An "OATS" or an open area test site is the most common radiated emissions test site.

They are most commonly constructed to comply with the requirements of ANSI C63.4 (for North America and elsewhere) and CISPR 16-1-x (for Europe and elsewhere).

The distance between the antenna and the equipment under test (EUT) is typically 3m, 10m or 30m. The measurement distance is important because you want to ensure that you are measuring the field strength in the far field as opposed to near field.

At 30 MHz, the wavelength is 10m. As you approach the near field or fresnel region (region between near and far field), the electric field may not yet be stable and the measurements will be less accurate.

Some standards mandate a specific separation, while others allow the use of 2 or more different separations. Because the strength of the electromagnetic field varies with distance, the limits are re-calculated at each measurement separation.

Although it is usually acceptable for test measurements to be compared between different measurement distances by factoring in an extrapolation factor of approximately 20dB/decade, in reality there are issuesthat make comparisons of measurements at different types of facilities problematic.