Semi Anechoic Chamber

Description

A Semi Anechoic Chamber (SAC) is a specialized electromagnetic test facility critical for Over-the-Air (OTA) performance evaluation of radio equipment, such as User Equipment (UE) and base stations (gNBs, eNBs), as specified in 3GPP conformance testing standards. Unlike a Full Anechoic Chamber (FAC) which is absorptive on all surfaces, a SAC features radio-frequency (RF) absorber material on its walls and ceiling to minimize reflections, but has a reflective metallic ground plane floor. This design intentionally creates a controlled reflection from the floor to simulate a realistic, ground-reflected propagation environment similar to typical real-world scenarios where devices operate above ground level. The chamber is also fully shielded from external RF interference, ensuring that measurements are not contaminated by outside signals.

The architecture of a SAC includes a shielded enclosure, pyramidal or wedge-shaped RF absorbers lining the interior surfaces (except the floor), a turntable to rotate the Device Under Test (DUT), and a stationary or movable measurement antenna connected to a vector network analyzer or communication tester. The reflective ground plane is typically a large, smooth metal surface. During testing, the DUT is placed on the turntable at a specified height above the ground plane. The measurement antenna, positioned at a fixed distance, transmits or receives signals. As the turntable rotates, the system measures the radiation pattern, Total Radiated Power (TRP), and Total Isotropic Sensitivity (TIS) of the DUT. The reflective ground creates a constructive and destructive interference pattern (multipath) that must be characterized, making the measurements more representative of actual use than a FAC.

SACs are used extensively for 3GPP RF conformance tests, especially for MIMO OTA performance evaluation in LTE and 5G NR. They support testing of key metrics like spherical coverage, beamforming gain, and spatial channel emulation. The chamber must be carefully calibrated to account for path loss and chamber reflections. The presence of the ground plane means that the measured radiation pattern includes both the direct path and a single reflected path, which is a simplified but valuable model for many deployment environments. This makes SAC testing a balance between the controlled simplicity of a FAC and the complex realism of a reverberation chamber, providing repeatable and standardized results for regulatory type approval and device certification.

Purpose & Motivation

The Semi Anechoic Chamber exists to provide a standardized, repeatable test environment for OTA performance measurements of wireless devices that accurately reflects a common real-world propagation condition: operation with a ground reflection. Prior to standardized OTA testing, devices were often evaluated only via conducted tests (cabled connections), which ignored the effects of antennas and device enclosure on real radiated performance. This gap meant devices could pass lab tests but perform poorly in actual use. The SAC was adopted to solve this by enabling realistic, yet controlled, radiated testing.

The motivation for SAC standardization in 3GPP stemmed from the need to ensure consistent device quality and network performance as mobile systems evolved with MIMO and advanced antenna systems. A Full Anechoic Chamber eliminates all reflections, which is an ideal free-space condition not typical for handheld UE use. The SAC's reflective ground plane introduces a controlled multipath element, making it more suitable for testing devices as they are typically held or placed above a surface. It addresses the limitation of FACs by providing a test environment that balances reproducibility with a more realistic physical model, which is crucial for evaluating metrics like TRP and TIS that directly impact user experience and network coverage. Its specification across multiple 3GPP technical reports (e.g., 38.113 for NR) ensures global harmonization in device certification.