**Abstract**

The state of the art on the open-area test site (OATS) has been introduced. Key technologies on the design and validation of a high-performance OATS have been provided. Some famous OATS in the world regarding their structure, the dimensions of the ground plane (GP), the location of the control room, and performance are listed in a table. A case study is provided on NIM's high-performance OATS. Many details are open for the first time, which show the fine design. A measurement uncertainty example has been provided in measuring the free-space antenna factor of biconical antennas. These results are based on the author's many years of experience, with lots of valuable data and photos. It is intended for calibration laboratories, for EMC antenna users, for writing EMC standards, as well as for the assessors in EMC.

**Keywords:** antenna factor, antenna calibration, open-area test site, calculable dipole antennas, site validation, site insertion loss

### **1. Introduction**

An open-area test site (OATS) is one of the key facilities in EMC. An OATS is a basic facility to measure the antenna factor [1] and the "benchmark" for semianechoic chambers over (30–1000) MHz [2].

When we make radiated disturbance measurements, both antennas and anechoic chambers are required. An OATS is the best choice for measuring the antenna factor over 30 MHz to 1000 MHz. We use antennas to "capture" the invisible electric fields (E-fields) or magnetic fields (M-fields) by converting the fields into voltage signals, which can be measured by an EMI receiver. However, the converting capabilities are different for different antennas. Thus, we need a parameter to "eliminate" this difference; thus, the antenna factor is defined as the ratio between the magnitude of E-field and the voltage induced on the load (usually 50 Ω) connected into the feed of an antenna [1]. One of the major activities for a calibration laboratory is to measure their value. In order to get a very accurate value for the antenna factor over 30 to 1000 MHz, OATS is the best choice since it can provide the most accurate value for broadband EMC antennas over this frequency band.

An OATS is the "benchmark" for semi-anechoic chambers over (30–1000) MHz. An EMC chamber can isolate the radiated emission by an EUT from others by a shielding enclosure outside the antenna and EUT. Usually, absorbers are lined inside to reduce the reflections along the four sides of an EUT and from the ceiling. However, it is very difficult to reduce totally due to the limits of technology and cost. Therefore, a quality verification process called validation is needed. The basic idea of this validation procedure is to measure the site attenuation (SA) between a pair of broadband antennas in the EMC chamber over (30–1000) MHz and compared with some standard value, which is defined as antenna pair reference site attenuation, *A*apr [3]. If the difference is less than some value (e.g., 4 dB or less), then the EMC chamber can be regarded as a qualified "compatible test site" (COMTS). Thus the accuracy of *A*apr is significant for the suppliers and end-users of the EMC chamber. Up to now, the OATS is the only way to measure the *A*apr accurately. "Normalized site attenuation" (NSA) can be used to validate the performance of EMC chambers [3], too; however, OATS is also required to measure the free-space antenna factor and the mutual coupling, etc., are very difficult to get [4, 5].

Due to the significance of OATS in the EMC area, it is worthwhile reviewing the technologies of design, construction and validation of a high-performance OATS for the following:


The start of the art on the OATS is introduced in Section 2; a detailed case study is provided on the design, construction, and validation in Section 3. An uncertainty example in measuring *A*apr is provided in Section 4.

## **2. The state of the art on OATS in the world**

The theoretical model of an OATS is listed in **Figure 1**. The electromagnetic fields (EM fields) radiated by a transmit antenna (Tx) are reflected only by the ground plane (GP), and the GP is infinitely large, consisting of a perfect electric conductor. Then, there are no other reflections. The receive antenna (Rx) received the EM fields radiated directly from the Tx and the reflections from the ground plane. Any other reflections are regarded as uncertainty sources. This means the better the ground plane size, the better agreement with an ideal OATS. Standard CISPR 16–1-5 states the minimum should be 30 m by 20 m. The most common ground plane size is 60 m by 30 m. The flatness of the metal ground plane is also important, especially above 700 MHz [6].

There are basically two types of structures on the large ground plane, as shown in **Figure 2**. In the solid type, the metal ground plane is placed directly on the surface of the concrete layer. This structure makes sure the metal ground plane can hold heavy EUT. In order to adjust the flatness of the ground plane, some space is designed between the metal ground plane and the concrete layer, which is called the floating type.

*Design, Construction and Validation of a High-Performance OATS DOI: http://dx.doi.org/10.5772/intechopen.99727*

**Figure 1.**

*The theoretical model of an OATS—Half-space model.*

**Figure 2.**

*The structures of the ground plane. (a) Solid type and (b) floating type.*

**Figure 3.**

*The design of stainless wire mesh (showing the corner of an OATS). (a) Inverted L-shape (vertical cut), (b) belt (bird view), and (c) triangular (bird view).*

The impedance transition between the edge of the ground plane and the soil is also important. Usually, stainless wire mesh is designed for this purpose. **Figure 3** shows some typical designs for the stainless wire mesh. In (a), the wire mesh is mounted vertically down to the soil. In (b), a wire-mesh belt is mounted around the edge of the metal ground plane. In (c), the belt is replaced by some triangular mire mesh. Simulation shows design (c) agrees best with the theoretical mode in **Figure 1**, under the same ground plane size and wire mesh area.

**Figure 4** shows the surface current induced. A pair of dipole antennas are resonated at 30 MHz and VP; the dipole antennas are separated by 20 m (R = 20 m)

*The comparison of triangular wire mesh. (a) No wire mesh and (b) triangular wire mesh.*

**Figure 5.** *The deviation of finite-size GP at VP (R = 20 m).*

both the Tx and the Rx are 1.3 m above the perfect electric conductor (PEC) ground plane. The height of the isosceles triangles is 10 m, and the bottom width is 5 m. The inner area (25 m by 15 m) of the ground plane is simulated with physical optics (PO), while others are simulated with Method of Moment (MoM). The simulation model is set up in free space; in other words, no consideration is taken into account on the effect of the grounding of wire mesh into the soil. **Figure 4** shows that the surface current fluctuated rapidly near the edge of the ground plane or the tips of the mesh [7].

The deviation *ΔA* is shown in **Figure 5**. The means for the legends are shown below:


As shown in [7], a GP of 30 m by 20 m is not qualified for measurements at R = 20 m, whose deviation is larger than 1.5 dB. However, it can be reduced to 1.2 dB at VP by introducing some triangular wire mesh shown in **Figure 4(b)**. If the GP is increased to 40 m by 30 m, the deviation is less than 1 dB. Less deviation at 30 m separation can be achieved by increasing the GP to 60 m by 40 m.

Further simulation shows that the triangular wire mesh is also effective for HP at R = 20 m, as shown in **Figure 6**.

The performance of a practical OATS should be as close as possible to the halfspace model shown in **Figure 1**. The IEC standard CISPR 16–1-5:2014 provides a detailed method to validate this, which is briefly summarized as in Eq. (1).

$$
\Delta A = |A\_{\rm m} - A\_{\rm c}| < T\_{\rm SIL} - \Delta A\_{\rm m}.\tag{1}
$$

where *ΔA* is the absolute deviation of the OATS from an ideal OATS, in dB, at the specified frequency; *A*<sup>m</sup> is the measured site insertion loss (SIL), in dB, between a pair of calculable dipole antennas; *ΔA*<sup>m</sup> is the SIL measurement uncertainty (*k* = 2), in dB, *A*<sup>c</sup> the calculated SIL, in dB; *T*SIL is the allowed tolerance in SIL, in dB. For a calibration test site (CALTS), *T*SIL = 1.0 dB at horizontal polarization (HP).

For a reference test site (REFTS), *T*SIL = 1.0 dB at HP and *T*SIL = 1.5 dB at vertical polarization (VP). **Table 1** lists some famous OATS in the world. Where CR stands for the control room (CR), some data are not available (N/A) due to the limited knowledge of the author.

**Figure 6.** *The deviation of finite-size GP at HP (R = 20 m).*


#### **Table 1.** *Some high-performance OATS.*
