I have run into the equation below in various literature/app notes. (example). I have never seen the derivation for such an equation and don't understand why round apertures are more effective at shielding than slot apertures. Could someone clarify or point me to some more study material?

enter image description here

  • \$\begingroup\$ The short answer is that 'square corners' and flat sides reflect back much RF energy, while round shapes offer far less reflection. Polygon shapes are best as the sharp angles trap RF waves inside them, which dampen out as a tiny bit of heat. \$\endgroup\$ – Sparky256 Feb 20 at 1:45
  • \$\begingroup\$ @Sparky256 Are you sure that is correct? The above equation shows that round apertures are more effective at shielding than rectangular. Seems to me you want more reflection to have an effective shield. \$\endgroup\$ – EasyOhm Feb 20 at 3:29
  • \$\begingroup\$ You are not reading what I wrote. I am saying the opposite of what your thinking. I said NOTHING about rectangular shapes, but they are less effective at absorbing standing waves than round or polygon shapes (diamond cut). Any flat side needs to be less than 1/4 wavelength to be effective. \$\endgroup\$ – Sparky256 Feb 20 at 3:42
  • 1
    \$\begingroup\$ @sparky This isn't so much about radar reflectance, like the angular shapes on the F-117 aircraft, which are >> Lambda. It's more about how much RF leaks through a hole in a continuous metal sheet, where the hole is << lambda. There's no absorption. OP's formula is widely quoted online but not referenced to any paper. I also can't understand why this would be so - a slot only radiates one polarisation, and it has capacitance across the gap, so it feels like it should leak less. \$\endgroup\$ – tomnexus Feb 21 at 6:50

Antenna efficiency decreases as it gets smaller relative to a half wave. This means shielding effectiveness (SE) improves as the radiator shrinks relative to its 1/2 wave dimension.

We know that slots, gaps and apertures radiate unintended RF proportional to these fractional wavelengths well below 1/4,1/2 wave. But apertures needed for intentional radiation antenna also leak out unintended radiation.

These experiments prove the average SE follows this well-known behaviour. If there must be an aperture for an internal antenna, a circle is an optimal shape for unintended emissions as the diameter is constant and a square or rectangle.

Slots have always had higher impedance for grounds, cabinets and doors than circles which degrades unintended efficiency by raising the impedance in the air gap surrounded by ground. This is why they are attached at least every 1/2 wavelength or PCB dual ground planes attached by micro vias every 1/10th wavelength.

It also verified that internal shields can be coupled to small apertures and increase radiation coupling by orders of magnitude either unintended or intended by Patch antenna.

This is just my experience, not a theoretical answer.


Babinet's Principle states that apertures in a metal enclosure function as an antenna in comparable ways to a piece of metal of the same shape as the aperture. Slots impact shielding more than round holes because they make better antennas. I learned this from the textbook Introduction to Electromagnetic Compatibility.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.