## 2 System Configuration of Open-Area EMC Testing
### Near-Far EMC Testing Configuration  
Based on the EM radiative characteristic, our open-area EMC testing system is designed by using a near-far EMC testing configuration similar to that used in the literature [9; 10], which is shown in Figure 1. The symbols \"G\", \"M\", and \"D\" refer to the typical internal interferences of a plane. G denotes the electric generator, M denotes the motor, and D denotes the other electric devices, such as the digital signal processor (DSP) and digital flight controller (DFC).  
There are two omnidirectional measurement antennas connected to two spectrum analyzers (SA), which are synchronized by setting one of them as the internal trigger and the other one as the external trigger. One of the measurement antennas (named the near-site antenna) is placed near the radiant source (or device under test (DUT)) (1 m or 3 m), and the other one (named the far-site antenna) is placed far away from the radiant source (9 m or further).  
Then, the waterfall figures of the two spectrum analyzers are captured, which contain both the signal and environmental interference. According to the EM radiative characteristic, when the signal is near the testing site, i.e., the DUT, the received signal falls off significantly with the distance. As a result, the signal strength of the near-site antenna is larger than the far-site antenna. In contrast, when the signals come from far away from the DUT, we consider them to be environmental interference, and the distances to the near-site and far-site antennas are almost the same. As a result, the signal strengths of the near-site and far-site antennas are similar. Based on this feature, the environmental interference can be clearly recognized from the internal signal and easily suppressed.  
A shortcoming of the near-far EMC testing configuration is the need to deal with the frequency spectrum superposition problem. In this case, the internal interference occupies the same frequency band as the environmental interference. Thus, the two spectrum analyzers (SAs) acquire the superposition of the internal and environmental interferences. The effect of the frequency spectrum superposition depends on the relative intensities of the internal and environmental interferences. If they possess the same intensity, the superposed  
Figure 1: Near–far EMC testing configuration.  
intensity will theoretically range from 0 (exact cancellation) to 2 times the original intensity (coherent accumulation).  
For independent Gaussian stochastic signals, the variance of the superposed signal is the sum of the variances of the internal and environmental interferences. When the variance of the internal interference is 10 and the variance of the environmental interference is 1, the superposed variance will be 0.41 dB larger than that of the desired one (internal interference), which is acceptable in practice. Conversely, when the variance of the internal interference is 1 and the variance of the environmental interference is 10, the superposed variance will be 10.41 dB larger than that of the desired one (internal interference). In this case, the superposition phenomenon violates the assumption of electromagnetic (EM) radiative characteristics and greatly deteriorates the recognition and suppression of internal and environmental interferences. Thus, the frequency spectrum separability should be examined before employing the near-far EMC open-area testing method.