What type of tag would I need to use in order to locate an item below the surface using an RFID reader 20 feet away?
Unfortunately, there is no RFID tag that can be used underwater to detect an object located 20 feet away. Passive low-frequency (LF) and high-frequency (HF) tags can be read underwater, but only from a distance of about 3 feet or less. Passive ultrahigh-frequency (UHF) tags have a longer read range, but UHF energy is absorbed by water, making it impossible to achieve such a long read range underwater. Sorry.
—Mark Roberti, Founder and Editor, RFID Journal
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