While heat insulating windows are a great thing, they have one big disadvantage: They also block RF signals. That means that cellular reception inside buildings and trains becomes very difficult. Back in 2016, I first heard about the concept of modifying the heat insulation layer of windows so they would let RF signals through. At the time I had little hope that I would see this in trains in my lifetime. But I was wrong, it has really happened now!
When I recently took a new ICE high speed train in Germany, they advertised that it was equipped with RF permeable windows for better cellular reception. And indeed, reception was quite o.k. in the train and when I looked more closely at the window, I could detect grooves in the insulation layer of the glass. I tried to take a picture, but it was quite difficult to get the camera to focus on the glass. But I sort of managed and you can see the structure of the grooves in the picture on the left. When looking directly at the window, you don’t see the structure unless you focus your eyes on the window. Just like the camera.
So how effective are those windows? That’s difficult to measure, but here’s a good indication: Two very low power technologies that can only cope with very little attenuation go through those windows without any problems: The GPS receiver in my smartphone and Iridium satellite text messaging with my Garmin inReach Mini 2. Really cool!