The read/write distance of an RFID system is not a fixed value but a dynamic range affected by multiple factors. The theoretical maximum read distance for passive UHF RFID can reach 12~25 meters, and for active RFID over 100 meters, but actual applications need to be evaluated accordingly.

The main factors affecting RFID read/write distance include:

(1) Reader transmission power: The greater the reader transmission power, the larger the read distance. It is usually expressed in dbm, for example, Fuen Electronics' D2184B reader uses the impinj E710 chip with a transmission power up to 33dbm=2W. However, countries have regulatory limits on transmission power, for instance, China's UHF band regulations limit to ≤2W ERP.

(2) Reader antenna gain and beamwidth: The higher the antenna gain and the narrower the beamwidth, the greater the read distance, but the read range becomes narrower and the read area control is more precise. It is usually expressed in dbi, for example, Fuen Electronics' UA209 antenna is a 9dbi circularly polarized antenna.

(3) Tag chip and reader sensitivity: The higher the sensitivity, the farther the read/write distance. It is usually expressed in dbm, for example, the NXP Ucode 9 chip has a sensitivity of -23.5dbm.

(4) Antenna size, directivity, and placement: The performance of the antenna plays a crucial role in the overall system performance. Factors such as antenna size, directivity, antenna position, electrical characteristics in the operating frequency band, and surrounding environment all affect the read/write distance. Both the reader's antenna and the tag's antenna are critical; the tag antenna has many designs and must be flexibly selected based on the material, size, and application environment of the object to which it is attached. It is best to conduct actual measurements to determine usability.

(5) Tag orientation: When the tag is rotated 90°, the reader may only be able to read the tag within a very narrow frequency range, and the read distance may sharply decrease to about 1 meter.

(6) Environmental factors: Metal reflects/absorbs RF signals; water and the human body also absorb RF energy, severely interfering with the read distance.

(7) Multiple tag count: In a multi-tag environment, the reader's multi-tag read/write performance also affects the number of tags read per unit time.

The effective identification distance of a passive system depends on the smaller of two parameters: the maximum distance at which the tag can obtain sufficient turn-on power from the reader (tag activation distance) and the maximum distance at which the reader can detect the tag's backscatter signal.