Measurements, Testing, and Calibration Advanced Measurement Topics Informational

How do I measure the radiation pattern of an electrically large antenna at millimeter wave frequencies?

Q: Which method is best for mmW?

For high-gain antennas (D > 100 mm) at 60+ GHz: planar near-field scanning is the most practical (the far-field distance is too large for indoor far-field, and CATR reflectors at 60+ GHz are very expensive and require sub-0.1 mm surface accuracy). For small antennas (D < 50 mm): far-field measurement in a small anechoic chamber is simple and accurate. For phased arrays requiring full hemisphere patterns: spherical near-field scanning. For production testing: CATR provides the fastest measurement (real-time, no post-processing transformation).

Q: What are the alignment challenges?

At 60 GHz: the wavelength is 5 mm. Mechanical alignment errors of 0.25 mm (lambda/20) introduce 18-degree phase errors. The scanner, AUT mount, and probe must be aligned to sub-millimeter accuracy. At 200 GHz: alignment to < 0.1 mm is required. Solutions: laser interferometer-based scanner position feedback, precision kinematic mounts, and post-measurement alignment correction algorithms.

Q: How long does a near-field scan take?

For 20,000 scan points with 50 ms VNA measurement per point: approximately 17 minutes. With fast scanning (continuous motion, triggered VNA acquisition at 1 kHz): approximately 20 seconds for a single frequency, or several minutes for a frequency sweep at each point. Spherical near-field scans take longer (more points for full sphere coverage): typically 30-120 minutes depending on angular resolution and frequency points.

Measuring the radiation pattern of an electrically large antenna at millimeter-wave frequencies requires addressing the unique challenges of mmW measurement: the far-field distance is very large (R_ff = 2D^2/lambda, where D is the antenna diameter; for a 300 mm antenna at 60 GHz: R_ff = 36 m), making traditional outdoor or indoor far-field ranges impractical for large antennas. The measurement approaches are: compact antenna test range (CATR, using a shaped reflector to create a plane wave in a shorter distance, typically 3-10 m; the reflector collimates a spherical wave from a feed horn into a uniform plane wave across the antenna under test; quiet zone quality: amplitude uniformity < +/- 0.5 dB, phase uniformity < 10 degrees across the test zone), near-field scanning (a probe scans a planar, cylindrical, or spherical surface close to the antenna, measuring the amplitude and phase of the radiated field; a mathematical near-field-to-far-field transformation computes the radiation pattern; planar scanning is most common for high-gain antennas; the scan plane must be at least lambda/2 sampled, which at 60 GHz means 2.5 mm probe spacing over a scan area larger than the antenna aperture plus 10-20 wavelengths of margin), and far-field range (for smaller antennas where R_ff < 10-20 m: a standard anechoic chamber with a separation distance exceeding R_ff; at 60 GHz with a 50 mm antenna: R_ff = 1.0 m, easily achievable in a benchtop anechoic chamber).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Probes, Chambers, Signal Generators

Millimeter-Wave Antenna Pattern Measurement

Antenna pattern measurement at mmW frequencies combines the challenges of precision microwave measurement with the complexities of mmW propagation (high path loss, sensitivity to alignment, atmospheric absorption, and small wavelength requiring fine mechanical positioning).

Measurement Techniques

Planar near-field scanning: The antenna under test (AUT) is fixed, and a small probe (typically a standard gain horn or open-ended waveguide) scans across a planar surface in front of the antenna. At each scan point: the VNA measures the amplitude and phase of the received signal. The probe spacing must be <= lambda/2 (2.5 mm at 60 GHz, 0.75 mm at 200 GHz). Scan area: typically 1.5-2x the AUT aperture. After scanning: a 2D FFT transforms the near-field data to the far-field pattern. Accuracy: +/- 0.3-0.5 dB in main beam, +/- 1-2 dB in sidelobes
Spherical near-field scanning: The probe scans a spherical surface surrounding the AUT. Measures the complete 3D radiation pattern. More complex mechanically but captures all radiation including rear lobes. The gold standard for full pattern characterization
CATR: A parabolic reflector (or shaped dual-reflector) creates a plane wave in a compact space. The AUT is placed in the quiet zone and rotated on a positioner to measure the pattern. Advantages: direct far-field measurement (no transformation needed), real-time results. Challenges: the CATR reflector must be very smooth (surface error < lambda/50 for good quiet zone quality; at 60 GHz: < 0.1 mm surface accuracy)

Antenna Pattern Measurement Parameters

Far-field distance: R_ff = 2D²/lambda
At 60 GHz (lambda=5mm), D=300mm: R_ff = 36 m
Near-field probe spacing: dx = dy = lambda/2 = 2.5 mm at 60 GHz
Scan area: (D + 10lambda) × (D + 10lambda) = 350 × 350 mm
Number of scan points: (350/2.5)² = 19,600 points

Common Questions

Frequently Asked Questions

Which method is best for mmW?

For high-gain antennas (D > 100 mm) at 60+ GHz: planar near-field scanning is the most practical (the far-field distance is too large for indoor far-field, and CATR reflectors at 60+ GHz are very expensive and require sub-0.1 mm surface accuracy). For small antennas (D < 50 mm): far-field measurement in a small anechoic chamber is simple and accurate. For phased arrays requiring full hemisphere patterns: spherical near-field scanning. For production testing: CATR provides the fastest measurement (real-time, no post-processing transformation).

What are the alignment challenges?

At 60 GHz: the wavelength is 5 mm. Mechanical alignment errors of 0.25 mm (lambda/20) introduce 18-degree phase errors. The scanner, AUT mount, and probe must be aligned to sub-millimeter accuracy. At 200 GHz: alignment to < 0.1 mm is required. Solutions: laser interferometer-based scanner position feedback, precision kinematic mounts, and post-measurement alignment correction algorithms.

How long does a near-field scan take?

For 20,000 scan points with 50 ms VNA measurement per point: approximately 17 minutes. With fast scanning (continuous motion, triggered VNA acquisition at 1 kHz): approximately 20 seconds for a single frequency, or several minutes for a frequency sweep at each point. Spherical near-field scans take longer (more points for full sphere coverage): typically 30-120 minutes depending on angular resolution and frequency points.

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