrete is important for electromagnetic pulse protection of critical infrastructure. Conventional measurements require large shielding chambers and are difficult in the low-frequency (LF) range because of long wavelengths and weak antenna coupling. This study proposes a compact system comprising LC-resonant ferrite-rod antennas and a 0.2 m × 0.2 m × 0.8 m below-cutoff rectangular cavity. Point-by-point measurements were conducted at 24 discrete resonance frequencies from 10 kHz to 160 MHz. For a copper-foil specimen, the measured SE showed an average deviation of 1.65 dB from theoretical values. The system was then applied to 0.2 m cubic specimens of plain concrete, a rebar grid, and reinforced concrete, yielding average SE values of 0.2, 15.5, and 16.3 dB, respectively. The similar frequency-dependent behavior of the rebar-grid and reinforced-concrete specimens indicates that the reinforcing-bar structure is the principal contributor to SE improvement under the tested configuration. Three repeated measurements of the reinforced-concrete specimen yielded a mean point-by-point standard deviation of 1.4 dB and a maximum of 2.64 dB. Relative to two previously reported systems, the proposed cavity reduces volume by approximately 96.3–99.98%, providing a compact approach for LF SE characterization of representative reinforced-concrete specimens.