How to calculate Distance between RFID Reader and tag?

Question

I have an UHF RFID reader with Circular Polarized Antenna integrated in it. I would like to get the distance between reader and Passive tag.. I am using Visual Studio 2013 C# for this. I revised the concept of Friis Power Relation to calculate the distance. But the relation in terms of power may not get the true distance between reader and tag. I would appreciate if anyone can help me..

Thanks.

Answer 1

This is in general a very challenging problem. While the Friis equations might sound like a good plan, they don't take a few things into account:

• Due to the frequencies used at UHF RFID, there is a lot of interference, fading, reflection etc. that causes the signal strength not to degrade in a very nice way, according to the theory.
• The orientation of the tag has a huge influence on the signal strength that is received by the tag, and later back by the reader.
• The signal strength returned by the tag depends significantly on the type of tag and chip that is on it.

You might be able to get some results, if the tag is always the same, in a fixed orientation, without a lot of reflecting materials around it.

What is your application, and what are you trying to achieve?