sense penetration of a plane via 1 loop or 2 parallel antennae

Other Hardware Sensors
1

I just bought a jeep wrangler soft top and want to make an intrusion deterrent system. I've seen lots of discussion about 2 different proximity sensors and how they don't really work. They are either too sensitive or not sensitive enough. I understand why a manufacturer makes those fickle little devices. They are easy to install and "work" in any situation. I'm a hacker and I don't need a solution in a tiny package.

What I'd like to do is use a concept similar to http://www.instructables.com/id/Arduino-EMF-Detector/?ALLSTEPS to either detect something passing through a window. Because of the roll bars on the Jeep, I could use either a loop antenna or a top and bottom antenna. I imagine having 5 to 7 of these vertical "barrier planes" around the vehicle. I could tune each one of them separately to avoid false positives.

Any ideas how to sense penetration of a plane?

Here are those sensors for reference:

2

Most instructables are either misleading or nonsense, and the one for an "EMF detector" is a perfect example.

A floating input on an Arduino responds to the presence of nearby charged objects, but the effect depends on so many factors that in practice, it is unpredictable and useless. It is also possible to destroy input pins or even the Arduino itself by connecting unterminated wires in the manner shown.

Sorry I can't be more constructive, but the only intrusion detection system that comes to mind might be laser tripwires.