24 GHz Radar PCB: ISM-Band Sensor Fabrication

What Is a 24 GHz Radar PCB?

A 24 GHz radar PCB is the circuit board at the heart of ISM-band radar sensors operating in the 24.00–24.25 GHz narrow band or the 24.05–24.25 GHz ultra-wideband (UWB) segment. It carries the transmit and receive antenna elements, the FMCW or Doppler transceiver IC, the IF signal path, and the power regulation—all on a single substrate that must maintain controlled impedance and low dielectric loss at K-band frequencies. These boards appear in industrial motion detectors, traffic-monitoring radar, drone altimeters, level-measurement sensors, and automotive ADAS sensor modules.

At 24 GHz the free-space wavelength is about 12.5 mm, which means a quarter-wave microstrip patch element is roughly 1.6–1.8 mm wide depending on effective permittivity. Trace width tolerances, dielectric thickness control, and surface roughness directly set antenna gain, impedance match, and ultimately detection range. Getting the PCB fabrication wrong by even 1 mil (0.025 mm) on trace width can shift the antenna center frequency by several hundred megahertz—enough to push the sensor out of the ISM allocation.

Why 24 GHz? Frequency Allocation and Use Cases

The 24 GHz ISM band is license-free in most jurisdictions under ETSI EN 302 264 (Europe) and FCC Part 15.245/15.249 (United States). Two distinct segments matter:

Compared to 79 GHz imaging radar, the 24 GHz band offers longer detection range per unit EIRP because free-space path loss is roughly 10 dB lower. However, the narrower available bandwidth limits range resolution to about 0.75 m in the 200 MHz UWB mode. Many industrial and building-automation sensors accept that trade-off because they need reliable detection at 20–50 m, not centimeter-level resolution.

Material Selection for 24 GHz Radar PCBs

Key Laminate Parameters

At 24 GHz, substrate loss becomes a primary design constraint. The two laminate parameters that matter most are dielectric constant (Dk) stability over frequency and temperature, and dissipation factor (Df). A Df above 0.005 at 24 GHz noticeably degrades receiver noise figure and antenna efficiency.

For most 24 GHz radar PCBs, Rogers RO4835 laminates hit the sweet spot: they process on standard FR-4 equipment (no special PTFE handling), tolerate lead-free reflow, and keep Df below 0.004 at K-band. When the antenna layer demands tighter Dk tolerance (±0.02 vs. ±0.05), moving to RO3003 or Isola Astra MT77 for the top two layers while keeping FR-4 or Megtron 6 for the digital core is a proven hybrid stackup strategy.

Surface Roughness and Conductor Loss

Copper roughness that is acceptable at 2.4 GHz becomes a measurable loss mechanism at 24 GHz. Standard electrodeposited (ED) copper has an RMS roughness of about 1.8 µm (the “standard” profile). At 24 GHz the skin depth in copper is only about 0.42 µm, so the current crowds into surface features and effective resistance rises. Specifying “very low profile” (VLP, ≤ 0.9 µm RMS) or “hyper very low profile” (HVLP, ≤ 0.4 µm RMS) foil on the antenna layer can reduce conductor loss by 0.3–0.5 dB/cm—a significant budget recovery when total antenna feed-line length may be 10–15 mm.

Stackup Architecture

A typical 24 GHz FMCW radar module uses a four- or six-layer stackup. The critical constraint is keeping the antenna layer and its ground reference as the outermost pair, with tightly controlled dielectric thickness.

Four-Layer Stackup Example

• Layer 1 (Top): Patch antennas, TX/RX microstrip feed lines, transceiver IC land pattern — RO4835, 0.254 mm (10 mil) core, HVLP copper.
• Layer 2 (Ground): Continuous ground plane beneath the antenna layer. Minimal via penetrations in the antenna footprint zone.
• Layer 3 (Signal/Power): IF traces, power distribution, SPI/I²C digital control — FR-4 or Megtron 6 prepreg, 0.36 mm.
• Layer 4 (Bottom): Connector footprints, decoupling, shielding ground.

The 10 mil RO4835 core on Layer 1–2 produces a 50 Ω microstrip width of approximately 0.58 mm (23 mil) with Dk 3.48. That width is comfortable for standard PCB etching and leaves room for a grounded coplanar waveguide (GCPW) variant with 0.15 mm gaps if additional mode control is needed.

Six-Layer Hybrid for Complex Modules

When the radar module integrates a microcontroller, CAN/LIN transceiver, and power management alongside the RF front end, a six-layer hybrid stackup separates the noisy digital section from the sensitive RF layers. Layers 1–2 remain the RF pair on low-loss laminate; layers 3–6 use standard high-Tg FR-4 bonded through a compatible prepreg. This approach is common in in-cabin radar modules where a single board must handle 24 GHz sensing plus CAN-FD communication to the body controller.

Antenna Integration on the PCB

Patch Antenna Arrays

Most 24 GHz radar PCBs use microstrip patch arrays etched directly on the top copper layer. A single rectangular patch on RO4835 (Dk 3.48) resonates at 24.125 GHz with a patch length of approximately 3.55 mm and width of 4.3 mm. A 1 × 4 series-fed array yields about 12 dBi gain with a half-power beamwidth of roughly 20° in the E-plane and 70° in the H-plane—suitable for a traffic radar that needs a narrow forward cone.

Element spacing should be close to 0.76 λ₀ (about 9.5 mm) to suppress grating lobes while keeping mutual coupling below −20 dB. The corporate feed network that distributes power to each element is itself a set of quarter-wave transformers and T-junctions; any impedance mismatch in the feed degrades sidelobe levels. Designers sometimes add a Wilkinson divider at the first split for better isolation between the TX and RX sub-arrays.

Radome and Cover Effects

A plastic radome placed over the antenna detunes the patch resonance. At 24 GHz, a 1 mm ABS cover (Dk ≈ 2.7) sitting 0.5 mm above the patch can shift the center frequency down by 200–400 MHz. The standard mitigation is to simulate the antenna with the radome in place and pre-compensate the patch dimensions, or to use a radome material whose electrical thickness is an integer multiple of λ/2 at the operating frequency so it becomes transparent. Polycarbonate at 1.6 mm thickness is a near half-wave window at 24 GHz.

Critical Fabrication Tolerances

Loose tolerances that are fine for a 2.4 GHz Wi-Fi board become failure modes at 24 GHz. The table below summarizes the fabrication parameters that most affect 24 GHz radar performance and the tolerances a capable RF fabricator should hold.

Surface Finish Considerations

Immersion silver (ImAg) is often preferred over ENIG for 24 GHz antenna layers because it avoids the lossy nickel barrier. The skin depth in nickel at 24 GHz is only about 0.6 µm, and a 3–5 µm nickel layer fully absorbs the RF current before it reaches the copper beneath. ImAg provides a thin (0.15–0.3 µm) silver layer directly on copper, yielding lower insertion loss. The trade-off is a shorter shelf life and sensitivity to tarnish, so boards should be assembled promptly or stored in sulfur-free packaging.

Via Transitions and Back-Drilling

Signal vias that transition from the antenna layer to an inner stripline must be carefully managed. A through-hole via in a 1.6 mm board leaves a stub below the target layer that resonates near 24 GHz—exactly the wrong frequency. Back-drilling (controlled-depth drilling from the opposite side) removes the stub. Alternatively, blind or buried vias eliminate the stub entirely but add fabrication cost. For volume production, back-drilling to within 0.1 mm of the target layer is the most cost-effective approach.

Transceiver IC Integration

Modern 24 GHz radar sensors typically use a monolithic transceiver such as the Infineon BGT24MTR11 or the Texas Instruments AWR2243 (which also covers 76–81 GHz). These ICs integrate the VCO, PLL, power amplifier, LNA, and mixer on a single die, leaving the PCB responsible for the antenna, IF filtering, ADC interface, and power supply.

The IC ground paddle must connect to the Layer 2 ground plane through a dense array of vias (typically 0.2 mm drill, 0.35 mm pad, on a 0.5 mm pitch grid) to minimize inductance. Any ground inductance at the PA output raises the reference impedance seen by the on-chip matching network and degrades output power and efficiency.

Testing and Validation

Bare-board verification for a 24 GHz radar PCB goes beyond standard continuity and impedance coupon testing:

• TDR impedance profiling: Measure the antenna feed lines with a time-domain reflectometer to confirm 50 Ω ± 2 Ω along the full trace length.
• Dk verification coupons: Include a resonant ring or a long microstrip line on a test coupon to back-calculate effective Dk at 24 GHz and compare against the laminate datasheet.
• Antenna pattern measurement: After assembly, measure the far-field pattern in an anechoic chamber or compact range. A gain drop of more than 1 dB from simulation indicates a fabrication or radome problem.
• Return loss: S11 at the antenna port should be below −10 dB across the 24.00–24.25 GHz band; below −15 dB is preferred for margin against production variation.

24 GHz vs. 60 GHz and 77/79 GHz Radar PCBs

Choosing between 24 GHz and higher-frequency radar bands depends on the application’s resolution, range, and cost requirements.

For applications requiring fine-grained presence detection or gesture recognition, a 60 GHz radar PCB is the better fit. For long-range automotive cruise control, 77/79 GHz dominates. The 24 GHz band remains the workhorse for industrial sensing, building automation, and cost-sensitive traffic radar where range matters more than resolution.

Design and Fabrication Checklist

1. Select a laminate with Df ≤ 0.004 at 24 GHz and specify VLP or HVLP copper foil on the antenna layer.
2. Define the stackup so the antenna-to-ground dielectric thickness is tightly controlled (±0.5 mil).
3. Use grounded coplanar waveguide or microstrip with solid ground reference—no splits, no routing underneath the antenna feed.
4. Back-drill or use blind vias for any signal transition to eliminate stubs resonant near 24 GHz.
5. Choose immersion silver over ENIG for the antenna layer to avoid nickel-induced loss.
6. Include Dk verification coupons and TDR impedance coupons in the panel.
7. Simulate the antenna with the radome in place; pre-compensate patch dimensions for the cover’s dielectric loading.
8. Verify S11 < −10 dB across the full 24.00–24.25 GHz band on first-article samples before volume production.

Frequently Asked Questions

Can I use FR-4 for a 24 GHz radar PCB?

Standard FR-4 has a dissipation factor around 0.020 at 10 GHz, which climbs further at 24 GHz. The dielectric loss alone would consume most of the link budget, and the Dk variation (±0.3 typical) makes repeatable antenna design impractical. FR-4 is acceptable for the digital and power layers in a hybrid stackup, but not for the antenna or RF feed-line layers.

What trace width gives 50 Ω at 24 GHz on RO4835?

On a 0.254 mm (10 mil) RO4835 core with Dk 3.48 and ½ oz copper, a microstrip width of approximately 0.58 mm (23 mil) yields 50 Ω. Always verify with a 2.5-D field solver that accounts for the actual copper profile and solder-mask dielectric.

How does the 24 GHz ISM band differ from the 24 GHz UWB band?

The narrow-band ISM allocation (24.00–24.25 GHz) permits up to +20 dBm EIRP for CW Doppler speed sensing. The UWB allocation within the same range allows wideband modulation (FMCW) at much lower power (−41.3 dBm/MHz), enabling range measurement at the cost of shorter detection distance. The PCB design is essentially the same; the difference is in the transceiver firmware and regulatory certification.

Is the 24 GHz band being phased out for automotive use?

In the European Union, the 24 GHz UWB automotive allocation expired in 2022, pushing new vehicle platforms to 77/79 GHz. However, the narrow-band 24 GHz ISM allocation remains available worldwide for industrial, scientific, and building-automation radar. Non-automotive 24 GHz sensor demand continues to grow. For next-generation automotive radar, see 79 GHz radar PCB fabrication.