How do I measure the antenna pattern of a millimeter wave antenna in a compact range?
Compact Range mmWave Antenna Measurement
Compact ranges are the preferred facility for mmWave antenna testing because the far-field distances at mmWave frequencies are prohibitively large for conventional outdoor or indoor far-field ranges.
| 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 |
Calibration Procedure
(1) Reflector types: parabolic cylinder: focuses in one plane only. Simpler but creates a line focus, not a point focus. Suitable for fan-beam antennas. Focal-fed paraboloid: creates a point focus with a plane wave in both planes. Most common for general-purpose CATRs. Dual-reflector (Cassegrain or Gregorian): allows the feed to be behind the main reflector, reducing blockage and improving the quiet zone quality. (2) Reflector surface accuracy: the surface RMS error (sigma) must satisfy Ruze equation for the impact on quiet zone quality: the gain loss of the reflector = exp(-(4×pi×sigma/lambda)^2). For 0.5 dB gain loss: sigma < 0.036 × lambda. At 60 GHz: sigma < 180 um. At 140 GHz: sigma < 77 um. At 300 GHz: sigma < 36 um. Manufacturing large reflectors (1-3 m) with 36 um RMS accuracy requires diamond-turned aluminum or composite panels with precision machining. (3) Feed horn: the feed horn illuminates the reflector with a known beam pattern. For even quiet-zone amplitude: the feed taper at the reflector edge should be -10 to -15 dB. The feed horn pattern must be known precisely (calibrated pattern measurement). Common feeds: corrugated horns (symmetric, low cross-pol), standard gain horns (simpler, -25 to -30 dB cross-pol).
Error Sources
(1) Atmospheric absorption: at 60 GHz, the oxygen absorption line causes 15 dB/km attenuation. Even in a 10 m CATR: the path loss due to absorption is 0.15 dB. At 183 GHz (water vapor line): attenuation is 30 dB/km, causing 0.3 dB loss in 10 m. This must be accounted for in the antenna gain measurement. At 77 GHz (automotive radar): absorption is minimal (~0.5 dB/km). (2) Cable and connector losses: cable loss at mmWave is significant. A 1 m SMA cable at 60 GHz: 3-5 dB loss. Use waveguide connections or short, low-loss coaxial cables (1.85 mm, 1.0 mm connectors). (3) Positioner accuracy: the AUT is mounted on a positioner that rotates it through 360° in azimuth and elevation. At mmWave: the beamwidth is narrow (a 10 cm antenna at 60 GHz has a 3.5° beamwidth). The positioner angular accuracy must be < 0.1° to accurately locate the beam peak and nulls. (4) Phase stability: the cables and feed path must be phase-stable during the measurement. Temperature changes during a long pattern measurement (30-60 minutes) can cause phase drift of several degrees, distorting the measured pattern. Use phase-stable cables and control the chamber temperature (±1°C).
- 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
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Fixture Considerations
(1) Calibrate: measure a reference antenna with known gain (standard gain horn) to determine the system path loss and establish the gain reference. The gain of the AUT is calculated relative to the reference: G_AUT = G_ref + (P_AUT - P_ref) dB. (2) Mount the AUT on the positioner with the phase center aligned to the center of the quiet zone. Alignment is critical: the phase center must be within ±1 mm of the rotation axis for accurate pattern measurement. (3) Measure: the positioner rotates the AUT while the VNA or receiver records the received amplitude and phase. A full 3D pattern requires cuts in multiple planes (typically every 1-5° in phi and theta). (4) Data processing: compute the antenna gain pattern, directivity, efficiency, beamwidth, sidelobe levels, front-to-back ratio, and polarization (from the co-pol and cross-pol pattern measurements).
Frequently Asked Questions
How does a compact range compare to a near-field scanner?
CATR: directly measures the far-field pattern (the plane wave simulates far-field conditions). No post-processing required. Fast measurement (real-time pattern acquisition). Limitations: the quiet zone size limits the maximum AUT size; the reflector is expensive and large. Near-field scanner: measures the near-field (amplitude and phase) on a scanning surface close to the AUT. The far-field pattern is computed via near-field to far-field transformation (NFFT). Advantages: compact (no large reflector needed), can measure very large antennas. Limitations: requires precise phase measurement at every scan point (slow for large arrays), and the NFFT computation introduces truncation and sampling errors. For mmWave: CATR is generally preferred because the far-field distance is large but the AUT physical size is small (fits easily in the quiet zone). Near-field scanning at mmWave requires extremely fine scan resolution (lambda/2 ≈ 2.5 mm at 60 GHz) and very stable phase measurement.
What absorber do I need for a mmWave chamber?
Pyramidal absorber (carbon-loaded polyurethane foam): standard for anechoic chambers. At mmWave: 6-inch pyramidal absorber: reflectivity < -30 dB at 30+ GHz at normal incidence. 12-inch absorber: < -40 dB. Performance degrades at grazing angles: < -15 to -20 dB at 70° incidence. For mmWave: the absorber tips (pyramids) must be sharp (< 3 mm point radius) for good high-frequency performance. Flat absorber with lossy coatings: very thin (< 5 mm) but provides only -15 to -25 dB reflectivity. Used for back walls where space is limited. Wedge absorber: used for grazing-incidence walls (side walls of the chamber). Provides -20 to -30 dB at angles up to 60°.
Can I measure phased-array antennas in a compact range?
Yes, but with considerations: (1) The quiet zone must be large enough to cover the full array aperture. For a 30 × 30 cm phased array at 28 GHz: quiet zone > 40 cm. (2) The phased array is typically tested at multiple beam steering angles. The positioner rotates the array, and at each position, the beam is steered electronically. This measures the combined effect of the element pattern and the array factor at each steering angle. (3) Active phased arrays require DC power and control signals, which must be routed through the positioner without introducing RF interference. Use filtered DC feedthroughs and shielded control cables. (4) Over-the-air (OTA) testing of 5G mmWave devices: 3GPP specifies CATR as one of the approved measurement methods for 5G NR FR2 device testing (TS 38.521-3).