Unfortunately, there is no ideal solution for this application. Passive high-frequency (HF) tags would require that each student present his or her card to a reader. Unless there was a way to restrict entry to only one student at a time, you would not get accurate data. Students would forget to present their RFID badges to the reader if there was nothing forcing them to do so—and even with a gate, several students could enter using a single person’s badge.
Ultrahigh-frequency (UHF) tags would present a challenge, since radio waves in the UHF spectrum are absorbed by water, of which the human body is mostly composed. This would make it difficult to get enough energy to the tag to enable it to respond. Data would be inaccurate if one or more bodies shielded the tag from the interrogator.
Active tags broadcast a signal, so it would be easier to pick up that signal and receive accurate data. However, such tags are expensive, and would require you to replace batteries (or the entire tag) every few years.
Perhaps the best compromise would be a battery-assisted passive (BAP) tag, which communicates with a reader like a passive tag, but uses a battery to obtain extra read range and consistence. Convergence Systems Ltd. just introduced a new BAP tag for just this application. See CSL Solves Long-Range Event and Personnel Monitoring With the Affordable BAP ID Card.
—Mark Roberti, Founder and Editor, RFID Journal
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