On the Mitigation of Radiation from Plc Networks

An improvement to an existing technique for the reduction of emissions from PLC networks is proposed. Using numerical simulations, the technique is shown to reduce anyone of the components of the radiated magnetic field by up to 80 dB for a theoretical case. The applicability of the technique to real networks is discussed. INTRODUCTION Electromagnetic radiation from PLC signals may cause interference to services operating in the same frequency range. The regulation of electromagnetic radiation from PLC signals is currently the subject of investigation in various international standardization bodies [1]. The aim of this paper is to propose an improvement to an existing technique for the reduction of electromagnetic emissions from PLC networks. MITIGATION OF EMISSIONS FROM INDOOR PLC NETWORKS A mitigation method using an auxiliary current was recently proposed in [2]. The idea is to take advantage of the presence of the additional ground conductor to inject a ° 180 out-of-phase version of the PLC wanted signal into the ground-neutral circuit. If all three wires run parallel and in close proximity to one another, the expectation is that the radiated fields from both circuits will have similar amplitudes and that they will be approximately ° 180 out of phase. Under these conditions, destructive interference should lead to partial or total cancellation of the radiation. Indeed, numerical and experimental tests carried out on simple network geometries show that reductions in the emissions of more than 20 dB can be achieved [2]. Measurements made on more realistic geometry networks showed, however, less good results and even a worsening of the emissions level for certain frequencies at which the interference between the radiation from the wanted and from the compensation signals (also called auxiliary signals) appeared to be constructive [3, 4]. We tested the technique using the setup shown in Figure 1. Generator/Receiver Splitter coupler coupler 3-conductor NUT Magnetic loop antenna Figure 1. Schematic measurement setup. We used a simple extension cord as the Network Under Test (NUT). We carried out measurements for two different cable layouts: 1) a 2-meter, straight layout and 2) a random layout. Photographs of the two layouts are shown in Figure 2. In both cases the NTU was terminated in two resistances of Ω 150 , one between phase and neutral and the second between neutral and ground. (a) (b) Figure 2. Photographs of the NUT layouts used. a) Straight 2-m cable layout. b) Random layout. 1 10 -10 -5 0 5 10 15 20 25 Frequency [MHz] M ag ne tic fi el d [d B -u A /m ] Using compensation Without compensation (a) 1 10 10 15 20 25 30 35 Frequency [MHz] M ag ne tic fi el d [d B -u A /m ] Using compensation Without compensation (b) Figure 3. Magnetic field measured with and without the compensation signal for (a) the 2-meter straight-segment setup shown in Figure 2a, and (b) the random layout shown in Fig. 2b. As can be seen in Figure 3a, the compensation signal significantly reduces the emissions from a straight cable over a wide range of frequencies. However, for the random configuration layout in Fig. 2b, the compensation signal does not help reduce the emissions level over the whole band and, even at those frequencies at which a reduction is in fact observed, it is considerably smaller than in the straight cable case. This result is in agreement with the results reported by Marthe et al. in [3, 4]. In the next section, we will introduce an improvement to this mitigation technique that allows the reduction of emissions for complex, realistic NTUs. IMPROVED MITIGATION METHOD In the new method, instead of a constant ° 180 phase shift, the phase and the amplitude of the auxiliary signal are selected at each frequency to minimize emissions at the physical region of interest [5, 6]. We begin by writing the equations governing the system consisting of the pair of conductors used for the transmission of the PLC signal, the pair of conductors used to carry the auxiliary or compensation signal (one conductor being common to both pairs) and a point where the radiation fields are to be mitigated. The geometry is illustrated schematically in Figure 4.