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Why do LF/HF RFID tags use the near field (magnetic coupling) and not the far field? Well, I guess it's easier. The two antennas can just be modeled as a "transformer" and power transfer is straight forward.

But would it be possible to use the far field as well (assuming we antenna size is not the issue)? How would the energy transfer change?

Why are "long distance" RFID tags UHF tags?

According to Frii's transmission equation, the received power is indirectly proportional to frequency. So a lower frequency would mean more power at the tag.

Is the reason just the large required antenna or something else?

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It's about catching the radiating energy - transverse electromagnetic wave, which is very small at long distance. Therefore a long range RFID has to have its own power supply, battery like. The received signal is a weak radio signal.

Meanwhile the LF tag works in near field, where magnetic coupling is possible, so it can be powered from the field without using its own supply. Further the transmit antenna could be also sensing the receiver within near field, which for far field isn't possible at all. When the EM wave is forming it is still binded to antenna in the conductive region- near field, but when it is 100% formed it becomes a transverse EM wave and it leaves for ever, the transmitter can't retrieve it back anymore.

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At 125 kHz, the far field begins at a distance of several km. There’s no chance a passive tag will work at this distance. At UHF, the far field begins at around a metre (300 MHz).

The Friis equation implies lower frequencies work better but that is only because a dipole or monopole is proportionately longer and receives more signal.

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  • \$\begingroup\$ Any particular reason for the downvote? \$\endgroup\$ – Andy aka Sep 21 '19 at 9:57

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