Shielding Effectiveness of Naval Waveguide-Below-Cutoff Ventilation Arrays: Full-Wave Simulation and Experimental Validation

This study investigates the electromagnetic shielding effectiveness (SE) of ventilation structures based on the waveguide-below-cutoff principle using a combined full-wave numerical and full-scale experimental approach. Although waveguide-below-cutoff ventilation designs are widely adopted for electromagnetic pulse (EMP) protection, existing standards, such as MIL-STD-188-125, prescribe conservative waveguide length guidelines that are often impractical for space- and weight-constrained naval platforms. Full-wave simulations were conducted using the Method of Moments accelerated by the Multilevel Fast Multipole Method (MoM–MLFMM), and broadband experimental measurements were performed in accordance with MIL-STD-188-125. The shielding performance was characterized using transmission coefficient (S21) measurements over a frequency range of 10 kHz–1 GHz using loop, biconical, and log-periodic antennas. The test specimens consisted of full-scale 20 × 30 waveguide-below-cutoff ventilation arrays with 50×50mmsquare apertures and waveguide depths of 140 mm, 175 mm, and 210 mm. The close agreement between the simulations and measurements validates the full-wave modeling framework and enables quantitative evaluation of the shielding performance for electrically large configurations. For all tested cases, the measured SE exceeded 80 dB across the entire frequency band, satisfying the MIL-STD-188-125 requirement. The results further demonstrate that compliant shielding performance can be achieved with waveguide depths that are approximately two to three times the aperture diagonal, with an enhanced attenuation observed at higher frequencies. These findings provide full-scale experimental evidence that the conservative waveguide length guideline in MIL-STD-188-125 can be satisfied with a substantially reduced waveguide depth, thereby establishing experimentally validated design flexibility for compact EMP-compliant naval ventilation structures.