ADAS (Advanced Drive Assistance Systems) Automobile Guidance System RF Frequencies: Do They Penetrate Glass?


Clients often ask about automobile RF. What are the frequencies, and is this harmful? This offering will share the RF frequencies for the various systems, their ability to penetrate through glass, and more.

Common RF Frequencies in ADAS Guidance Systems

1. Automotive Radar Systems (primary RF guidance technology)

  • 24 GHz band: Older short- to mid-range radar (used for adaptive cruise control, blind-spot monitoring).

  • 77–81 GHz band: Current industry standard for long-range radar, adaptive cruise, collision avoidance, lane keeping, and autonomous emergency braking.

  • 79 GHz: Increasingly used for high-resolution short-range radar (parking assist, cross-traffic alert).

These radar systems are key in object detection, speed estimation, and distance measurement.

2. LiDAR Systems (not RF, but near-infrared light)

  • Typically, 905 nm or 1550 nm wavelengths (infrared) are not part of the RF spectrum, but they are sometimes confused because they also provide distance and guidance data.

3. Ultrasonic Sensors (low-frequency, non-RF)

  • Typically, 40–60 kHz sound waves are used for short-range tasks like parking sensors, not RF.

4. Vehicle-to-Vehicle (V2V) / Vehicle-to-Infrastructure (V2I) Communication

  • 5.9 GHz DSRC (Dedicated Short-Range Communications) → older standard.

  • 5.9–7.1 GHz C-V2X (Cellular Vehicle-to-Everything) → newer systems using cellular LTE or 5G bands for networked driving and cooperative safety.

Key Point

The primary RF frequencies for guidance (not communication) are:

  • 77–81 GHz automotive radar, especially for advanced functions like autonomous driving, collision avoidance, and lane keeping.

Can 77–81 GHz RF Signals Penetrate Automobile Window Glass?


Penetration is minimal and often effectively blocked, especially by modern coated or laminated automotive glass. At 77–81 GHz (the primary automotive radar band):

  1. Wavelength: ~3.7–3.9 mm → very small, highly sensitive to surface roughness, material interfaces, and especially metallic layers.

  2. Standard clear glass (uncoated) allows some mmWave penetration but with high attenuation—typically 20–40 dB loss, depending on thickness. (FYI, 20 dB is a 100-times reduction, and 40 dB is a 10,000-times reduction.)

  3. Laminated windshields (with polymer layers) or tempered side windows add more loss through scattering and absorption.

  4. Metallic or Low-E coatings (standard for UV blocking, heating, or energy efficiency) create near-total reflection at these frequencies, effectively blocking the signal.

Why This Matters in ADAS

  • Automotive radar at 77–81 GHz is typically mounted outside the glass, behind bumper plastic, in exterior mirror housings, or in specialized radar-transparent panels, not behind windows.

  • This is because:

    • Radar signals travel in straight lines and need a clear path to detect vehicles, pedestrians, and obstacles.

    • Even small reflections or phase shifts from passing through glass degrade radar imaging and distance estimation.

Most automotive radars are intentionally placed outside glass surfaces to avoid these losses. Trying to run 77–81 GHz signals through vehicle glass is usually not practical or reliable.

At these millimeter-wave frequencies, signal penetration through automotive window glass is severely limited due to:

  1. High frequency = short wavelength (~4 mm)

    • Highly sensitive to dielectric loss and reflections.

    • Much attenuation through any material, especially glass, is greater than in the lower RF bands.

  2. Automotive glass is rarely plain

    • Most windows are laminated, tinted, or metallic-coated (e.g., for UV, heating, or noise reduction).

    • Metallic coatings can reflect >99% of millimeter-wave energy.

    • Even uncoated glass introduces 20–40 dB or more losses at 77–81 GHz.

  3. Internal reflections and scattering

    • Multiple internal reflections in laminated glass cause signal dispersion.

    • Moisture or dirt on glass can further disrupt wave propagation at these frequencies.

Glass Type Estimated Attenuation (dB) Plain, clear, uncoated window glass provides a 15–25 dB reduction. Tinted or heat-treated glass offers 25–35 dB. Metalized or Low-E coated glass is >40–60 dB (nearly opaque). A laminated windshield (multi-layered) offers a 30–50 dB reduction.

(Please note: 60 dB is the goal in shielding effectiveness terms. 60 dB is one-million times reduction.)

For comparison: A signal loss of 20 dB means only 1% of the signal gets through.

Practical Implications in Vehicles

  • Automotive radar systems at 77–81 GHz are always mounted externally, typically:

    • Behind plastic bumper covers (which are transparent to mmWave).

    • Inside rear-view mirrors or body panels specifically designed to be RF-transparent.

  • Never behind glass, unless the glass is specially designed to be RF-transparent at mmWave, which is rare and expensive.

Conclusion

Standard automotive glass does not reliably pass 77–81 GHz RF. Radar systems at those frequencies must be placed outside the cabin or behind carefully engineered radar-transparent materials, not glass.

Different types of automotive glass show significant attenuation at millimeter-wave frequencies, especially 73–81 GHz (used by automotive radar). Here’s what the measured data shows:

  • Clear glass at 73 GHz has an approximate specific attenuation of 14.4 dB per centimeter. This means that you lose about 14.4 dB of signal for each centimeter of thickness, a very steep loss.

  • Clear glass at 91 GHz (slightly higher) shows even more loss, with around 18.8 dB per centimeter. As frequency increases, attenuation rises.

  • Tinted automotive glass at 81 GHz (common in side and rear windows) shows total penetration losses around 26.5 dB, even without precise per-centimeter values. That’s a considerable loss, meaning most of the signal is blocked or reflected.

  • Standard laminated automotive glass (typically around 5 mm thick) will experience similar or even higher losses, mainly if it contains embedded materials like heating wires or metallic coatings.

Key Takeaways

  1. Even plain, uncoated glass sharply attenuates 77–81 GHz radar signals.

  2. Any added tinting, metallic coatings, or laminations increase signal loss dramatically.

  3. This is why automotive radar units are never placed behind glass; they are mounted in positions with clear RF paths, like behind radar-transparent plastics or embedded in bumpers and mirrors.