Do you have any information about RFID chips or other small devices that could emit a sound when they were at a predetermined distance from each other? This is very important to me, and your expertise would be appreciated. Or, if not, can you please provide information on who else I might contact?
Unfortunately, no RFID devices can do what you seek. The only RFID solutions that provide fairly precise location data are UWB systems, which can tell you where tags are located to within 10 centimeters (3.4 inches). A UWB system could tell you the locations of two tags and issue an alert when they were a specified distance apart, but this would only be accurate to within 10 centimeters. There might be mesh-network devices that could do it, but I am unaware of any.
One big issue you would face is that radio waves bounce off of things, making it very difficult to use them to determine precise distances. The UWB systems mentioned above use multiple frequencies to overcome the problem of multipath, but most other RF technologies cannot determine distances very precisely.
—Mark Roberti, Founder and Editor, RFID Journal
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