CATR
Understanding CATR
Simulating the Far-Field Indoors
To accurately measure an antenna's radiation characteristics (such as gain, sidelobe levels, and phase patterns), the test must occur in the antenna's far-field region. In this region, the spherical waves radiating from the antenna have expanded to resemble flat, parallel plane waves. The standard mathematical threshold for the far-field is the Fraunhofer distance, which is proportional to the square of the antenna's aperture diameter divided by the wavelength. For high-frequency millimeter-wave antennas (like 5G mmWave arrays or satellite reflectors), this distance can be tens or hundreds of meters, making indoor chambers impractical and outdoor testing difficult.
A Compact Antenna Test Range (CATR) solves this challenge. It uses a high-precision collimating reflector (or a dual-reflector system) to fold the electromagnetic path. A feed horn placed at the reflector's focal point radiates a spherical wave. When this wave reflects off the parabolic surface, it is collimated into a plane wave. This creates a volume of space, known as the quiet zone, where the wavefront is flat and uniform, simulating the far-field within a short physical distance.
Quiet Zone and Reflector Edge Design
The performance of a CATR is defined by the quality of its quiet zone, which must exhibit minimal amplitude and phase variation (typically less than 0.5 dB and 5 degrees). The main design challenge is edge diffraction. When the spherical wave hits the boundary of the reflector, the sharp edge scatters the waves, causing ripple interference in the quiet zone. To prevent this, CATR reflectors use specialized edge treatments, such as serrated edges (which distribute the diffracted energy away from the quiet zone) or rolled edges (which smoothly guide the energy away). This enables high-accuracy indoor testing of aerospace, defense, and automotive radar systems.
Key Mathematical Relations
Technical Specifications Comparison
| Measurement Method | Required Chamber Size | Operational Frequencies | Main Measurement Focus | Key Advantage / Disadvantage |
|---|---|---|---|---|
| Direct Far-Field (DFF) | Very Large (needs Fraunhofer distance) | Low to Mid frequencies (< 6 GHz) | Real far-field patterns directly | Simple setup; high chamber cost for large antennas |
| Near-Field Scanning (NSI) | Small (probe moves close to antenna) | All bands (requires mathematical transform) | Phase/amplitude maps, computed far-field | Compact chamber; slow measurement speed due to grid scan |
| CATR (Compact Range) | Moderate (folds path with reflector) | High frequencies (6 GHz to 300 GHz) | Direct far-field patterns in quiet zone | Fast real-time measurements; high reflector fabrication cost |
Frequently Asked Questions
Why is CATR preferred for 5G mmWave testing?
Because mmWave frequencies (24 GHz to 40 GHz and above) have short wavelengths, the Fraunhofer distance is long. CATR collimates the waves to create a plane-wave quiet zone indoors, enabling rapid, controlled over-the-air (OTA) testing.
What is a 'quiet zone' in a compact range?
The quiet zone is the spherical or cylindrical volume of space in front of the test position where the amplitude and phase of the collimated plane wave are uniform enough to simulate true free-space conditions.
Why are the edges of CATR reflectors serrated?
The serrated (saw-tooth) edges are designed to minimize edge diffraction. By breaking up the sharp boundary of the reflector, the serrations scatter the diffracted waves away from the quiet zone, preventing interference ripples.