Effect of rainfall on link quality in an outdoor forest deployment

Existing work has shown that rainfall has an effect on link quality. Some authors report a positive effect in moist conditions, whereas others demonstrate a significant decrease in link throughput as a result of rainfall or fog. The precise cause of these variations has not yet been conclusively established. This paper reports on long term (26 day) link quality results from 12 nodes deployed in a forest. We found that rainfall has the effect of decreasing the performance of 28% of good links (classified as those having above 90% packet reception), but simultaneously increasing the performance of 34% of poor links (those having below 50% packet reception). In addition, it was found that variations in link quality persisted for a few days after rainfall. This suggests that link variations are not a result of rain induced fading, but rather due to water sitting on node packaging. We present experimental evidence which demonstrates that changes in link quality (both positive and negative) are indeed due to the presence of water, capacitively loading the antenna, altering its radiation pattern.