**Abstract**

Multiprobe spherical near-field measurement is a potent tool for fast and accurate characterization of electrical properties of antennas. The use of fast switching in one axis, an azimuth positioner, and a near- to far-field transformation allows a substantial time reduction in antenna measurements while maintaining high-quality results. On the other hand, conventional emissions EMC measurement systems are typically based on detecting the radiated spurious emissions by a device at different frequencies. The systems usually work in far-field (or quasi-far-field conditions), performing the measurements either at 3 or 10 meters. Measurements under these conditions take space and time. Moreover, the systems are not costeffective for pre-compliance purposes where pre-testing of the device should provide valuable information and confidence about the DUT before performing a compliance test. This chapter analyzes the possibility of cost and space reduction for EMC systems based on multiprobe near-field measurement systems in combination with OTA (over the air measurements), reference-less systems, spherical near-field transformation, phase reconstruction, modal filtering, source reconstruction, and software-defined radio receivers.

**Keywords:** electromagnetic compatibility, measurements, near-field, multiprobe, over-the-air

### **1. Introduction**

Electromagnetic Compatibility systems are well established and have been widely studied during the last decades. There are a lot of references in this area, like [1]. On the other hand, multiprobe spherical near-field measurements systems have been extensively used for antenna characterization, becoming a potent tool for fast and accurate characterization of electrical properties of antennas. The use of fast switching in one axis, an azimuth positioner, and a near- to far-field transformation allows a substantial time reduction in antenna measurements while maintaining high-quality results. During the last years, combining these kinds of tools with postprocessing techniques to increase the accuracy of the measurements and reduce some spurious effects like noise, leakage or echoes, has shown promising results [2]. As explained in the same book, the combination of measurements with simulations has shown the possibility of considering some effects of the measurement scenario.

On the other hand, conventional EMC systems are typically based on detecting the maximum power radiated by a device at different frequencies. The systems

usually work in far-field (or quasi-far-field conditions), performing the measurements either at 3 or 10 meters, but sometimes the device's position under test (DUT) in the setup affects the measurement. Even if there are very well-established standards for EMC measurements to get these peak values and check good performance (low radiation) of the electronic devices, the measurement uncertainty is higher than the one found for antenna measurements.

Some practical radiation emission measurement solutions try to overcome the high cost of pre-compliance chambers that typically can go up to hundreds of thousands of euros. These solutions are based on small systems implemented by robotic arms to scan the volume around the DUT [3–7]. The solution is well suited for diagnostics since the field can be measured very close to the DUT. Moreover, the low-cost systems typically are not shielded, and the phase recovery capabilities are strongly setup dependent. Nevertheless, just a few of them are able to reconstruct the phase and thus compute the field at 3 or 10 meters from the near-field information.

This chapter analyzes the possibility of cost and space reduction for radiation emission EMC measurement systems based on the use of techniques already used in near-field antenna measurements, including near to far-field transformation algorithms, OTA (over the air measurements), reference-less systems, multiprobe arrays, phase reconstruction, modal filtering, source reconstruction, and softwaredefined radio receivers. During the last years, the authors have been working on all those topics for the complete EMC system, as can be observed in different papers in journals and conferences [8–13], and this chapter summarizes all the work included in those previous research works. The chapter describes the advantages of low-cost near-field measurement systems that could be used for EMC pre-compliance measurements, showing some practical results.

The following sections will focus on several of the essential aspects of this kind of system. Section 2 will explain the possible near-field EMC system architectures, explaining each subsystem (hardware or software). Section 3 will explain the configuration of multiprobe array systems for over-the-air (OTA) systems. Section 4 will explain the amplitude and phase calculation using cheap and integrated Software Defined Radio (SDR) receivers. Section 5 will focus on the effects of near to far-field spherical transformation algorithms, and Section 6 will introduce some of the post-processing techniques that can be included for EMC systems. As the reader can observe, all these techniques have been widely used in antenna measurements, although there is still an open research line to redefine their limitations for EMC measurements, where the objective is to detect the radiated power peak values instead of the 3D radiation pattern.

#### **2. EMC system architecture**

The near-field EMC measurement system proposed is based on the architecture of a multiprobe near-field antenna measurement system. In this case, we are using some of the conventional Microwave Vision Group measurement multiprobe setups [14]. These systems are based on wideband dual-polarized probes located on arch systems. The receiver is switching between probe and probe in order to acquire the amplitude and phase of the electromagnetic field generated by the probe. Instead of using a conventional vector network analyzer, in this case, we have replaced the receiver with an SDR platform, whose performance will be shown in the following section. For this application, the DUT is self-transmitting, and, therefore, it is not necessary to include a specific transmitter, or in that case, the transmitter is not synchronized with the receiver, as in the conventional antenna measurement

#### *EMC Measurement Setup Based on Near-Field Multiprobe System DOI: http://dx.doi.org/10.5772/intechopen.99604*

systems. This is called over-the-air (OTA) systems, although the procedure we use for this specific application is called reference-less system [11, 12] since the reference is not extracted from the transmitter but from a fixed probe.

Two different architectures that can be used for phase reconstruction in a multiprobe system are presented. The first one is based on an external fixed probe whose relative position is kept fixed with respect to the DUT (**Figure 1**). In this case, the reference antenna is sensitive to amplitude and phase changes (proximity to mast, motor, cable, and multiple reflections). Nevertheless, it will be seen that excellent performance can be achieved with such a simple setup while keeping the advantages of a multiprobe system. Moreover, the solution is scalable and could be implemented in multiprobe systems of different sizes to cover larger DUTs or lower frequencies.

The second solution is based on a reference channel for phase reconstruction that uses one of the probes of the multiprobe arch. This solution does not need any additional hardware, but the complexity is transferred to the post-processing techniques. The solution is described in **Figure 2**. In this case, the setup is simplified, and the coupling between probes reduced since the reference channel belongs to the multiprobe arch.

Some extra post-processing steps are needed in order to retrieve the phase information [11]. The results obtained with this setup can be better than using an external reference antenna; nevertheless, the complexity arises for situations in which the elevation arch needs to be rotated. In this case, the top-probe is not onaxis, and further mathematical derivations are needed in order to reconstruct the near-field phase. This can be done by a non-convex iterative optimization algorithm, as explained in [15].

The next step is the near to near-to-far-field transformation. The theory included in [16], proposed by Prof. Hansen in 1973, and widely used in antenna

**Figure 1.** *Measurement setup based on external reference antenna.*

near-field measurement is used. This theory is based on the decomposition of the electromagnetic field in spherical mode coefficients, using as input information the amplitude and phase of both orthogonal tangential electric field components (theta and phi) in a sphere enclosing the DUT. Once the spherical mode coefficients are calculated, the field can be computed at any desired distance, in particular 3 or 10 meters.

Some post-processing techniques appeared during the last year that use the information available of the antenna under test and the measurement system, could also be used to improve the results of these EMC measurements. These techniques are based on the filtering of the electromagnetic field in other domains: time domain, antenna electromagnetic sources, spherical modes, or cylindrical modes. A summary of all these techniques can be found in [2], and some of them have been used for extracting the results presented in this work.

Finally, the results are compared with the conventional EMC standards [17, 18] for the different cases to ensure that the DUT passes the final compliance testing in terms of radiation. In comparison with other compact and low-cost solutions, the use of a shielded and anechoic environment that is typically used for antenna characterization [14] provides accurate results when calculating the radiated field at a finite distance. For source reconstruction, the solution proposed here is based on the commercial software INSIGHT [19]. The measured tangential components of the near-field can be exported, and the equivalent currents of the DUT

**Figure 2.** *Measurement setup based on the top probe as a reference antenna.*

**Figure 3.** *EMC spurious emissions characterization with near-field multiprobe solutions.*

reconstructed in order to determine which areas of the DUT might be responsible for the non-desired radiated emissions.

**Figure 3** summarizes the process, and in the following sections, the different parts will be explained.
