EMI shielding Plate

Why Punch Plate Fails EMI Tests – It's a Sieve, Not a Shield

I get asked this all the time.“Why can’t I just use perforated plate for EMI shielding? It’s metal, it has holes, and the holes are small.”

And every time, I want to sigh. Yes, it’s metal. Yes, it has holes. It looks like it should work. But here’s the thing – it’s a sieve, not a shield. It’ll stop your fingers. It’ll stop a cockroach. But any RF that actually matters? It won’t stop a thing.

At 1 GHz, a few millimeters is a wide-open door. At 2 GHz, it’s even worse. Those little holes become slot antennas – they radiate out and couple in just as easily.

Then there’s the thickness problem.

A thick plate with holes acts like a bunch of tiny waveguides. If the hole is long enough relative to its diameter, RF can’t make it through – that’s the waveguide‑below‑cutoff effect. But a perforated plate? It’s thin sheet metal. The holes are shallow. No waveguide effect. RF punches straight through.

A honeycomb vent has real depth – typically half an inch or more. That depth kills the signal. Perforated plate just doesn't have that.

And eddy currents.

When RF hits a shielded surface, it induces eddy currents that create opposing fields. Sharp edges – like those punched holes, especially square or triangular ones – break up those current paths. Round and hex holes are a little better, but still not great. Honeycomb cells have smooth, continuous walls. The currents flow freely, and the shield stays effective.

We’ve tested both – same hole size, same open area.

A 1/8‑inch perforated plate at 1 GHz? Maybe 10‑15 dB shielding. At 2 GHz, you’re lucky to get 10 dB. At 5 GHz, it’s essentially zero.

A 1/8‑inch honeycomb vent at 1 GHz gives you 40‑60 dB. At 2 GHz, still 40‑50. At 5 GHz, 35‑40. In some designs, it can hit 100 dB well into the GHz range.

That’s the gap. One is a screen. The other is a shield.

MIL‑HDBK‑419A says it plainly: perforated metal isn’t good enough for high‑frequency shielding. Among non‑solid shielding materials, metal honeycomb is the best performer – high attenuation at microwave frequencies with very low pressure drop.

Some people worry that honeycomb vents choke airflow. They don’t. A good honeycomb has 80‑95% open area. Perforated sheet typically runs 30‑50%. The straight cells give laminar flow and low pressure drop. Perforated sheet has sharp edges that create turbulence and higher resistance. So you’re not giving up airflow – you’re actually improving it while getting real shielding.

Bottom line – ordinary punch plate is a cost compromise. It works for low‑frequency stuff and low‑EMI environments. But for modern electronics – digital circuits, wireless, 5G – it’s not enough. The holes leak. The edges disrupt eddy currents. There’s no waveguide depth.

If you’re failing EMI tests, that punch plate is probably your weak link. Swap it for a real honeycomb waveguide vent. Pass the test. Stop chasing ghosts.

We make the good ones. That’s what we do – but I’ll save the sales pitch. Just wanted to set the record straight. Again.