# A New Approach to Estimating the Path Loss in Underground Wireless Sensor Networks
## 3 TDR and Field Trials
### Trial Setup  
Due to the challenges of conducting robust field trials, very few trials have been carried out investigating RF communication through soil [4, 16]. During this research, the attenuation of RF signals in soil were measured at three separate locations (labelled A, B, C in the following sections) to provide a varied set of data for testing the performance of the models. The locations of the trials were selected to have different soil types and conditions. In addition, all of the locations had a soft surface (i.e., grass) in order to ease the process of digging and reinstating the ground, thus facilitating the implementation of the trials. The trial consisted of first excavating vertical holes in the ground with approximate diameters of 300-400 mm, depth of 500 mm and spacing of 500-600 mm, created using hydraulic post hole borers and/or vacuum excavation techniques. The depth of these holes was selected to avoid reflections from the surface [9]. Although 2.4 GHz is a common frequency band used in terrestrial  
Figure 2: The proposed method of calculating the rise time in TDR waveforms with end reflections [10]. wireless sensor networks, it is not suitable for environments where attenuation is high (due to higher losses). Previous studies on underground transmission using 2.4 GHz showed that due to the high attenuation of soil this frequency band is not feasible [5]. In these environments (i.e., soil) lower frequency bandwidths are preferred due to their lower losses and therefore longer potential range. During this project a WUSN node developed during the research [29] was used as the transmitter with fixed frequencies of 433 MHz and 868 MHz. The node was placed at the bottom of the first hole and the hole was backfilled and compacted with the same soil. Handheld RF spectrum analysers were used to detect the signal strength at various distances from the transmitter by burying them or their antenna at the same depth at a fixed horizontal spacing from the transmitter in the other excavated holes. The soil was compacted over the receiver for each location. The RF spectrum analysers were connected by a shielded USB cable to a laptop where the results were logged. Figure 3 shows a schematic of the trials. Samples of soil were taken from each of the holes at multiple depths to analyse the characteristics of the soil. The particle size distribution (PSD) and the gravimetric water content of the samples were measured during the laboratory tests [10]. The dry sieving method based on British Standard 1377-2 [30] was used to obtain the PSD of the samples. The conductivity and permittivity of the samples were also measured in the laboratory using Time Domain Reflectometry (TDR) [11, 12]. Care was taken during the removal of the soil samples from the ground in order to minimise disturbance to the natural compaction state of the soil. The main parameters that can potentially negatively affect RF trials are antenna orientation, soil disturbance and instrument characteristics [13]. The high attenuation of EM signals in soil makes the orientation of the antennas a crucial parameter in the tests. For the purposes of these tests, omnidirectional antennas were used and all of the antennas were placed in the soil parallel to each other by ensuring that they were vertical to the horizon before the compaction stage. In addition, based on the parameters used in these tests (i.e., frequency, distance, antennas) and the Fraunhofer distance (\(d_{f}=\frac{2D^{2}}{\lambda}\)) the measurements are considered to be in the far field region (\(>\)\(d_{f}\)) [31]. Any disturbance of the soil will alter its dielectric properties. As the soil is excavated from the holes, irrespective of the excavation method, it will lose its original structure and density. Therefore, in order to ensure repeatability and reliability of the results it was very important that the excavated soil from each hole was compacted back on top of the transmitter/receiver to restore the soil to its original condition as much as possible.