How do I design the antenna for a short range millimeter wave radar sensor?
Short-Range Radar Antenna
Short-range radar antennas are simpler than long-range radar antennas because the lower gain requirement relaxes the array size, but the wide-beam requirement introduces its own design challenges.
On-Package Antenna Design
(1) eWLB (embedded wafer-level ball-grid) antenna: the antenna is fabricated on the package redistribution layer (RDL) of the radar IC. The antenna is within 50-100 um of the radar die. This eliminates the package-to-board-to-antenna transitions (lowest loss possible). Examples: Infineon BGT60TR13C (antenna on eWLB package RDL). (2) PCB antenna: the antenna is designed on the radar module PCB. The radar IC is soldered to the board, and the antenna traces are on the same board. More design flexibility (larger antenna area available) but adds package-to-board transition loss (0.3-1 dB). Examples: TI AWR1843 evaluation module (patch antennas on Rogers 4003C PCB). (3) Waveguide-slot antenna: for higher gain and improved side-lobe performance. The waveguide is formed in the package or board substrate. More complex fabrication but provides excellent pattern control. Used in some industrial radar designs.
Beamwidth Control
(1) Azimuth: the azimuth beamwidth is controlled by the number of elements in the horizontal direction. Single element: beamwidth ≈ 70-90°. 2-element array: beamwidth ≈ 40-50°. 4-element array: beamwidth ≈ 20-25°. For short-range radar: 1-2 elements per channel (wide beam). (2) Elevation: the elevation beamwidth is controlled similarly. For parking assist (targets at ground level): a narrow elevation beam (20-30°) directed at the ground plane is ideal (reduces returns from irrelevant objects above the sensor). For presence detection (targets at any height): a wide elevation beam (60-90°) is needed. (3) Sidelobe suppression: for ADAS applications, the sidelobes from the antenna pattern can cause false detections (a strong reflector in the sidelobe direction appears as a weak target in the mainlobe direction). Target sidelobe level for automotive: < -15 dB below the mainlobe. Achieved through: amplitude tapering of the array (Taylor or Chebyshev weighting applied through the beamforming gain control). This costs approximately 1-2 dB of peak gain (the tapered array has lower gain than a uniform array of the same size).
Beamwidth: θ ≈ λ/(N×d)
MIMO virtual array: N_v = N_TX × N_RX
Single patch: 5-7 dBi, ~80° beamwidth
FOV: 60-120° azimuth for short range
Frequently Asked Questions
Can I use a single TX and single RX antenna?
Yes, for the simplest applications (presence detection, level sensing): a 1TX/1RX radar detects targets but cannot determine their angle (no angular resolution). The radar provides range only (and velocity from Doppler). Used in: simple motion sensors, liquid level sensing, and industrial distance measurement. For any application requiring angular information (ADAS, gesture recognition, people counting): at least 2 TX or 2 RX antennas are needed to form a baseline for angle estimation. More antennas = better angular resolution.
How do I prevent ground bounce in a parking sensor?
Ground bounce (reflections from the road surface directly below the sensor) is a major problem for downward-facing parking radar: (1) Tilt the antenna beam downward at 15-30° (not straight down). This directs the mainlobe toward the detection zone (1-5 m ahead of the bumper) and places the ground directly below in the sidelobe region. (2) Absorber: place absorber material behind the antenna (between the sensor and the bumper) to prevent reflections from the bumper structure. (3) Signal processing: the ground bounce appears at a fixed range (the height of the sensor above the road, approximately 0.3-0.5 m). Apply a static clutter cancellation that removes this fixed-range return. Any target at a different range (> 0.5 m) is a valid detection. (4) Elevation beamforming: use a 2-element elevation array to independently steer the beam upward (away from the ground) in the near range and downward in the far range.
What PCB substrate should I use for a 77 GHz radar antenna?
Standard choices: (1) Rogers RO4003C (Dk = 3.55, tan_d = 0.0027 at 10 GHz, ≈ 0.004 at 77 GHz): the most popular choice for automotive radar PCB antennas. Good balance of performance and cost. Widely available and well-characterized. (2) Rogers RO3003 (Dk = 3.00, tan_d = 0.001 at 10 GHz): lower loss than RO4003C. Used for radar designs where the antenna efficiency is critical. Higher cost. (3) Megtron 7 (Dk = 3.4, tan_d = 0.001 at 10 GHz): a hybrid material from Panasonic. Good for designs with both mmWave radar and digital (the digital section uses the Megtron layers, the radar antenna uses the same material). Lower cost than Rogers alternatives. (4) LTCC (Dk = 5-7, tan_d = 0.002-0.004): used for some AiP-style radar modules. The higher Dk reduces the antenna element size (making the array more compact) but also reduces the bandwidth and efficiency. Important: use VLP or HVLP copper foil for 77 GHz, and avoid ENIG surface finish on the antenna traces (the nickel increases the loss by 50-200%).