##### Open Area Test Site

Open area sites are only required to meet site attenuation at the center of the turntable, while chambers are required to meet volumetric site attenuation. In other words, chambers are required to meet site attenuation not only at the center of the turntable, but also around the perimeter of the chamber's quiet zone, which is a much more stringent requirement.

Open area sites could be 100% in compliance with the requirements, and still have technical problems that would result in the artificial failure of a product. Conversely, since chambers are held to more stringent standards, not only does this reduce the possibility that a product will fail due to a problem with the test site, it also increases the repeatability and reproducibility of the data.

The Open Area Test Site (OATS) is a 3 and 10 meter emissions test range. The ground plane is perforated sheet steel for a durable consistent surface.

All cable runs are underground. All control, power and signal lines are isolated from one another and extra isolated conduits are available for support equipment.

The turntable is a large, custom built, all aluminum, flush mounted 10 foot diameter table.

PNK offers a Equipment to testing EMI and Antenna Characteristic which covers a large range of electromagnetic in the Low frequency Spectrum and also cover high frequency EMI.

## OATS design

The first task was to investigate the performance of different sizes of OATS ground-planes. An important part of the construction of an OATS is the metallic surface, which is laid onto the ground to stabilize the grounds reflection co-efficient. Advice on the construction of OATS can be found in a number of standards documents such as ANSI C63.7 [1] or EN55 022 [2]. The size of ground-plane needed to give the required reflection characteristics is described in both [1, 2]. [1] in particular goes into much detail in assessing the size needed for a good reflecting surface by using the theory of Fresnel Zones.

The first Fresnel Zone is the area of reflecting surface for which the path length for a reflection from the edge of the Zone is half a wavelength longer than a reflection from the center of the Zone. Referring to Figure 1, the mathematical description of the Fresnel Zone is:

d2 - d1 = l/2(1) where d2 describes the path taken when a reflection occurs from a point on the edge of the Fresnel.

Zone, and d1 is the ray-tracing path of reflection. [1] considers that the first Fresnel Zone is the minimum area needed to provide a good reflection but gives no indication as to how the reflection characteristics of this surface compare with the characteristics of the ideal case of an infinitely large surface. The use of the Fresnel Zone is also lacking in that only the phase difference between the reflections from various parts of the reflecting surface are used, and that the path length differences are not.

The investigation attempted to find a more detailed analysis of the required size of reflecting surface than that in [1].

## Fresnel Zones

The theory of Fresnel Zones is taken from books on optical theory, for example [3]. The problem from the point of view of optical theory is shown schematically in figure 2. The light from the source of radiation, which reaches the sink of radiation, travelling through the aperture, is treated using the Huygens principle. This states that each element of area at the aperture acts as a point source of radiation. The following equation is the description of light travelling from source to sink:

Equation 2 is Fresnel’s equation. The term accounts for the phase difference between the rays passing through different points of the aperture. Similarly the term 1/rs describes a decay in intensity due to the path length difference between rays passing through different points of the aperture.

The two objectives were to account for the path length difference between the reflection from various points on the ground-plane and to give an indication of the quality of the reflection from particular sizes of ground-plane.