A technique to narrow down radiation patterns of broad beam antenna operationally and its application to security gate to prevent shoplifting based on monopulse system

Antennas for use of security gates to prevent the shoplifting have been researched and developed based on the monopulse system at 920 MHz. Because the radiation patterns of the conventional antenna with the security gates are not enough sharp, many false operations are pointed out to recognize the RF-tags located far from the security gates. Ideas for using large antennas or radio wave absorbers were however rejected from the ladies' apparel shops because the security gate became large and the entrance became unlovely. In this paper, a technique to narrow down the radiation patterns of the antenna operationally was proposed while keeping antenna size to be small. The main emphasis was placed on the analog signal processing using the monopulse system having antennas with broad radiation patterns. The sensitivity of arrival angle detection from the RF-tags was newly defined and it was evaluated using the microstrip patch antenna pair with a half power beam width of 150
^o
as a function of the antenna pair distance. From the numerical and experimental investigations, it was obvious that the antenna pair distance from fortieth part to tenth part of a free space wavelength guaranteed the narrow beam operation to be less than the range of the arrival angle of 10°.