# Intelligent Measurement of Coal Moisture Based on Microwave Spectrum via Distance-Weighted kNN
## 2 Materials and Methods
### Measurement System and Procedure  
The measurement system based on the microwave free-space transmission, which was used in this study, is shown in Figure 2. A portable Vector Network Analyzer (VNA, MS46122B-020, Arnitsu, Japan) was used to generate and measure broad bandwidth microwave signals. A mechanical calibration kit (TOSLKF50A-20, Anritsu) was employed to calibrate the VNA using the short-open-load-through (SOLT) method to correct the system error. A pair of identical standard-gain horn antennas (EIA standard-WR90 waveguide type) were fixed on a linear slide rail and connected to the transmitting and receiving ports of VNA through a coaxial cable. A rectangular container (material: Polymeric Methyl Methacrylate, PMMA) with an open-top was used to hold coal. The VNA was connected to the computer through a USB-interface data transmission line, and the measurement control and further signal processing were performed by the computer.  
The coal samples used throughout this experiment were manually collected commercial coals from six different regions in China, including Henan province (Region-A), Shandong province (Region-B), Shanxi province (Region-C), Inner Mongolia (Region-D), Guizhou province (Region-E) and Shaanxi province (Region-F). According to the standard for measurement of coal moisture content [6], the commercial coal samples of each group were sieved through a 13-mm sieve to obtain the experimental coal samples. To obtain coal samples with different MCs, three steps were performed on the experimental coal samples: (1) Sample drying. The experimental coal samples were dried in a forced-air oven at 108 \({}^{\circ}\)C for two hours to obtain completely dry coal samples. (2) Moisture addition. An appropriate amount of water was added to the dry coal samples in order to obtain coal samples with specific moisture content. Constant stirring was required at this point to mix the coals and water evenly. (3) Static diffusion. Put the coal samples obtained in the previous step into a sealed bucket for 12 h to ensure that the water was fully diffused in the coal samples. The processes of static diffusion and subsequent microwave signal acquisition were both carried out at the normal temperature of 25 \({}^{\circ}\)C.  
The coal samples after diffusing were taken out and packed into the rectangular container for microwave signal acquisition. During the signal acquisition, the VNA gen  
Figure 2: The measurement system based on microwave free-space transmission, including 1. Vector Network Analyzer for generating and measuring broad bandwidth microwave signals, 2. Computer for controlling measurement progress and processing signals, 3. Rectangular Container for holding coal samples, 4. Horn Antenna for receiving and transmitting signals, and 5. Linear Slide for fixing the antennas.  
erated microwave signals containing 133 frequency points from 8.05 GHz to 12.01 GHz with a 30-MHz interval. When microwaves penetrated the coal sample successively, the microwave scattering parameters of coal were sequentially recorded. After the signal acquisition, the MC of the coal samples was measured according to the method of standard [6] and used as the standard reference value. The above processes of sample preparation were repeated to obtain the sample for the next moisture level. In this experiment, a total of 294 original microwave spectrum curves were obtained, as shown in Figure 3.