The distance of 140 meters is chosen based on the authors’
previous experiments on the communication distance and data
loss statistics. It is a distance approaching the limit of acceptable
transmission for the specific equipments (i.e., Imote2 and
antenna) in this research. Data transmission at distances larger
than 140 meters suffers from severe unreliability and data loss
that sometimes goes beyond 50%. Such excessive communication
distances should be avoided in properly deployed SHM
systems. However, if such weak links are indeed unavoidable,
the re-transmission based communication method can be firstly
used to reduce data loss to the extent where redundant coding
can take effect.