According to [1] (which has more info on the antenna design), the resonant cavity antenna was designed by a company called Proant [2]. I remember reading the the antenna is the same on the Pi 3 and Zero W. There are a few options for PCB antenna design software out there, but I had never seen this type of PCB antenna before the Zero W. Not sure what software they use, or whether it's off the shelf or proprietary. Any antenna designers out there?

[1]: https://www.raspberrypi.org/magpi/pi-zero-w-wireless-antenna...

[2]: http://mob.proant.se/en/home.htm