## 6 Field Test of the WSN Platform
### Issues during Field Deployment  
#### 6.2.1 RF Range Issues  
Although the size of the test field was relatively small compared to the communication range in ideal conditions of about 17 km for the 433-MHz band with a 110 dBm link budget, we noticed during deployment that some nodes at more than 50-70 m from gateways were received little or not at all.  
Several design decisions reduced the communication range. First, the placement of the gateway antenna for field communications between the solar panels of the birdhouse (see Figure 9) reduced its efficiency. We expected this, but aesthetic considerations prevailed, since we had not the time to measure the actual loss before the test. After the test, we measured about a 24-26 dBm loss, a penalty that led us to install external sockets for all aerials in the new birdhouse project.  
After the test, we discovered two additional RF attenuation causes on the PCB of the sensor node. As shown in Figure 3, the antenna is just a few millimeters away from two large ferromagnetic items: the battery side seal and the case of the quartz crystal resonator. The typical loss of the NMHA is about 6-8 dBm, but these two elements combined were adding about a 10 dBm loss. Thus, in the next version of the sensor node PCB, we will use a different case for the crystal resonator and move the battery away from the antenna to reduce its RF attenuation.  
The layout of the baluns of both the sensor node and the gateway (shown in Figures 3 and 4) was compacted to reduce the size of the nodes, size being especially important for the sensor node. One of the purposes of the node prototype was to determine the loss of the compact balun layout, which we determined after the test to be around 3 dBm. Thus, we accepted the larger size of a straight balun layout.  
The package of the sensor node (shown in Figure 8) was introducing an attenuation of about 2 dBm, but this cannot be easily avoided.  
Another attenuation cause was due to terrain geometry. The test field shown in Figure 6 had a very gently slope at the bottom-left corner, almost imperceptible to the eye. To increase the sensitivity to heat released by ground fire, we installed the sensor nodes low on the tree trunks, about 0.5 m above the ground. The gateways, on the other hand, were placed higher in the trees to improve both energy harvesting from sunlight and RF reception from low-placed sensor nodes. However, Node 1 and, even more, Node 56 were attenuated too much by this small terrain undulation, which was easily discovered by the checks performed during deployment. Thus, we decided to install the affected nodes higher, at 1 m from the ground. Nevertheless, this incident shows that even small terrain undulations may restrict the coverage of the gateways, especially for distant nodes, and this should be taken into account when choosing the positions of the gateways in the field. We also decided to develop a repeater node that can be deployed to improve the gateway coverage of the field.  
In general, the deployment positions of the gateways on hilly terrain should be carefully analyzed during planning. Nevertheless, there can always be propagation issues that are discovered only during deployment (e.g., due to fading from multipath propagation or obstacles). Some of these issues can be fixed by changing the position of the sensors (a different position on the tree trunk, higher from the ground or on a nearby tree). In other cases, we can always add a gateway to cover these areas or, even better cost-wise, a repeater device.