How RFID tags work for door access
The RFID applications for personnel identification typically operate at quite a low frequency, almost 140 kHz, for badge detection. The information of a cardholder, object, or reader is electronically stored in the RFID cards or tags, which can contain small pieces of information such as identification numbers, prices, or codes.
Door readers use this same technology. Each RFID reader is equipped with a small antenna, which sends its own radio waves to detect any RFID tag or card within its range.
This range can vary for each reader depending upon the frequency of the radio waves it emits, anywhere from about 10 centimeters (~4") to around one meter (~1'). The RFID reader decodes the info from the RFID tag and sends the signal to its host software, which either grants or denies access to the user.