What Are Open Area Test Sites?

Open area test sites (OATS) are outdoor facilities constructed for measuring the radiated electromagnetic emissions of electronic equipment under controlled, standardized conditions. They simulate free-space propagation by providing an unobstructed, flat ground plane and a clear measurement path between a device under test and a receiving antenna, free from walls, ceilings, and other reflective structures. OATS are a foundational element of electromagnetic compatibility (EMC) testing, used to verify that products meet regulatory emission limits before they enter commercial markets.

The concept emerged from mid-twentieth-century radio interference research, when standards bodies recognized the need for a reproducible outdoor environment that could serve as a reference against which other test facilities would be calibrated. Two dominant standards now govern OATS requirements: CISPR 16-1-4, developed by the International Electrotechnical Commission's Special Committee on Radio Interference, and ANSI C63.4, the North American equivalent. Together these standards define the site geometry, ground-plane dimensions, ambient noise limits, and validation procedures that a facility must satisfy to be used for regulatory compliance testing.

Site Design and Physical Specifications

An OATS consists of a metallic ground plane surrounded by an area large enough to keep reflecting objects outside the measurement zone. CISPR 16-1-5 specifies a minimum ground plane of 30 m by 20 m, with 60 m by 30 m being the most common operational size at accredited facilities. The ground plane is typically constructed from solid steel or aluminum sheets, with surface flatness tolerances that become more stringent above 700 MHz. Some facilities use a floating construction approach, in which panels rest on adjustable supports to compensate for surface irregularities, as described in the design and validation study of a high-performance OATS published by IntechOpen. Impedance transition features at the ground-plane edges, including wire mesh skirts, reduce diffraction-related measurement errors.

The measurement antenna is mounted on a mast that raises and lowers over a defined height range (typically 1 m to 4 m) while the receiving antenna sweeps the same range, enabling the test to find the maximum received emission level. Tests are conducted at standard distances of 3 m, 10 m, or 30 m between the equipment under test and the receiving antenna, depending on the emission class and frequency range.

Normalized Site Attenuation Validation

Normalized site attenuation (NSA) is the primary metric used to validate an OATS and to qualify other test sites, including semi-anechoic chambers, as equivalent to an ideal outdoor location. NSA is computed by comparing the measured signal between a transmitting and receiving antenna pair, normalized for the specific antenna factors, against a theoretical value calculated for a perfect OATS. CISPR 16-1-4 requires that the measured NSA fall within ±4 dB of the theoretical value across the test frequency range of 30 MHz to 1 GHz. The NSA method as described in CISPR 16-1-4 sets the reference against which all shielded enclosure test facilities are benchmarked.

Comparison with Alternative Test Facilities

Semi-anechoic chambers (SACs) are the main alternative to open area test sites for radiated emissions measurements. A SAC uses RF-absorbing material on walls and ceiling to replicate free-space conditions indoors, solving the ambient noise problem that can invalidate outdoor measurements in urban or industrial environments. However, NSA validation ties the chamber's performance directly to OATS reference values, meaning the outdoor site remains the ultimate reference standard for the field. EMC testing guidance comparing OATS and SAC facilities notes that OATS measurements remain preferred for certain frequency ranges and for equipment that is too large for chamber facilities.

Full-anechoic rooms (FARs) and reverberation chambers serve specialized testing needs but require separate validation methodologies not derived from OATS reference conditions.

Applications

Open area test sites have applications in a range of fields, including:

• Consumer electronics compliance, verifying products meet FCC Part 15 and CISPR 32 emission limits
• Automotive EMC, testing vehicle electronics in the 30 MHz to 1 GHz range
• Military and aerospace, measuring emissions from communication and navigation systems
• Antenna calibration, providing a reference environment for determining antenna factors
• Research and metrology, serving as the reference standard for national measurement institutes