//---電波在空氣的衰減
若將單位改為距離是公里, 頻率是 GHz, 則
Loss in KM & GHz
= -147.558 + 20 LOG (D * 1000) + 20 LOG (F * 1000000000)
= -147.558 + 20LOG D + 60 + 20LOG F + 180
= 92.442 + 20LOG D + 20LOG F

2.4G , 0.01km=10m
Loss
= 92.442 + 20Log(0.01) + 20Log(2.4)
= 92.442 + 20*(-2) + 20*(0.380211)
=60 (dbm)

2.4G , 0.1km=100m
Loss
= 92.442 + 20Log(0.1) + 20Log(2.4)
= 92.442 + 20*(-1) + 20*(0.380211)
=80 (dbm)

2.4G , 1km=1000m
Loss
= 92.442 + 20Log(1) + 20Log(2.4)
= 92.442 + 20*(0) + 20*(0.380211)
=100 (dbm)

//---BT無線電功率等級
Class 1 = 100 mW = 20dBm =(100 meter)
Class 2 = 2.5 mW = 4 dBm
Class 3 = 1mW = 0 dBm =(10 meter)

dBm
20 dBm=100mW
0 dBm= 1mW
-10 dBm= 0.1mW
-70 dBm=100pW
-80 dBm=10pW
-90 dBm=1 pW

頻率.功率與距離

Bluetooth Device Sensitivity    -70dBm
2.4Ghz Wave that travel 10m will loss of 60dB

Receive Power=Transmit Power(0 dBm)
             – Transmission Loss(60 dBm)
             – Receiver Loss(5 dBm)
            = -65dBm
2.4Ghz Wave that travel 100m will loss of 80dB
Receive Power=Transmit Power(20 dBm)
             – Transmission Loss(80 dBm)
             – Receiver Loss(5 dBm)
            = -65dBm

//---

變化

2倍是6dB

3倍是9dB

4倍是12dB

5倍是14dB

10倍係20dB