Why Is It Difficult to Shield Low-Frequency Magnetic Fields

1. Introduction
In scenarios such as electronic equipment R&D, military classified system testing, and precision instrument calibration, low-frequency magnetic field interference (typically referring to magnetic fields below 30Hz) can seriously affect equipment performance and test accuracy. For example, when an oscilloscope measures weak electrical signals, external low-frequency magnetic fields can cause waveform baseline drift and signal distortion, directly reducing the reliability of test data. However, shielding low-frequency magnetic fields is far more difficult than shielding high-frequency electromagnetic waves, primarily due to differences in their physical properties and propagation laws. Traditional shielding materials have limited attenuation effects on low-frequency magnetic fields, making it difficult to meet the strict requirements of high-precision testing for the electromagnetic environment. Targeting this pain point, the LISUN SDR-2000B/SDR-800S EMI Shielding Rooms are optimally designed based on multiple international and domestic standards. Through collaborative work with the EMI-9KA/EMI-9KB system, they achieve comprehensive isolation of electromagnetic interference including low-frequency magnetic fields, ensuring the stable operation of precision instruments such as oscilloscopes. In-depth analysis of the difficulties in low-frequency magnetic field shielding and the solutions provided by the LISUN SDR series shielding rooms is of great practical significance for improving the accuracy of EMC testing.

2. Why Is It Difficult to Shield Low-Frequency Magnetic Fields: Physical Principles and Technical Bottlenecks
2.1 Core Physical Properties of Low-Frequency Magnetic Fields
The challenge of low-frequency magnetic field shielding stems from its unique physical nature: on the one hand, low-frequency magnetic fields have extremely long wavelengths (e.g., the wavelength of a 50Hz power frequency magnetic field is approximately 6000km), much longer than the size of conventional shielding rooms. This causes the magnetic field to act on the shielding material in a “penetrating” rather than “reflecting” manner, rendering the “reflection loss” mechanism relied upon by traditional high-frequency shielding almost ineffective. On the other hand, the skin effect of low-frequency magnetic fields is extremely weak, with current only able to penetrate deep into the conductor (e.g., the skin depth of a 50Hz magnetic field in copper is approximately 9.3mm). Ordinary thin metal plates cannot effectively attenuate magnetic field energy through “absorption loss”. In addition, low-frequency magnetic field sources are mostly industrial equipment (such as motors, transformers), power lines, etc., which have high magnetic field strength and wide distribution, further increasing shielding difficulty.
2.2 Limitations of Traditional Shielding Methods
Traditional electromagnetic shielding mainly uses high-conductivity materials (such as copper, aluminum), but their low magnetic permeability results in limited attenuation capacity for low-frequency magnetic fields. While high-permeability materials (such as permalloy) can improve magnetic field absorption, they suffer from high cost, easy saturation, and poor mechanical strength, making them difficult to apply in the manufacture of large shielding rooms. At the same time, gaps, doors, windows, interfaces and other parts of the shielding room are prone to forming “magnetic field leakage channels”. Low-frequency magnetic fields can penetrate the shielding body through these weak links, leading to a decline in overall shielding effectiveness. For example, gaps in ordinary shielding room doors can increase low-frequency magnetic field leakage by tens of times, seriously affecting shielding performance.

3. Low-Frequency Magnetic Field Shielding Design of LISUN SDR-2000B/SDR-800S Shielding Rooms
3.1 Design Basis and Core Standards
The LISUN SDR-2000B/SDR-800S EMI Shielding Rooms are strictly designed and manufactured in accordance with standards including GB/T12190 (Measurement Methods for Shielding Effectiveness of Electromagnetic Shielding Rooms), GJB5792 (Classification and Measurement Methods for Electromagnetic Shielding Bodies of Military Classified Information Systems), IEEE std299 (Standard for Measuring the Effectiveness of Electromagnetic Shielding Enclosures), and EN50147. Targeting the particularity of low-frequency magnetic field shielding, special optimizations have been made in material selection, structural sealing, filtering systems, etc., to ensure that the shielding effectiveness meets the high-precision testing requirements of military and civilian applications.
3.2 Key Structural and Material Design
Optimization of Shielding Materials: The shielding room shell is made of 2mm galvanized cold-rolled steel plate, which has both high conductivity and magnetic permeability. The absorption loss of low-frequency magnetic fields is improved by increasing the material thickness. The internal floor adopts a composite structure of “10mm wood + 2mm galvanized steel plate”, further enhancing low-frequency magnetic field blocking capacity and preventing magnetic field penetration through the ground.
Sealing and Interface Design: The shielding room door adopts a precision structure of 0.9×1.7m (SDR-2000B) and 0.6×0.6m (SDR-800S), equipped with conductive gasket sealing to minimize gap leakage. For interfaces such as power supply and internet lines, 30A/220V power filters and RJ-45 internet line filters (SDR-5000B/SDR-2000B) are configured to attenuate interference signals coupled by low-frequency magnetic fields through filtering, ensuring no magnetic field leakage at the interfaces.
System Collaboration Mechanism: The shielding room works collaboratively with the EMI-9KA/EMI-9KB system to form a comprehensive anti-interference system of “shielding + filtering + monitoring”. For the SDR-2000B, the EMI receiver, computer, DUT (Device Under Test), and tester are all placed inside the shielding room to directly isolate external low-frequency magnetic field interference. The SDR-800S is connected to an external EMI receiver through a BNC interface to ensure that the signal transmission process is not affected by low-frequency magnetic fields, guaranteeing the accuracy of test data.
3.3 Analysis of Core Performance Parameters
The LISUN SDR series EMI Shielding Rooms cover three core models, whose structure and performance parameters are optimally designed for different application scenarios. The specific parameters are shown in the following table:

As can be seen from the table, all three models adopt a unified high-permeability shielding material and filtering system. The core differences lie in size and coordination method: the SDR-5000B is suitable for large-scale equipment and multi-person operation scenarios, equipped with a three-loop antenna to enhance signal reception capacity; the SDR-2000B is adapted for small and medium-sized equipment testing, balancing flexibility and shielding effectiveness; the SDR-800S is a compact design, suitable for precise testing of small DUTs, achieving internal and external signal isolation through a BNC interface.

4. Application of Oscilloscopes in Shielding Rooms and Verification of Shielding Effectiveness
4.1 Interference-Free Testing Scenarios for Oscilloscopes
As a core instrument in EMC testing, the measurement accuracy of oscilloscopes is highly sensitive to the electromagnetic environment. In LISUN SDR series shielding rooms, oscilloscopes can be used for two types of key tests: first, EMI emission testing of DUTs. The shielding room isolates external low-frequency magnetic field interference, ensuring that the signals captured by the oscilloscope only come from the DUT, avoiding baseline drift and waveform distortion. Second, verification of the shielding room’s own shielding effectiveness. The oscilloscope measures low-frequency magnetic field coupled signals before and after shielding to quantitatively evaluate the shielding effect. For example, when testing a military classified device, the device and oscilloscope are placed in an SDR-2000B shielding room. The EMI-9KA system monitors the device’s electromagnetic emissions, and the oscilloscope synchronously collects signal waveforms. Since the shielding room effectively isolates external power frequency magnetic field (50Hz) interference, the waveform baseline is stable, and the signal amplitude measurement error is controlled within ±1%.
4.2 Quantitative Verification of Shielding Effectiveness
The “signal comparison method” is used to verify the shielding effect of the shielding room on low-frequency magnetic fields: a 50Hz low-frequency magnetic field generator is placed outside the shielding room, and the oscilloscope measures the amplitude of the external magnetic field signal (recorded as V₁). The oscilloscope is moved into the shielding room, the generator position is kept unchanged, and the indoor signal amplitude is measured (recorded as V₂). The shielding effectiveness SE = 20lg(V₁/V₂). Test results show that the shielding effectiveness of the LISUN SDR-2000B at 50Hz can reach more than 80dB, and the SDR-800S can reach more than 75dB, far exceeding the performance indicators of conventional shielding equipment, proving its efficient blocking capacity for low-frequency magnetic fields.

5. Typical Application Scenarios and Practical Value
5.1 Military Classified Equipment Testing
Military classified information systems have extremely high requirements for electromagnetic shielding. Low-frequency magnetic field interference may cause data transmission leakage or equipment misjudgment. The LISUN SDR-2000B shielding room is designed in accordance with GJB5792 standards, providing a military-standard electromagnetic environment for EMC testing of classified equipment. Oscilloscopes in the room can accurately measure the low-frequency magnetic field emission values of the equipment, ensuring that the equipment meets confidentiality requirements.
5.2 Calibration of Precision Electronic Instruments
During the calibration of precision instruments such as oscilloscopes and spectrum analyzers, low-frequency magnetic field interference must be avoided to ensure calibration accuracy. The compact SDR-800S shielding room can place both the calibration standard source and the oscilloscope indoors. The EMI-9KB system monitors environmental interference, ensuring the accuracy and reliability of calibration data.
5.3 EMC Testing of Industrial Equipment
Industrial equipment such as motors and transformers generate strong low-frequency magnetic fields during operation. Their EMI emission testing must be carried out in an environment free of external interference. The SDR-5000B shielding room can accommodate large industrial equipment and test systems. Oscilloscopes in the room capture electromagnetic signals during equipment operation, and combined with EMI receivers to analyze interference sources, providing data support for the optimization of equipment EMC.

6. Conclusion
The physical properties of low-frequency magnetic fields, such as strong penetration and weak skin effect, make them a technical challenge in the field of electromagnetic shielding, and traditional shielding methods are difficult to achieve efficient isolation. Based on core standards such as GB/T12190 and IEEE std299, the LISUN SDR-2000B/SDR-800S EMI Shielding Rooms have built a comprehensive low-frequency magnetic field shielding system through high-permeability material selection, precision sealed structure design, and collaborative work with the EMI-9KA/EMI-9KB system, effectively solving the shielding dilemma. The application of oscilloscopes in the shielding room not only verifies the shielding effectiveness but also reflects the role of the shielding room in ensuring the testing accuracy of precision instruments. With the continuous improvement of electronic equipment’s requirements for the electromagnetic environment, the LISUN SDR series shielding rooms, with their excellent low-frequency magnetic field shielding performance, flexible model configuration, and customization capabilities, will play an important role in military, civilian, industrial and other fields, providing strong support for the development of EMC testing technology.