A resonant circuit for use with a radio-wave detection system for the prevention of shoplifting or the like, which is formed on a flexible substrate and has a coil and capacitor circuit whereby the capacitor has an indented area in the dielectric of the capacitor to promote disablement of the circuit when exposed to a strong electromagnetic field. Upon exposure to a strong electromagnetic field, an electrical short forms across the dielectric of capacitor in the indented area. Because the short is fragile and can be opened by flexure of the circuit, an island of reinforcing material is formed in the indented area, such that when the substrate is flexed, the indented area remains rigid to protect the short. The indented area can be further stress-relieved by introducing a gap in the conductors forming the capacitor plates.