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
My reading now:
root@Arduino:~# iwconfig 2>&1 | grep 'Signal level'
Link Quality=70/70 Signal level=-21 dBm
Adjust router antenna and make router and Yun closed together as well as add high gain antenna into Yun will bring Signal level up.
Should not bring 'Signal level' too high, It could be saturation the RF circuit.