Yes, there are passive RFID systems available. Transcore, for instance, introduced its ultrahigh-frequency (UHF) eGo pass system in 2003 (see Army Tests RFID for Access Control). Sirit is also a leader in providing passive UHF vehicle-identification systems (see Sirit, 3M Join Forces in Electronic Vehicle Registration).
We haven’t done any case studies, but here is one relevant article you might find useful: Condos Using RFID to Keep Access in Check. I hope that helps.
—Mark Roberti, Editor, RFID Journal
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