The development of RFID and thus growing number of options for its applications puts the actual problems to read a large number of RFID tags attached to objects or items that are in the region of space bounded by metallic walls. Such a situation occurs when there is a need for a reliable, automated inventory and real-time monitoring of items or objects (important documents, jewelry, weapons, etc.) that are typically placed in relatively small closed metal boxes (in racks, cabinets or safes, for example). In addition, as a rule, this situation does not require large range of tags reading.
For reliable and automated real-time monitoring of objects it is necessary that the EMF exists with intensity exceeded tag's sensitivity at any point of space where the objects with passive RFID tags are placed. There is some problem, however. If at least one of the linear dimensions of closed box with metal walls exceeds the signal wavelength, the box has the properties of a multimode cavity .