How well does RFID energy pass through objects?

15 June 2005 Telecoms, Datacoms, Wireless, IoT

RFID (radio frequency identification) has the ability to identify tags that are not optically visible from the reader. Passive tags extract their operating energy from the energising field radiated by the reader, which available energy decreases as it travels away from the reader, placing a limit on the operating range that can be achieved with a specified radiated power and transponder sensitivity.

What happens when the energy has to pass through a ream of paper, or a plank of mahogany wood, before getting to the transponder. The short answer is that only 37% of the energy is going to pass through the paper, or 53% through the wood. And this does not matter whether the object is only a couple of centimetres thick, or more than 10 cm thick.

The reason for this loss of energy transmission relates to a property of the material called 'dielectric constant'. Wherever the energy wave encounters a change in dielectric constant (eg, air-to-paper, paper-to-air), part of the energy is reflected at the transition layer and never reaches the tags. Engineers who have experience in high resolution short range radar, such as ground penetrating radars, use this principle to detect objects buried in the ground.

The reflected energy caused by the transition of dielectric constant, results in a benefit to users of UHF RFID in that the radio signals also reflect off walls and floors, allowing tags that might normally be screened via a direct path approach, to get their energy from redirected reflected energy.

If you would like to know more, a detailed presentation, together with tables showing the reflection and transmission percentages for a variety of materials can be seen at www.trolleyscan.com/refltran.html

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