How do I measure the radiation pattern of an electrically large antenna at millimeter wave frequencies?
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).
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
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.