Wi-Fi Signal

Wi-Fi signals are transmitted through radio waves; therefore, interference and attenuation are inevitable.

Wi-Fi standards impose strict restrictions on the transmit power of devices. An indoor antenna gain cannot exceed 5 dBi. In addition, Wi-Fi signals attenuate exponentially over distance. That is, signals attenuate by 20 dBm when the unobstructed distance increases by tenfold.

Distance (m) Signal Power (mW) Relative Strength (dBm)
1 100 +20
5 4.0 +6
10 1.0 0
25 0.16 –8
50 0.04 –14
100 0.01 –20

The gain of an indoor antenna is within 5 dBi. Therefore, a longer antenna can provide stronger signal strength.

The antenna gain depends largely on the antenna size. There are 1-dbi, 3-dbi, and 5-dbi antennas. Between two 5-dBi antennas, the one larger in size can offer greater receiving sensitivity if not a larger gain. If one terminal receives Wi-Fi signals at a strength of 1 bar but cannot access the network, this is because the antenna in use has poor receiving sensitivity.

The main causes of signal attenuation are metal objects, walls (especially load-bearing ones), and large furniture or household appliances.

 

The following table lists the empirical penetration loss values when 2.4 GHz electromagnetic waves penetrate various construction materials.

Material Penetration Loss
Reinforced concrete partition (load-bearing wall) 20–40 dB
Ceiling 30 dB
Concrete wall 10–18 dB
Hollow brick wall 4–6 dB
Plasterboard wall 3–5 dB
Common glass door and window 2–4 dB
Wooden door 3–5 dB
Coated glass door and window 12–15 dB
Wooden furniture, door, and other wooden partition 2–15 dB
Metal object Total reflection
Water Full absorption

Wireless signals are transmitted at the 2.4 GHz frequency (802.11b/g/n) or 5 GHz frequency (802.11a/n/ac). These frequencies are also used by microwave ovens, refrigerators, wireless mice, and some cordless phones, resulting in co-channel interference. In addition, the closer an interference source, the stronger the interference. Moreover, if there are multiple wireless signals around, wireless signals on the same channel or adjacent channels interfere with each other, making transmission unstable.

Interference Source Frequency Power Evaluated Interference
Microwave oven S-band (2.4–2.5 GHz) > 800 W Severe: The network speed decreases sharply when the distance is between 2 m and 4 m, and the network is disconnected occasionally when the distance is less than 2 m.
Cordless phone 2.4 GHz 3 W Fairly severe: The network speed decreases when the distance is between 0.5 m and 1 m, and the network is disconnected when the distance is less than 0.5 m.
Wireless camera 2.4 GHz 0.5–1 W Slight, but it is better to keep the interference source far away.
Bluetooth device 2.4 GHz 1 mW Insignificant