Hengshi (Langfang) Precision Machinery Manufacturing Co., Ltd.

Electronic systems generate heat and require ventilation.

At the same time, openings in a metal enclosure can allow electromagnetic energy to enter or escape.

This creates a basic conflict:

• a completely closed metal enclosure may provide good electromagnetic continuity but poor ventilation; and
• a large open ventilation grille may provide airflow but reduce shielding effectiveness.

An EMI honeycomb vent is designed to balance these requirements.

It allows air to move through a large number of small conductive passages while attenuating electromagnetic energy across a specified frequency range.

What Is an EMI Honeycomb Vent?

An EMI honeycomb vent normally consists of:

• a metallic honeycomb core;
• a conductive frame;
• electrical bonding between the core and frame;
• a conductive surface finish when required;
• a gasket or welded interface to the enclosure; and
• optional dust filters or protective grilles.

The cellular core divides one large ventilation opening into many small metallic channels.

These channels are commonly described as waveguides below cutoff.

What Does Waveguide Below Cutoff Mean?

A metallic passage can support electromagnetic-wave propagation only under particular dimensional and frequency conditions.

When the operating frequency is below the passage’s cutoff frequency, the electromagnetic field does not propagate through the channel in the same way as it would through an open aperture. Instead, it decays along the length of the passage.

This allows the cellular array to perform two functions:

1. air passes through the open cells; and
2. electromagnetic energy is attenuated by the conductive passages.

The concept is simple, but the performance of a finished vent is not determined by one formula alone.

Theoretical attenuation can be reduced by:

• poor electrical contact;
• gaps between cells;
• frame discontinuities;
• damaged plating;
• corrosion;
• incorrect installation;
• conductive objects passing through cells; and
• frequency and polarization effects.

This is why shielding claims should be supported by a defined test method.

Why Use Many Small Cells?

A single large opening provides an easy path for electromagnetic coupling.

Dividing the opening into smaller cells changes the electromagnetic behaviour while preserving a relatively high ventilation area.

The main benefits include:

• high airflow compared with a solid shield;
• repeated, controlled conductive passages;
• compact panel thickness;
• scalable panel dimensions;
• compatibility with duct and enclosure frames; and
• availability in different conductive materials.

The honeycomb geometry also gives thin metal foil enough structural organization to form a large vent panel.

The Main Design Variables

1. Cell Size

Cell size affects both shielding and airflow.

Smaller cells generally increase the cutoff frequency and can support stronger attenuation over a wider frequency range, provided that the cells and frame are electrically continuous.

However, smaller cells may also:

• increase pressure loss;
• increase blockage;
• increase contamination sensitivity;
• increase manufacturing complexity; and
• increase cost.

Cell size should therefore be selected against the required shielding frequency range and airflow requirement.

2. Core Depth

The electromagnetic field decays along the channel, so greater depth can increase attenuation below cutoff.

Depth also affects:

• pressure drop;
• weight;
• installation space;
• manufacturing stability; and
• cost.

Specifications frequently describe the relationship between channel length and cell dimension. However, a nominal length-to-cell ratio should not be treated as a complete shielding guarantee.

The finished assembly still needs proper electrical bonding and verification.

3. Core Material

The core must provide a conductive path.

Common options include:

• aluminum honeycomb vent structures;
• steel;
• stainless steel;
• copper honeycomb core structures;
• brass;
• tin-plated steel; and
• nickel-plated structures.

Material selection depends on:

• electrical conductivity;
• corrosion environment;
• joining method;
• weight;
• temperature;
• mechanical strength; and
• compatibility with the enclosure.

Highly conductive copper and brass are attractive for many shielding applications, while stainless steel may be selected for strength, corrosion resistance or welded construction.

Surface treatment can be used to improve contact, solderability or environmental resistance.

4. Foil Thickness

Foil thickness influences:

• mechanical strength;
• open area;
• weight;
• brazing or soldering stability;
• cell-to-cell bonding; and
• damage resistance.

Very thin foil can maximize open area but may be difficult to handle or reliably join. Thick foil improves strength but reduces effective ventilation area.

5. Electrical Continuity

Electrical continuity is one of the most important—and most frequently underestimated—requirements.

The following interfaces should be considered:

• cell wall to adjacent cell wall;
• core to frame;
• frame corner joints;
• frame to enclosure;
• gasket to mating surface;
• fastener locations; and
• duct transitions.

A high-performance core installed through a poorly bonded frame may fail to achieve the expected shielding effectiveness.

6. Surface Treatment

Possible treatments include:

• electroless nickel plating;
• tin plating;
• conductive conversion coating;
• passivation; and
• conductive coating of appropriate external components.

The selected finish should remain compatible with the base material and mating enclosure.

Dissimilar metals can create galvanic-corrosion risks, especially in humid or marine environments.

7. Pressure Drop

An EMI vent is also an airflow component.

Pressure drop depends on:

• face velocity;
• cell size;
• core depth;
• foil thickness;
• open area;
• protective screens;
• dust filters; and
• frame blockage.

Dust filters can become the dominant source of resistance, particularly after contamination.

Shielding and thermal design should therefore be evaluated together.

Why Installation Is Part of the Shielding System

Shielding effectiveness belongs to the complete installed system, not to the honeycomb core alone.

The vent may be attached by:

• continuous welding;
• conductive brazing or soldering, including vacuum-brazed honeycomb vent construction;
• bolted conductive flanges;
• EMI gaskets; or
• conductive finger stock.

The mounting method should maintain low-impedance electrical contact around the perimeter.

Paint, oxide, sealant or uneven surfaces between the vent and enclosure can interrupt the conductive path.

For high-performance rooms and enclosures, installation details may be as important as the nominal vent specification.

Testing and Standards

Shielding performance should be expressed with:

• frequency range;
• required attenuation in decibels;
• test method;
• test configuration;
• polarization when applicable; and
• condition of the installed assembly.

United States Department of Defense guide specifications require waveguide-below-cutoff ventilation penetrations to be designed as part of the electromagnetic barrier and call for factory or acceptance testing in accordance with specified procedures, including IEEE 299 in relevant applications.[1]

The same specifications emphasize:

• conductive construction;
• controlled cell dimensions;
• sufficient passage length;
• corrosion-resistant design; and
• complete integration with the shielded enclosure.

This highlights an important purchasing rule:

Do not compare two EMI honeycomb vents only by cell size and price. Compare their materials, joining, frame, plating, installation method and verified test conditions.

Common Applications

EMI and RFI honeycomb vents are used in:

• shielded rooms;
• MRI rooms;
• anechoic chambers;
• military shelters;
• electronic cabinets;
• radar equipment;
• communication systems;
• data centres;
• test laboratories;
• medical electronic systems; and
• power and control enclosures.

Each application can have a different frequency range, shielding target and environmental condition.

An MRI vent, for example, should not automatically be treated as identical to a military HEMP ventilation penetration.

Common Specification Mistakes

Mistake 1: Specifying Only “100 dB Shielding”

A decibel value without a frequency range and test method is incomplete.

Mistake 2: Calculating the Core but Ignoring the Frame

Leakage often occurs at seams, corners and interfaces rather than through the ideal cell geometry.

Mistake 3: Adding a Nonconductive Coating

An unsuitable coating may interrupt electrical contact between the vent and enclosure.

Mistake 4: Passing Wires Through the Cells

A conductor passing through a waveguide opening can compromise the penetration design and create an unintended coupling path.

Mistake 5: Ignoring Corrosion

Corrosion can gradually increase contact resistance and degrade long-term shielding performance.

Mistake 6: Ignoring Pressure Drop

A vent that meets shielding requirements but prevents adequate cooling is not a successful design.

Manufacturing Perspective

HengShi manufactures custom EMI shielded honeycomb vents, vacuum-brazed honeycomb vent assemblies, copper honeycomb cores and aluminum honeycomb fan vents for EMI and RFI applications.

Available configurations may include:

• stainless-steel honeycomb;
• copper or brass honeycomb;
• nickel-plated surfaces;
• tin-plated surfaces;
• circular or rectangular panels;
• custom frames and flanges; and
• soldered, welded or brazed construction.

Typical variables include:

• cell size;
• core depth;
• foil thickness;
• overall panel dimensions;
• frame material;
• mounting-hole pattern;
• surface treatment; and
• required open area.

Actual shielding effectiveness should be confirmed against the customer’s complete specification and test conditions.

Information Needed for a Quotation

Please provide:

1. application;
2. required shielding effectiveness;
3. frequency range;
4. test standard;
5. panel dimensions;
6. cell size;
7. core depth;
8. core and frame material;
9. surface treatment;
10. maximum airflow;
11. maximum pressure drop;
12. operating temperature;
13. corrosion environment;
14. mounting method;
15. drawing; and
16. required quantity.

When the cell dimensions are not yet defined, provide the shielding and airflow requirements first.

Conclusion

An EMI honeycomb vent solves a difficult engineering problem: it allows an enclosure to exchange air without leaving one large electromagnetic opening.

The vent works by converting the opening into an array of conductive passages operating as waveguides below cutoff.

Its performance depends on the complete assembly:

• cell dimensions;
• core depth;
• conductivity;
• cell bonding;
• frame construction;
• surface treatment;
• enclosure interface;
• installation quality; and
• test conditions.

A reliable EMI vent is therefore not simply a piece of honeycomb cut to size.

It is an electrically continuous ventilation assembly designed and tested as part of the shielding system.

Related Products

• EMI Shielded Stainless-Steel Honeycomb Vent
• Vacuum-Brazed Honeycomb Vent
• Copper Honeycomb Core
• Aluminum Honeycomb Fan Vent
• Contact HengShi Engineering

References

1. U.S. Army Corps of Engineers, Naval Facilities Engineering Systems Command and Air Force Civil Engineer Center, UFGS 13 49 20.00 10: RFI/EMI Shielding, Section 2.9, “Waveguide Assemblies.”
2. J. E. Casper, D. R. Rutt and D. E. Tremain, “Performance of Standard Aperture Shielding Techniques at Microwave Frequencies”, Proceedings of the IEEE 1988 International Symposium on Electromagnetic Compatibility, 1988.
3. U.S. Department of Defense, MIL-HDBK-1195: Radio Frequency Shielded Enclosures, 30 September 1988.
4. IEEE, IEEE Std 299-2006: IEEE Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures, 2006.
5. IEEE, IEEE Std 299.1-2013: IEEE Standard Method for Measuring the Shielding Effectiveness of Enclosures and Boxes Having All Dimensions Between 0.1 m and 2 m, 2013.

Request an Engineering Evaluation

HengShi manufactures customized metallic honeycomb structures and framed vent assemblies for EMI and RFI shielding applications.

Please contact HengShi Engineering and send your required frequency range, shielding effectiveness, dimensions, material, cell size, core depth, airflow conditions, mounting method and quantity for review.