Question 1

What is free space path loss?

Accepted Answer

Free space path loss (FSPL) is the energy a radio wave loses simply by spreading out as it travels away from its source, with nothing in the way. In Wi-Fi it is calculated as FSPL(dB) = 20·log10(d) + 20·log10(f) + 92.45, with the distance d in kilometres and the frequency f in GHz. It depends on only two things: how far the signal has travelled and what frequency it is on.

Question 2

How much does a Wi-Fi signal lose in the first metre?

Accepted Answer

About 40 dB at 2.4 GHz, 47 dB at 5 GHz and 48 dB at 6 GHz. Precisely: 40.2 dB on channel 6 (2437 MHz), 47.3 dB on channel 100 (5500 MHz) and 48.3 dB on 6 GHz channel 49 (6195 MHz). The higher the frequency, the higher the loss, which is why a 2.4 GHz signal reaches further than a 5 or 6 GHz signal at the same transmit power.

Question 3

How much signal is lost every time the distance doubles?

Accepted Answer

6 dB. The inverse square law says that doubling the distance spreads the same energy over four times the area, which is a quarter of the power, or −6 dB. So a signal measured at −40 dB after 1 metre is at −46 dB at 2 m, −52 dB at 4 m, −58 dB at 8 m and −64 dB at 16 m.

Question 4

How do I calculate the signal level in dBm at a given distance?

Accepted Answer

Take the EIRP of the AP — transmit power in dBm plus antenna gain in dBi — and subtract the path loss. An AP radiating 14 dBm EIRP on 2.4 GHz gives 14 − 40.2 = −26.2 dBm at 1 m, −32.2 dBm at 2 m and −44.3 dBm at 8 m. Every wall the signal crosses is subtracted on top of that.

Question 5

How much does a wall attenuate a Wi-Fi signal?

Accepted Answer

As a first pass: about 3 dB for drywall, 2 dB for a clear glass window, 6 dB for brick, 12 dB for concrete, 18 dB for reinforced concrete and 15 to 20 dB for a metal door or an elevator shaft. A wall's attenuation applies once, where the signal crosses it, and it stays lost for the rest of the path — which is why the curve steps down at the wall and never recovers.

Distance	Walls	2.4 GHz dB	5 GHz dB	6 GHz dB
1 m	—	-40.2	-47.3	-48.3
2 m	—	-46.2	-53.3	-54.3
4 m	—	-52.2	-59.3	-60.3
8 m	—	-58.2	-65.3	-66.4
16 m	—	-64.3	-71.3	-72.4
30 m	—	-69.7	-76.8	-77.8

Free Space Path Loss

Path loss over distance

Reference distances