# Comparison of Radio Frequency Path Loss Models in Soil for Wireless Underground Sensor Networks
## 2 Experimental Methodology  
As it is difficult to conduct tests for various soil types and conditions in the field, a laboratory test set-up was developed to allow the investigation of the signal attenuation for a range of soil types and water contents. Several aspects need to be considered in the design of the test bed used to measure the signal attenuation in the laboratory to avoid negative/unwanted effects on the tests and ensure reliability of the measurements. The following are the main aspects which were considered for development of the test setup: (1) Pathway of RF signal in soil sample; (2) Soil container shape and geometry; (3) Elimination of the environmental interferences (i.e., reflection of signal); (4) Soil composition and compaction methods; (5) Measurement techniques. The ideal setup for RF attenuation tests would be to have an infinitely large container to remove any effect from the container on the measurements. However, this is not practical, as preparation of soil samples for a large container would not be feasible. Initially, a plastic container was selected for the purpose of the tests. However, initial tests showed that environmental EMI (Electromagnetic interference) contaminations adversely affected the readings. Therefore, for the purpose of these tests, an aluminium container with dimensions of 60 cm (length) \(\times\) 10 cm (diameter) was chosen as the sample mould for measuring the EM signal strength along specific distances (Figure 2). Six holes were  
Figure 1: Comparison of modified-Friis and CRIM-Fresnel models. placed along the container at 10 cm apart from each other for signal measurement. The container was designed to have five measurement points including the last position at 50 cm. However, after initial testing it was observed that at the data from the 50 cm point was contaminated by the environmental interferences. This was caused by less shielding and soil at the end of the container and therefore, external signals could affect the signals more easily. Due to the inconsistencies in the data, although measurements were taken at this point, they were not included in the paper. A cylindrical shape was used for the container in order to ensure a maximum contact between the compaction rammer (4.5 kg rammer with the diameter of 10 cm) and soil surface at the edges of the container (avoiding sharp corners which the rammer cannot get to). In addition, a diameter of 10 cm was selected for the container based on the size of the rammer used for compaction the soil sample. The soil used in this experiment was a mixture of Leighton Buzzard Sand and English China Clay with three different gravimetric water contents, 10%, 20% and 30%. The mixing and compaction of the samples as well as gravimetric water content (GWC) determination were carried out based on the British Standard [18]. The prepared soil sample was placed in the container and the signal strength was measured at certain distances (holes along the container) from the transmitter. For the purpose of these tests three separate soil samples were prepared for each soil condition and the measurements were repeated five times at each location (total of 15 samples for each location and soil condition). The logarithmic averages of these measurements were then calculated and used for the comparison. The soil was changed between each set of measurements (three soil samples for each condition) in addition, the delay between measurements at each location was minimised in order to avoid changes in soil condition and homogeneousness to affect the results. Due to the large attenuation of RF signals in soil higher frequency Industrial, Scientific and Medical (ISM) bands (i.e., 2.4 GHz) are not used in underground Wireless System Networks (WSNs). 434 MHz is one of the most commonly used ISM band for long range WSNs and was selected for the purpose of this research. The transmitter node used in this research was a previously developed 434 MHz node for underground WSN with a bandwidth of 150 KHz (Frequency Shift Keying, FSK modulation).