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The most direct action for mobile EMC engineers to solve radiation scattering or cavity resonance problems is often to "stick a piece of absorbing material nearby". This action usually lowers the noise peak, but sometimes it introduces a new problem in OTA testing: the antenna efficiency decreases.
The working principle of absorbing materials is to convert electromagnetic energy into heat. This mechanism itself does not distinguish whether the energy comes from the noise source or the antenna. A material with an absorption rate exceeding 90% at 24 GHz, if attached to the near-field range of an antenna, will also absorb a significant proportion of the antenna's radiated energy. The noise has been suppressed, and the useful signal has also been suppressed.
The bidirectional effect of absorbing materials on antennas: absorption and loading
The impact of absorbing materials on antennas has two levels, and they often occur simultaneously, but the mechanisms are different.
The first level is absorption loss.The higher the absorption rate of absorbing materials in the target frequency band, the greater the proportion of electromagnetic energy entering the material that is converted into heat. If the material is located on the near-field radiation path of the antenna, a portion of the energy radiated by the antenna will enter the absorbing layer and be dissipated. The definition of antenna efficiency is the ratio of radiated power to input power, and the part dissipated by the absorbing layer is directly included in the loss, resulting in a decrease in efficiency. In the millimeter wave frequency band, the antenna gain itself is high and the beam is narrow, but the near-field energy is concentrated. If the absorbing material is too close to the antenna aperture, the impact of absorption loss will be amplified.
The second level is dielectric and magnetic loading. Absorbing materials usually have a certain dielectric constant and magnetic permeability, and even if their loss tangent is not high in the target frequency band, their energy storage component will still change the field distribution around the antenna. For PIFA, monopole, or millimeter wave AiP antennas commonly found in mobile phones, the resonant frequency is sensitive to the dielectric constant of the surrounding medium. The absorption plate is attached near the antenna, which is equivalent to introducing an additional dielectric layer in the near-field region of the antenna. The resonant frequency may shift towards lower frequencies, and the real and imaginary parts of the input impedance may change simultaneously, resulting in a mismatch of the originally tuned matching network. This offset does not manifest as "signal absorption", but rather as "antenna non resonance", which is also reflected as a decrease in efficiency in OTA testing, but the reason and absorption loss are completely different.
Space constraints inside mobile phones: Why are antennas and noise sources so close together
In the era of feature phones, the physical distance between the antenna and the motherboard noise source is relatively sufficient, and absorbing materials can be placed in a position that is "far enough from the antenna and close enough to the noise source". The stacking density of smartphones has changed this premise.
Taking the millimeter wave AiP module as an example, the antenna array and RF front-end chip are packaged on the same substrate, and the switch noise of the chip and the radiation aperture of the antenna may only be separated by a few millimeters. In this layout, if the absorbing material is placed too close to the antenna, it will absorb the energy of the antenna; If it is placed too far away from the noise source and cannot effectively suppress the noise path coupled to the antenna. Engineers need to find a position within a very narrow spatial window that only affects noise and not the antenna. The existence of this window depends on whether the noise path and antenna radiation path are spatially separable. If the noise is coupled to the antenna through cavity resonance, the absorbing material attached to the inner wall of the cavity may be effective and not directly block the antenna aperture; If the noise is transmitted to the antenna in the form of surface waves through FPC or metal frame, the absorbing material needs to be attached to the transmission path, which may happen to pass through the near-field region of the antenna. At this point, "selectivity" cannot be achieved solely by location, but also requires the material itself to have frequency or spatial selectivity.
Three engineering methods for spatially selective absorption of waves
The first type is patterned mounting
Do not stick the absorbing material as a continuous piece, but instead, based on the distribution of the antenna near-field, make local mounting or openings in areas where the noise path passes but the antenna radiation field is weak. For example, leaving a blank space directly above the antenna aperture and only placing absorbing plates near the aperture edge or feeding point. This scheme requires data on the near-field distribution of the antenna as support, and requires high mounting accuracy. However, it can reduce the impact on antenna efficiency without changing the material system.
The second type is the composite of frequency selective surface (FSS) and absorbing layer
Add a frequency selective structure on the surface of the absorbing layer, so that the material exhibits absorption in the noise frequency band and high impedance or transmission in the antenna operating frequency band. For scenarios where the antenna and noise source frequencies are different (such as Wi Fi noise coupled to millimeter wave antennas), FSS can make the absorbing layer "noise effective and signal transparent". The cost of this solution is an increase in thickness and process complexity, and the design of FSS needs to be precisely matched with the antenna operating frequency band.
The third method is to use thin absorbing wave plates with low dielectric constant and low loss, and strictly control their distance from the antenna
If absorption is unavoidable, at least the loading effect should be controllable. A low dielectric constant matrix can reduce the pulling force on the resonant frequency of the antenna, while thinning shortens the coupling path of near-field energy. When selecting, the dielectric constant and loss tangent of the absorbing plate in the target antenna frequency band should be confirmed, not just the absorption rate in the noise frequency band.
The following table summarizes the applicable conditions and costs of three spatial selectivity methods:
Selection judgment based on antenna efficiency cost
Firstly, confirm the insertion loss or absorption rate of the absorbing material in the target antenna operating frequency band, rather than just looking at the absorption peak in the noise frequency band
A material with an absorption rate of 90% at 24 GHz, if the antenna also operates at 24 GHz, sticking it to the near-field of the antenna means that 90% of the antenna's radiated energy may be dissipated. When selecting, suppliers should be required to provide insertion loss data of materials in the antenna operating frequency band, and confirm the spatial relationship between the mounting position and the near-field of the antenna. If it is impossible to avoid the near-field, priority should be given to materials with lower insertion loss in the antenna frequency band, rather than materials with the strongest absorption in the noise frequency band.
Secondly, evaluate the influence of the dielectric constant of the absorbing material on the resonant frequency of the antenna
Even if the material has low losses in the antenna frequency band, its dielectric constant will still pull the resonant frequency. For narrowband antennas (such as NFC and certain Sub-6 GHz frequency bands), a frequency offset of several tens of MHz may lead to a deterioration in standing wave ratio. When selecting, the real part of the dielectric constant of the absorbing plate in the target frequency band should be confirmed, and the loading effect of the absorbing plate should be included in simulation or measurement during the antenna matching design stage.
Thirdly, when space permits, priority should be given to patterned or FSS schemes rather than using a continuous piece of material for "full coverage"
Continuous installation is simple, but it comes at the highest cost of antenna efficiency. If the spatial relationship between the antenna and the noise source is complex, patterned mounting can confine the absorbing material to the noise path, reducing the obstruction of the antenna aperture. The FSS scheme provides selectivity in the frequency dimension and is suitable for scenarios where noise and signal frequency bands are separated. Both solutions require the supplier to provide design support that matches the antenna structure, rather than just providing standard sheets.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of mobile phone absorbing materials covers thin absorbing plates, fine pore absorbing foam, and patterned die-cutting solutions.
The thickness of the thin absorbing wave plate can be reduced to the order of 0.1 mm, supporting low dielectric constant substrate options and providing dielectric constant and insertion loss data for the target frequency band; Fine pore absorbing foam facing 24-43.5 GHz cavity resonance suppression; The patterned die-cutting scheme can be designed for local mounting based on the near-field distribution of the antenna.
The joint evaluation of product support and antenna structure helps engineers find a quantifiable balance between noise suppression and antenna efficiency. For more technical information, please visithttp://www.avanzado.cn/.