Español
Français
Português
In the verification of samples of anti metal magnetic plates, there is an overlooked phenomenon: the same roll of material, cut into circular and square shapes, is attached to the same tag antenna with different reading distances. Circles usually perform better, while squares have weaker recovery of read distance near the four corners, and the overall read distance may be 10% -20% lower.
This difference is not caused by the material batch or the bonding state. It comes from the planar shape of the magnetic flux sheet - more precisely, from the geometry of the magnetic flux path determined by the shape.
The magnetic field lines do not travel in straight lines in the magnetic flux sheet, but rather in a "curved path"
The function of magnetic plates is to provide a low magnetic resistance path for magnetic lines to bypass metals. Magnetic field lines start from one side of the tag antenna, enter the magnetic plate, propagate along the magnetic plate, and then return to the antenna from the other side. This path is not a straight line from point A to point B, but a "detour curve" - magnetic field lines need to turn, diffuse, and converge inside the magnetic sheet.
Shape determines path
The shape of this curve depends on the planar contour of the magnetic plate. The closer the contour of the magnetic flux sheet is to the natural path of the magnetic field lines, the fewer "turns" of the magnetic field lines inside the magnetic flux sheet, the shorter the effective path, and the lower the magnetic resistance. The greater the deviation between the contour and the magnetic field path, the more turns and detours the magnetic field lines are forced to make inside the magnetic sheet, resulting in a longer effective path and an increase in magnetic resistance.
The right angle of a square magnetic plate: a "sharp turn" in the magnetic flux path
The four right angles of a square magnetic plate are discontinuous points on the magnetic flux path. After the magnetic field lines enter the magnetic plate from the edge of the antenna, they need to propagate along the plane direction of the magnetic plate to the opposite side. At the right angle, the magnetic field lines are forced to make a sharp 90 ° turn.
Firstly, the path becomes longer
The magnetic field lines at right angles need to bypass an "L-shaped" path, and the actual path length is greater than the straight-line distance between two points. The longer the path, the greater the magnetic resistance, and the lower the effective magnetic permeability of the magnetic sheet.
Secondly, there is an increase in local magnetic resistance
The geometric discontinuity at the right angle concentrates the magnetic flux density near the corners, and the local magnetic resistance is higher than the average level of the magnetic flux sheet. Magnetic field lines tend to avoid these high magnetic resistance areas and take shortcuts from the inside of the right angle, resulting in underutilization of the magnetic sheet material in the corner areas.
The effective area of the square magnetic plate is therefore smaller than its geometric area. The material of the four corners "exists nominally but has not been fully utilized in the magnetic circuit".
Circular magnetic plate: continuous curved path, magnetic flux "does not bend"
The contour of a circular magnetic plate is a continuous curve without right angles or geometric mutations. After entering the magnetic plate from the edge of the antenna, the magnetic field lines propagate along the direction of the arc, and the path is a smooth curve without sharp turns.
Shorter path and more uniform magnetic resistance
The benefits brought by this geometric feature are: the path of magnetic field lines inside the magnetic plate is shorter and smoother, the distribution of local magnetic resistance is more uniform, and the effective area of the magnetic plate is closer to its geometric area. The effective magnetic permeability of circular magnetic plates is usually higher than that of square magnetic plates with the same material and coverage.
Edge effect is more uniform
The four corners of the square are the concentrated locations of magnetic flux leakage - the geometric discontinuity at the corners makes it easier for magnetic field lines to "overflow" into the metal from the corners. The curvature of the circular edge is continuous, and the magnetic flux leakage is evenly distributed along the edge without concentrated leakage points.
Runway shape: elongates the magnetic flux path for a long strip antenna
Circular magnetic plates are suitable for circular or nearly circular tag antennas. But many UHF tag antennas are elongated dipoles, with the long axis of the antenna much larger than the short axis. For this type of antenna, the coverage of circular magnetic plates in the long axis direction may be insufficient, and magnetic field lines cannot be wound back at both ends of the long axis.
The track shaped (elongated) magnetic plate elongates the magnetic flux path in the long axis direction, providing more ample space for the long strip antenna to orbit. Both ends are still circular arcs, avoiding the problem of right angles in squares; The middle is a straight edge, providing a continuous magnetic flux path along the long axis direction. The long/short axis ratio of the runway shape should match the aspect ratio of the tag antenna - if the ratio is too small, the coverage in the long axis direction is insufficient; If the proportion is too large, the magnetic flux path in the short axis direction is "squeezed", and the edge effect becomes concentrated in the short axis direction.
Shape and antenna matching: not just 'covering'
The shape of the magnetic plate not only affects the magnetic resistance, but also affects the resonance conditions of the tag antenna. The edge of the magnetic plate is the boundary of sudden changes in dielectric constant and magnetic permeability, which will alter the field distribution around the antenna. The right angled boundary of the square magnetic plate introduces four "corner points" in the near-field of the antenna, each of which is a local impedance discontinuity; The continuous boundary of the circular magnetic plate makes the field distribution smoother, and the offset of the antenna resonance frequency smaller and more predictable. For narrowband applications like NFC, the difference in resonance offset caused by shape may directly affect the read distance. The resonant offset of a circle is usually smaller than that of a square because the dielectric loading on the continuous boundary is more uniform.
Selection and judgment of magnetic plate shape based on "shape x magnetic flux path x antenna"
Firstly, prioritize selecting the contour of the magnetic flux sheet that matches the shape of the tag antenna, rather than the default square shape
Circular antennas are equipped with circular magnetic plates, elongated dipoles are equipped with track shaped magnetic plates, and square antennas can be equipped with rectangular magnetic plates but need to accept a decrease in magnetic flux utilization in corner areas. The goal of shape matching is to make the path of magnetic field lines as smooth and short as possible.
Secondly, when space permits, use circular or runway shapes instead of square shapes, even if the die-cutting cost is slightly higher
The die-cutting of circular and runway shapes requires higher tool precision and discharge design, and the cost may be slightly higher than that of square shapes. But the effective magnetic permeability improvement and read distance recovery rate improvement brought by shape are usually worth it in harsh metal environments. If space is limited and square shapes must be used, performance degradation in corner areas should be accepted, and larger edge margins should be reserved near the corners.
Thirdly, the aspect ratio of the runway shape should match the aspect ratio of the tag antenna, rather than being arbitrarily elongated
The long axis direction of the runway shape provides space for magnetic flux to circulate, while the short axis direction provides edge constraints. When the aspect ratio deviates too much from the antenna ratio, the edge effect in the short axis direction will become concentrated, and the magnetic flux path in the long axis direction may also exceed actual needs. When selecting, the proportion of the runway shape should be determined based on the antenna aspect ratio, rather than the intuition of 'the longer the better'.
Fourth, compare the recovery rate of reading distance for different shapes during the sample stage, rather than just comparing material parameters
Cut the same roll of material into circular, square, and track shapes, and stick them on the same label to measure the reading distance. The difference in reading distance between the three shapes is a direct measure of the shape effect. If the difference is significant, it indicates that shape selection has a substantial impact on system performance and should not be ignored as a "die-cutting detail".
Advanced Institute (Shenzhen) Technology Co., LtdThe anti metal magnetic sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, supporting circular, square, track shaped, and customized special-shaped die-cutting according to the contour of the tag antenna.
The product can provide comparative data on effective magnetic permeability and read distance recovery rate under different shapes, helping engineers manage the shape as an independent variable in magnetic sheet design beyond material parameters. For more technical information, please visithttp://www.avanzado.cn/.