Waveguide-below-cutoff ventilation array for EMP protection: large 50 mm cells, 140–210 mm deep

01

Depth: 140–210 mm cells passed 80 dB where a five-diagonal rule would ask for about 350 mm

Cheng, Tsai and Hwang (IEEE Access, 2026) built full-scale 20 × 30 waveguide-below-cutoff ventilation arrays with 50 × 50 mm square apertures in an AA5083 aluminium alloy enclosure and measured them to MIL-STD-188-125 from 10 kHz to 1 GHz with loop, biconical and log-periodic antennas. [1] Their paper states that MIL-STD-188-125 traditionally recommends a waveguide depth of more than five times the aperture diagonal. [1]

The authors conclude that, for the configuration they tested, the depth can come down from about 350 mm (5 × the 70 mm diagonal) to 140–210 mm, a reduction of about 40–60%, and they observed stronger attenuation at higher frequencies. [1] They also cite earlier experiments showing that seams or gaps in ship hulls can cut shielding effectiveness by 15–30 dB, so the frame-to-bulkhead joint matters as much as the cells. [1]

Editorial calculation: a 50 mm square cell has a diagonal of 50 × √2 ≈ 70.7 mm; five diagonals is about 354 mm, and 140, 175 and 210 mm are 1.98, 2.47 and 2.97 diagonals.

The reduced depth is the authors' result for 50 mm square cells in a 20 × 30 array; ask the supplier for measured shielding effectiveness of the actual array and frame against the project's requirement.

02

Cell width sets the cut-off: 50 mm cells near 3 GHz, 15 mm near 10 GHz

For a rectangular tube of width a, the dominant TE10 mode has a cut-off frequency fc = v / 2a, and below cut-off the field decays as e^(−αz); the IEEE Access paper approximates a single tube's shielding effectiveness as SE ≈ 8.686 α d, so it grows roughly linearly with depth d. [1] A Siemens patent on ventilation insets for shielded cabins describes honeycomb insets as a multiplicity of waveguides side by side that attenuate below their cut-off frequency, and gives examples: welded, roughly rectangular sections for about 1 GHz, hexagonal or square cells for about 10 GHz and soldered capillary tubes for 35 GHz. [3]

Editorial calculation with the paper's single-tube formula, taking free-space wave speed: far below the 3 GHz cut-off of a 50 mm cell α approaches π / a ≈ 62.8 Np/m, giving about 76, 96 and 115 dB for 140, 175 and 210 mm depth; at 1 GHz α ≈ 59.2 Np/m, giving about 72, 90 and 108 dB. These single-tube estimates are not the array's measured values, which exceeded 80 dB in every case. [1]

03

Grid, honeycomb or multi-ply arrays: what 80–100 dB costs in air pressure

Pang, Ahn and Yook (Korean Society for Geothermal and Hydrothermal Energy journal, 2016) compared waveguide-below-cutoff arrays for EMP shielding by computational fluid dynamics: grid, honeycomb and multi-layer arrays of 2, 4, 6 and 8 plies. [2]

With 1 mm thick waveguide walls and a 30 mm module, the grid and honeycomb arrays reached the highest centre-line velocity, 17.5 m/s, about 2.5 m/s faster than at the inlet; at shielding effectiveness of 80 and 100 dB the grid array had the lowest average pressure drop and the 8-ply array the highest. [2] With 0.1 mm thick walls and 100 dB, the pressure drop of the 6-ply multi-layer array did not rise markedly with inlet velocity compared with the other types. [2]

State the airflow and allowed pressure drop of the ventilation system together with the shielding requirement; the array type, wall thickness and depth are then chosen to meet both.

04

Data for an EMP ventilation array enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Requirement: [standard; shielding effectiveness in dB; frequency range]
Opening: [duct size; wall or bulkhead type; space available for depth]
Array: [cell shape and width; depth; wall thickness; material]
Air: [airflow; face velocity; allowed pressure drop]
Installation: [frame and bonding to the wall; gasket or weld; test method and acceptance test]

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