Understanding Wifi Signal Strength:
Wireless signal strength is traditionally measured in either percentile or dBm (the power ratio in decibels of the measured power referenced to one milliwatt.) The level of 100% is equivalent to the signal level of -35 dBm and higher, e.g. both -25 dBm and -15 dBm will be shown as 100%, because this level of signal is very high. The level of 1% is equivalent to the signal level of -95 dBm. Between -95 dBm and -35 dBm, the percentage scale is linear, i.e. 50% is equivalent to -65 dBm.
Get Wireless signal strength in dBm at Yun:
iwconfig 2>&1 | grep 'Signal level'
Link Quality=70/70 Signal level=-37 dBm