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In the selection review of RFID magnetic separators, the most eye-catching number on the supplier's data sheet is usually "95% read distance recovery rate" or "3-fold increase in read distance". This number looks intuitive and comparable, and engineers can easily use it as a basis for comparing different supplier solutions horizontally.
But the read distance recovery rate is not a standardized material parameter. It is the ratio between the "reading distance after attaching a magnetic sheet" and the "free space reading distance without metal" or "metal environment reading distance without a magnetic sheet", and the denominator and numerator of this ratio vary with the testing conditions. The recovery rate of read distance may decrease from 95% to 60% or increase from 60% to 90% when replacing a metal substrate, tag antenna, or reader with the same magnetic separator.
Metal substrate size: The length of the eddy current path determines how much the magnetic separator needs to "resist"
The first conditional variable for reading distance recovery rate is the size of the metal substrate.
Eddy current circuit area and reverse magnetic field strength
The core mechanism of RFID tag failure when attached to metal is eddy current. Eddy currents form a closed loop on the metal surface, and the larger the loop area, the stronger the eddy currents and the stronger the reverse magnetic field. The eddy current path on a small-sized metal plate (such as 5 cm × 5 cm) is short, and the reverse magnetic field is limited. The magnetic separator only needs to suppress weaker eddy currents to restore the reading distance; The eddy current path on a large-sized metal plate (such as 30 cm x 30 cm or a whole metal shelf) is long, and the reverse magnetic field is strong. The magnetic separator needs to provide stronger magnetic flux guidance ability to achieve the same read distance recovery rate.
The supplier's read distance recovery rate data is usually measured on a metal plate of a specific size, and this size may not be indicated on the specification sheet. If engineers extrapolate this data directly to metal shelves or containers in practical applications, the actual recovery rate may be much lower than expected. When selecting, it is necessary to confirm the size of the metal substrate used for testing and whether this size is comparable to the metal structure in the actual application scenario.
Tag antenna types: coupling differences between dipole, spiral, and microstrip patch
The second conditional variable for read distance recovery rate is the type of tag antenna.
Dipole antenna
The dipole and folded dipole antennas commonly used in UHF RFID tags are sensitive to the surrounding dielectric environment, and the dielectric constant of the magnetic separator will pull the resonant frequency of the antenna.
microstrip patch antenna
The microstrip patch antenna itself has a ground layer, and the relative position and spacing between the magnetic separator and the patch determine the coupling strength.
helical antenna
The commonly used spiral antenna in the HF frequency band is mainly based on magnetic field coupling, and the magnetic permeability of the isolation sheet directly participates in the antenna inductance.
The same magnetic separator used on dipole labels may cause detuning due to dielectric loading, resulting in a decrease in read distance recovery rate; When used on microstrip patch tags, the coupling may be weak due to the presence of a ground layer, and the effect of the magnetic separator may be partially "shielded"; When used on spiral antennas, the magnetic coupling path is more direct and the recovery rate may be higher. The supplier's range recovery rate data is usually only measured for one antenna type, and direct comparison across antenna types is meaningless.
Reader configuration: The denominator of the read distance itself is constantly changing
The third conditional variable for the read distance recovery rate is the power of the reader/writer and the antenna gain.
Power and gain change reference read distance
The absolute value of 'read distance' depends on the output power of the reader/writer, antenna gain, path loss, and tag sensitivity. The reading distance of a fixed reader (output power 30 dBm, antenna gain 8 dBi) may be several times that of a handheld reader (output power 20 dBm, antenna gain 2 dBi). The read distance recovery rate of a magnetic separator is the ratio of the "post mount read distance" to the "reference read distance". If the reference read distance itself varies due to different reader/writer configurations, the comparability of the recovery rate will be greatly reduced.
Differences in the definition of testing distance
A more subtle issue is the definition of testing distance. Some suppliers measure the read distance recovery rate under the condition of "fixed distance between the reader antenna and the tag", while others measure it under the condition of "scanning to find the maximum read distance". The data of the former reflects the success rate of reading at a specific distance, while the data of the latter reflects the maximum achievable reading distance. The recovery rates under the two definitions cannot be directly compared.
Why is' reading distance improvement percentage 'easily misunderstood
The problem with the percentage increase in reading distance is that it compresses multiple conditional variables into one number. This number is valid under the testing conditions of the supplier, but it does not inform the engineer of the size of the metal plate used for testing, the type of tag antenna, the power of the reader/writer, or how the reference reading distance is defined.
Engineers can easily understand "a 300% increase in reading distance" as "three times the reading distance can be restored by sticking it on", but the actual improvement depends on their application conditions. If the metal substrate in the application is much larger than the testing conditions, or if the tag antenna type is different from the testing conditions, the actual improvement may only be 100% or even lower. This is not a supplier's "false label", but rather the indicator of "reading distance improvement percentage" itself does not have cross condition comparability.
Selection and judgment of RFID magnetic separators based on "test condition restoration"
Firstly, suppliers are required to provide complete testing conditions for the read distance recovery rate, rather than just providing percentages
The testing conditions should include: the material and size of the metal substrate, the type and size of the tag antenna, the output power and antenna gain of the reader/writer, the definition of the reading distance (fixed distance reading success rate or maximum reading distance scanning), and the mounting position and coverage area of the magnetic isolation sheet. Without these conditions, the recovery rate of reading distance can only be used as a qualitative reference and cannot be used as a quantitative comparison basis.
Secondly, during the sample stage, the recovery rate of the reading distance should be retested using actual metal structures and tag antennas, rather than directly using supplier data
If the actual application is metal shelves, metal containers, or metal equipment casings, the same or comparable sized metal substrates should be used for testing; If the tag antenna is a specific type of dipole or microstrip patch, the same type of tag should be used for testing. The purpose of retesting is to 'restore' supplier data to its own application conditions, rather than assuming that the data can be translated.
Thirdly, in cross supplier comparisons, priority should be given to comparing "relative performance under the same testing conditions" rather than comparing their respective read distance improvement percentages
If the testing conditions of two suppliers are different, their percentage improvement in reading distance is not comparable. A more reliable approach is to select a unified testing condition (fixed metal substrate, tag antenna, and reader/writer configuration), test samples from two suppliers under the same conditions, and compare the actual reading distance.
Fourthly, manage the installation position and coverage area of the magnetic separator as part of the testing conditions
The relative position between the magnetic separator and the tag antenna, whether the coverage area reaches 90% of the antenna projection area, and whether there is an air gap on the mounting surface will all affect the read distance recovery rate. When selecting, it is necessary to confirm whether these parameters in the supplier's testing conditions are consistent with the actual assembly. If they are inconsistent, they need to be revalidated during the sample stage.
Advanced Institute (Shenzhen) Technology Co., LtdThe RFID magnetic isolation product line covers three frequency bands: LF, HF/NFC, and UHF. It can provide a complete test condition description for read range recovery rate, including metal substrate size, tag antenna type, and reader/writer configuration.
The product supports retesting and verification of read distance recovery rate based on the actual metal structure and label model used by customers, helping engineers restore supplier data to their own application conditions. For more technical information, please visithttp://www.avanzado.cn/.