hilti PX 10 TRANSPOINTER working principle
A low RF induction field is created by the torroid in the sender.

The receiver torroid being PLL tuned to same frequency detects strongest signal when both are parallel and concentric. 

Thus you can determine ideal location for drilling on hard to get side with sender and locate center on the easy side. Range is given as 2" to 4ft.


parallel surfaces are essential with greater distance and may be influenced by iron near/between the path loss.