## 4 Results and Evaluation
### Assessing Signal Strength Measurements  
The methods followed for testing the effective communication range of the proposed monitoring system utilized the techniques presented in [27]. As already mentioned, the sink/gateway node, apart from soil-specific information for each node, collected auxiliary data, including time stamps and the received signal strength indicator (RSSI) per each sensor node. The latter information was collected from sensor nodes at various distances and locations from the sink/gateway node, inside the university campus, in the presence of vegetation, and with agricultural buildings nearby.  
Figure 12(a) illustrates the RSSI, in dBm, as a function of time, in min, for a 5-day period including soil wetting on day one for node #A1 (red curve) and node #A2 (blue curve). As can be observed, the placement of the antenna of #A1 underground reduces the RSSI by 8 dBm, approximately, while the intense wetting of the soil close to the sensor box further deteriorates the signal roughly by 6-8 dBm. These research findings are in accordance with the analysis and experimentation in [26; 36; 37]. Thanks to the high noise resilience and sensitivity of the LoRa technology, these cumulative signal degradation factors leave an efficient margin for justifying the application of the technique of placing everything underground. This option should be valuable in cases where either the protection of equipment from environmental stress and malicious actions or the ease of its in situ installation are parameters that really matter.  
Figure 13: Range coverage-related measurement arrangements and results: (**a**) RSSI, in dBm, as a function of time, in min, for a 5-day period including soil wetting on day one for node #A1 (red curve) and node #A2 (blue curve); (**b**) the area inside the university campus where the LoRa radio distance coverage experiments took place; (**c**) graphs corresponding to the radio distance coverage experiments, in a mixed environment, including trees, open areas/roads, and agri-buildings, as measured for sensor #A2.  
Figure 13b depicts the area where experiments took place, while Figure 13c illustrates the fading of the RSSI values as a function of the distance from the sink/gateway node, as measured for the setup of #A2. According to the results gathered, by using the LoRa modules with 2 dBi omnidirectional antennas for the sensor nodes and a 5 dBi antenna for the sink/gateway (i.e., as described in Section 3.1), the communication distance coverage was fluent up to the 450 m physical limit of the campus area in a mixed-obstacle environment for the LoRa radio settings of 14 dBm transmit power, 125 kHz bandwidth, coding rate 4/5, and 128 chirps/symbol (SF7) spreading factor. Given that, with these radio module settings, the sensitivity of the LoRa chip can reach the \(-130\) dBm level and that the transmit power of the transceiver can be increased up to 23 dBm, distances of more than 1 km can be achieved with the specific inexpensive omnidirectional antennas used. These results are in agreement with the LoRa radio characteristics presented in [19] and the extensive LoRa performance measurements previously conveyed in [27].