# Signal Strength and Distance

I will be very thankful if you could clarify something to me.

Assume I am standing at point A and scanning WiFi around me. My phone records an access point X with a signal strength, let's say -50 dBm. I use FSPL (Free Space Path Loss) formula and calculate that the AP (Access Point) X is somewhere in radius of 5 meters around me. Then I make another scan, but this time record an AP X with a signal strength -75 dBm. The same FSPL formula suggests that X is somewhere in radius 55 meters from me.

My question is - can I safely assume that X is no farther than 5 meters from me ? Because if not and it is let's say 10-20 meters away, how can I explain a signal strength of -50 dBm, which corresponds to 5 meters distance. Is it possible that signal strength from a farther location will be stronger than from the nearer location ?

Thank you in advance for all the answers.

• Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 9:30

## 1 Answer

In theory, a wireless signal loses power with the distance squared (due to the power distributing over the surface of a hypothetical, expanding sphere). Using at least three different measurements you could calculate the cross section of all three reception circles and find the source. (In three dimensions, you'd need to use reception spheres and at least four measurements. Most often the earth can replace the fourth sphere.)

In practice, this is much harder. Space isn't empty and anything inside the Fresnel zone between the transmitter and the receiver attenuates the signal - even the earth if you're not using masts. Additionally, other transmitters can disturb the signal and cause what might appear like attenuation.

So, unless you very carefully model the environment (nearly impossible) you're left estimating. With some experience, these estimates can be surprisingly accurate but then again they can be totally wrong.

Another, completely different aspect is the gain of a directional antenna. Depending on the antenna geometry, some directions will get much more power than others.

• thank you for your quick reply. I understand that the signal get distorted for many different reasons, what I still don't understand is whether I can conclude that the distance that corresponds to the strongest signal that was recorded at certain location is an upper bound of the distance of the AP from me, i.e the AP can not be (much) farther than this distance...cause if it would be farther, the signal strength would be lower. – TomerT Sep 19 '18 at 19:35
• If you know the transmitted power in your direction you could give a very good estimate of the maximum distance - but usually you don't know.that. – Zac67 Sep 19 '18 at 19:54