The rules does not say anything of the placements of the doorways. Some arenas have a doorway at the entrance of the evacuation zone, some arenas don't.
Some years ago almost all arenas was designed the same, with plexi glass, but it was expensive for schools and organizers, so we changed it to be more flexible. Some still use the old style arena, but it is no requirement.
So, since it is not in the rules and no requirement, teams can not rely on using an antenna. They should prepare their robot to detect the evacuation zone in som other way. Maybe reflective tape, maybe just detecting the absence of the line? Or maybe detect victims (and from that conclude that they are in the evacuation room)?
// Fredrik Löfgren, TC and OC member 2016 / 2017