Study of fighter jet canopy radar stealth by CAC in 2022.
Metalic structure mounted outside of coated glass, is a strong radio wave reflection on the top section where the illumination angle is 90 degrees. The other part is ignorable as the radio wave is reflected away.
Solution
Rivet/Bolt contribution to radar return
solution:
Use EM meta-material fixture. The hole is covered by non-metalic layer, the shaft of bolt/rivet is radio trasparent, the other side of the bolt/rivet is EM absorbian structure.
Note, the data diagram is from experiment setup, the result of real product is better because certain peak in the experiment would never happen in real life.
The canopy is apparently J-20S because of the external mounted frame. It is clear that J-20S took the solution in the paper.
The solution to the rivet/bolt is likely used from day one of J-20 like other application of meta-material (primary suspect is the copper colored DSI bump). This is probably why AVIC (J-20 and J-35) does not bother to tape these holes so much.