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In the design review of wireless charging receivers, the selection of magnetic chips usually revolves around three indicators: wireless charging efficiency, NFC reading distance, and temperature rise. If these three criteria are met, the magnetic chip scheme will be approved. GPS sensitivity, Wi Fi throughput, and cellular reception level are often not on the same evaluation form.
But the antenna inside the phone does not exist in isolation. The GPS antenna, Wi Fi antenna, cellular antenna, and wireless charging coil share the back and middle frame area of the body. After the magnetic patch is attached, it not only changes the magnetic field around the wireless charging coil and NFC antenna, but also changes the electromagnetic environment of all adjacent antennas. This kind of impact may not necessarily appear in the specification book of the magnetic sheet, but it will appear in the RF test report of the entire machine.
Triple effects of magnetic plates on adjacent antennas: magnetic loading, metal coupling, and near-field redistribution
There are three main paths through which the receiving end magnetic patch affects adjacent antennas.
The first is magnetic loading
After the high magnetic permeability material of the magnetic sheet enters the near-field of the antenna, it will change the magnetic flux distribution around the antenna, which is equivalent to adding an additional inductance component to the antenna inductance. This is similar to the mechanism by which magnetic tiles affect NFC antennas, except that the object is replaced with a GPS or Wi Fi antenna. If the magnetic disk is too close to the antenna, the resonant frequency of the antenna may shift towards lower frequencies, causing a mismatch in the originally tuned matching network and a decrease in efficiency. For narrowband antennas (such as GPS at 1.575 GHz), a frequency offset of several tens of MHz is sufficient to deteriorate the standing wave ratio.
The second is metal coupling
Magnetic sheets typically come with a metal backing (copper foil or metal layer) for shielding and structural support. This layer of metal appears as an additional conductor to the neighboring antenna. If the distance between the metal edge of the magnetic plate and the antenna radiator is too close, the metal edge may become a parasitic coupling path in the near-field of the antenna, changing the radiation pattern or input impedance of the antenna. In the millimeter wave frequency band, the scattering effect of metal edges is more significant.
The third is near-field redistribution
The high magnetic permeability of the magnetic sheet will "attract" magnetic field lines to concentrate on itself, which means that the magnetic flux distribution in the near-field of the antenna is redirected. For antennas with magnetic field components as the main component (such as NFC and some low-frequency cellular antennas), this redistribution may change the effective radiation volume of the antenna; For antennas dominated by electric field components, the impact is relatively small, but the dielectric constant of the magnetic sheet still participates in the near-field distribution.
Antennas in different frequency bands are affected to varying degrees
The impact of magnetic plates on antennas in different frequency bands is not equal.
GPS antenna
Working at 1.575 GHz (L1) and 1.227 GHz (L5), it belongs to narrowband circularly polarized antennas and is highly sensitive to the surrounding dielectric and conductor environments. The magnetic loading and metal coupling of the magnetic sheet may deteriorate the axial ratio and decrease the gain of the GPS antenna. The GPS signal itself is extremely weak (about -130 dBm), and if the antenna efficiency decreases by a few dB, it may prolong the positioning time or decrease the positioning accuracy.
Wi Fi antenna
Working at 2.4 GHz and 5 GHz, the bandwidth is relatively wide and the tolerance for frequency offset is higher than GPS. However, if the metal backing of the magnetic sheet is too close to the Wi Fi antenna, it may still reduce antenna efficiency through near-field coupling. In MIMO configuration, the impact of magnetic tiles on different antenna units is not consistent, and it may also change the correlation between antennas, affecting MIMO throughput.
Cellular antenna (Sub-6 GHz)
Usually located in the middle frame or bottom, the physical distance from the wireless charging coil is relatively far, and the degree of impact is usually less than GPS and Wi Fi. However, in some compact stacked models, the edge of the magnetic sheet may extend to the near-field region of the cellular antenna, and the impact still needs to be evaluated.
Spatial relationships determine the degree of influence: distance, area, and direction
The degree of influence of a magnetic patch on adjacent antennas depends on three spatial variables: distance, area, and direction.
distance
The most direct factor. The closer the distance between the edge of the magnetic plate and the antenna radiator, the stronger the magnetic loading and metal coupling. A common practice in engineering is to maintain a certain "safe distance" between the magnet and GPS/Wi Fi antenna, but there is no unified standard for this distance, which depends on the specific design of the antenna, the magnetic permeability of the magnet, and the size of the metal backing. In compact stacked models, the safety distance may be compressed to a few millimeters, and the influence of magnetic plates needs to be confirmed through simulation or actual measurement.
Area
It determines the range that the magnetic disk occupies in the near-field. The larger the planar size of the magnetic sheet, the more antennas it covers or approaches, and the wider the potential impact range. The rule of 90% coverage is for wireless charging coils, but it does not guarantee that the magnetic patch is also "safe" for adjacent antennas. The area design of magnetic chips needs to find a balance between wireless charging efficiency and compatibility with adjacent antennas.
direction
It determines which parameter of the antenna is affected by the magnetic patch. When the magnetic patch is located on the back of the antenna (on the ground plane side), it mainly affects the grounding conditions of the antenna; When the magnetic patch is located on the side of the antenna, it mainly affects the near-field distribution of the antenna; When the magnetic plate is located in front of the radiation direction of the antenna, it may directly block the radiation. The relative relationship between the mounting position of the magnetic patch and the antenna layout needs to be considered in the structural design stage.
Why does' magnetic selection 'need to include adjacent antennas in the validation
The specification sheet of the magnetic sheet only describes the magnetic properties and losses of the material itself, and does not describe the sensitivity of the adjacent antenna after being attached. This means that even if the magnetic chip meets both wireless charging and NFC indicators, its impact on GPS/Wi Fi antennas is still an unverified variable.
Even more troublesome is that this impact is not visible in incoming inspection and separate testing of magnetic plates. It is only exposed during the RF testing of the entire machine, and at this time, the magnetic chip scheme has been locked, with limited adjustment space. If GPS sensitivity decreases during overall testing, engineers' first reaction is often to check the GPS antenna itself or matching network, rather than necessarily tracing it back to the magnetic patch. The influence of magnetic disks is therefore easily underestimated or wrongly attributed.
Selection and judgment of magnetic plates at the receiving end based on the spatial relationship between antennas and magnetic plates
Firstly, before selecting the magnetic patch, draw a spatial relationship diagram between the patch and all adjacent antennas
Confirm the relative positions of GPS, Wi Fi, cellular antennas, and wireless charging coils, as well as whether the planar dimensions of the magnetic patch will extend into the near-field area of these antennas. If the distance between the edge of the magnetic sheet and the antenna radiator is less than the near-field radius of the antenna, the influence of the magnetic sheet needs to be evaluated during the design phase, rather than waiting until the entire machine is tested before investigating.
Secondly, use the RF indicators of adjacent antennas as independent verification items for the magnetic chip scheme
In the sample stage, compare the differences in GPS sensitivity, Wi Fi throughput, and cellular reception level between "with magnetic chips" and "without magnetic chips" conditions. If the difference exceeds the acceptable range, it is necessary to adjust the size, position, or material system of the magnetic sheet, rather than just optimizing the wireless charging efficiency.
Thirdly, in the selection of material systems, consider the "influence radius" of magnetic sheets on adjacent antennas
The influence range of high permeability materials on the near-field is usually larger, while the influence radius of low permeability or thin magnetic sheets is relatively smaller. If the magnetic disk must be close to the GPS or Wi Fi antenna, choosing a solution with moderate magnetic permeability and a smaller metal backing area may be more beneficial for overall RF compatibility than pursuing the highest magnetic permeability.
Fourth, reserve a transition zone between the magnetic sheet and the antenna in the stacking design
If space permits, leaving a transition zone between the edge of the magnetic plate and the antenna radiator that is free of metal and high permeability materials can weaken the strength of magnetic loading and metal coupling. There is no unified standard for the width of the transition zone, which needs to be determined based on the antenna frequency band, magnetic characteristics, and overall structure.
Advanced Institute (Shenzhen) Technology Co., LtdThe receiving end magnetic sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, and can provide different magnetic permeability levels and plane sizes based on the location and frequency band of adjacent antennas. It also supports comparative evaluation of RF indicators of adjacent antennas under "magnetic sheet/non-magnetic sheet" conditions during the sample stage.
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