EMC Question of the Week: January 30, 2023

Edge view of a pane of glass with and incident, reflected and transmitted electric field.

At 100 MHz, the plane-wave shielding effectiveness of a pane of glass (5 mm thick, εr = 8) is approximately

  1. 0 dB
  2. -3 dB
  3. 3 dB
  4. 6 dB

Answer

The best answer is “a.” A pane of glass that is thin relative to a quarter-wavelength provides virtually no attenuation. There is no absorption loss, because glass is a very poor conductor. And while the reflection coefficient off the front face is about 0.5, the reflection coefficient off the back face is about -0.5. For thin panes of glass these contributions cancel. 

Plane-Wave Shielding Effectiveness of 5-mm Glass Pane

Note that at 5.3 GHz, the 5-mm pane of glass is a quarter-wavelength thick. At this frequency, the contributions of the first reflection and the second reflection cancel and the shielding effectiveness peaks at about 4 dB. 

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