Abstract

A security thread detector for verifying the authenticity of banknotes. In the preferred embodiment, the invention comprises a housing with a passage through which banknotes can be passed, wherein the housing also comprises a waveguide, a microwave oscillator for generating microwaves and two resonating slots on a wall of the waveguide, and a microwave detector. After a banknote is inserted through the passageway, the microwave diode produces an analog signal that is proportional to the microwave strength. The diode and the slots are arranged such that the radiated power from each slot is one hundred eighty degrees out-of-phase. If a banknote has no security thread, then the detector receives a balanced signal. If the banknote contains a security thread, the thread interferes with one of the radiating slots. This interference causes an imbalance condition and a corresponding signal is sent from the detector diode. The resulting signal is then sent to a microprocessor which activates an appropriate indicator. This indicator notifies the user of the presence or absence of a security thread; thus, the user can determine whether the banknote is counterfeit.