User Quote: “If I put a mmWave radar behind a wall, will it still detect motion or breathing?” — Reddit r/AskElectronics
Introduction: Why People Care About Through-Wall Capability
The idea of “seeing through walls” has always fascinated both engineers and the general public. From Hollywood spy movies to real-world search-and-rescue scenarios, people want to know whether radar can actually detect motion or even vital signs through a wall. With the increasing availability of millimeter-wave (mmWave) radar sensors, especially at 60 GHz and 77 GHz, many are asking: If I place an mmWave radar behind a wall, can it still track motion or detect breathing?
The short answer is: it depends. mmWave radar does have some through-wall capability, but its effectiveness varies significantly with wall material, thickness, and environmental conditions. To truly understand what works and what doesn’t, we need to dive into the physics of wave propagation, examine real-world scenarios, and consider both applications and limitations.
The Physics of Wave Penetration
Electromagnetic waves interact with obstacles in three primary ways: reflection, absorption, and transmission. When a radar signal meets a wall, part of it is reflected back, part is absorbed by the material, and part passes through—attenuated.
Material Absorption
Different wall materials behave differently:
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Drywall and plywood allow a significant portion of mmWave energy to pass through with relatively low attenuation.
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Brick and concrete absorb and scatter much more, often reducing signal strength by more than 20–30 dB.
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Metal-reinforced walls effectively block most signals due to strong reflection and absorption.
A key reference here is the FCC technical reports on propagation loss, which consistently show higher attenuation for higher-frequency signals through dense building materials.
Frequency Dependence
The higher the frequency, the shorter the wavelength, and the less capable it is of penetrating thick or dense materials. mmWave radars (24–81 GHz) are much more sensitive to material absorption than lower-frequency radars (e.g., UHF or S-band). This is why military ground-penetrating radars often operate at lower frequencies: they trade resolution for penetration.
That said, mmWave has its advantage—extremely high resolution. Even if penetration is reduced, the fine detection of small motions (like chest movement from breathing) is still possible when signal strength is sufficient.
Real-World Performance and Limitations
Let’s translate the theory into practice:
Drywall vs. Concrete
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Drywall (common in U.S. houses): mmWave radar can detect walking motion, hand gestures, and sometimes breathing signals, provided the wall is not too thick. Many academic papers, including IEEE Xplore studies on 60 GHz sensors, confirm viable motion detection through drywall.
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Concrete walls (common in Asia and Europe): detection range drops dramatically. A concrete wall can reduce signal by up to 30 dB, making it nearly impossible for consumer-grade mmWave modules to detect fine movements like breathing.
Attenuation and Multipath Effects
Even when signals penetrate, they weaken and mix with multipath reflections. Multipath can sometimes help (by bouncing signals around obstacles), but it usually adds noise, reducing accuracy. Engineers often use advanced algorithms—like MIMO beamforming and FMCW radar signal processing—to separate true motion signals from noise.
Potential Applications
Despite these limitations, mmWave through-wall detection is not just a curiosity; it has practical applications:
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Search and Rescue
In post-earthquake or fire scenarios, first responders may use radar to detect survivors trapped behind debris. While mmWave cannot penetrate thick concrete slabs, it can work through lighter obstructions, identifying subtle breathing or motion signals. -
Indoor Monitoring
mmWave sensors are increasingly used for non-contact vital sign monitoring. For example, a radar installed in one room may detect movement or respiration of a patient in the next, if the wall is not too dense. Compared with cameras, radar provides higher privacy protection since it does not capture images. -
Industrial and Security Systems
In certain industrial environments, radar can monitor occupancy or detect unauthorized presence behind partitions. Combined with AI signal processing, this opens new use cases in smart buildings and security monitoring.
For further exploration of mmWave use cases, you can see Linpowave’s radar applications.
Legal and Ethical Considerations
Through-wall detection is powerful, but also sensitive.
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Regulatory restrictions: Both the FCC in the U.S. and ETSI in Europe regulate the maximum power and frequency bands for radar sensors. Developers must ensure compliance before deploying through-wall systems.
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Privacy concerns: Detecting human presence without consent raises ethical questions. Unlike standard occupancy sensors, through-wall detection is less visible and could be misused. Companies must adopt strict data protection measures and ensure transparency in deployment.
Frequently Asked Questions (FAQ)
1. Can mmWave radar really detect breathing through a wall?
Yes, under the right conditions. If the wall is thin and made of low-density material such as drywall, mmWave radar can detect micro-motions like chest movement during breathing. However, thick or dense materials such as concrete or brick make breathing detection nearly impossible due to high signal attenuation.
2. How far can mmWave radar detect motion through walls?
The effective range depends on both the radar’s transmit power and the wall material. Through drywall, detection is typically possible within 3–5 meters. Through concrete, the range drops dramatically, often to less than 1 meter—or no detection at all.
3. Is through-wall radar detection safe for humans?
Yes. Consumer and industrial mmWave radar devices are regulated by agencies like the FCC and ETSI. They operate at very low power levels (typically milliwatts), which are considered safe for continuous exposure.
4. What makes mmWave radar better than cameras for indoor monitoring?
Radar offers non-visual sensing. It does not capture identifiable images, making it much more privacy-friendly. Additionally, radar performs well in low light or obscured environments where cameras fail. For example, Linpowave’s radar solutions emphasize human presence detection without invading privacy.
5. Are there legal restrictions on using through-wall radar?
Yes. In most countries, you cannot legally use through-wall radar for surveillance without consent. Regulations cover both the permitted frequency bands and privacy protection. Developers and users must ensure compliance with national laws before deploying any through-wall application.
6. Can mmWave radar replace thermal imaging for search and rescue?
Not directly. Thermal imaging works well for detecting body heat through smoke or thin barriers, while mmWave radar excels at detecting motion—even tiny chest movements. In practice, they are complementary: thermal cameras provide fast detection of heat sources, while radar can confirm life signs behind debris.
Conclusion: What Works and What Doesn’t
So, can mmWave radar see through walls? The nuanced answer is:
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Yes, it works through thin, low-density walls like drywall and plywood. Motion and even breathing can be detected in controlled conditions.
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No, it struggles or fails through thick, dense materials like reinforced concrete or metal. The signal loss is simply too high.
The technology is not a “magic spy tool,” but rather a specialized sensing method with clear strengths and boundaries. By understanding its physics and real-world limitations, engineers and users can better apply mmWave radar where it truly makes sense—whether in healthcare monitoring, disaster response, or smart building automation.