An automatic data collection system tracks articles by providing a robust electromagnetic (EM) field within an enclosure in which the articles are stored. Respective data carriers, such as RFID tags, attached to each article respond to the electromagnetic field by transmitting unique data identified with each article. Where the frequency of the EM field coupled to the enclosure differs from the natural resonance frequency of the enclosure, the placement of a probe to couple energy to the enclosure is selected to have the reflected phase of the EM energy from walls equal at the probe location thereby producing a robust EM field to activate and/or detect data carriers.