Question about signal strength

[Enki]

Hello

If spec on the router says that output power is around 14dBm then I understand that when my phone is next to router then application like NetSpot (or any signal strength measure tool) on my phone should indicate that network power is positive so why I never go more than -20dBm?

Thanks

 

[Sanna1967]

read at the bottom of this page. -20 is excellent signal
https://www.metageek.com/training/resources/wifi-signal-strength-basics.html

 

[Enki]

I have red few articles like this one though it is not answering my question or I do not understand.
Following decibel scale from sound engineering I understand that in example if you have 10W speaker you need to have 100W speaker to be 2 times louder or 1000W to be 3 times louder. dB scale is just a reference which given starting point so 0 dB in sound is level of human hearing in example.
My question here is: I am using
http://www.proxim.com/products/knowledge-center/calculations/calculations-milliwatts-vs-dbm#calc
so this gives me that 14dBm (power of my router) is equivalent of 25mW saying that the wave will have 25mW power at the starting point?
Based on measurement from my phone which is -20dBm I want to know what is the power of the wave reaching my phone?

 

[Enki]

I think I found my answer here:
https://superuser.com/questions/298290/wireless-router-signal-strength-stats
Basically dBm value is useless to calculate real broadcast power.

 

[sfx2000]

Basically dBm value is useless to calculate real broadcast power.

dBm is actually valid, but one has to consider the Tx/Rx performance of both ends, along with loss in between - in freespace, 2D loss is a 10LogR function, and one has to consider the Rx Sensitivity of the client radio, along with the gains of the antenna and path loss to the actual radio...

Here's a better view... it's Link Budget, and the free air path loss drives things, and this, like I mentioned above, is generally a 10LogR function...

 

[Enki]

So assuming this 14dBm is real EIRP then I should be getting 25mW of power from antenna. Then if this signal goes only 10 cm why my phone says it is -20dBm. If -20dBm is 0.01 mW (according to some charts) does it mean that signal received is 2500 times lower?
Is there any math to explain why 14dBm router signal gives -20dBm signal strength on client? This is the link I am missing or understanding what is what here...

 

[degrub]

first off, those values are determined either by theoretical calculation or measurement in a Faraday cage - e.g. clean noise free environment.
What you are measuring with your phone is within a noise field - ie other wireless signals which further reduces the available margin.

 

[sfx2000]

first off, those values are determined either by theoretical calculation or measurement in a Faraday cage - e.g. clean noise free environment.
What you are measuring with your phone is within a noise field - ie other wireless signals which further reduces the available margin.

Most consumer WiFi NIC's top out at -20dB RSSI - they're not very precise...

 

[Enki]

I am not buying this Faraday cage argument - yes it will affect the result but not to this degree. I am not getting full understanding of "within a noise field" - it may affect some results but not completely make it wrong. Why the scale starts at 0dBm so 1mW? Edimax states they signal is 25mW, Asus ac1200g+ gives you 316mW ! - link here from FCC tests:
https://fccid.io/MSQ-RTACRH00/RF-Exposure-Info/RF-Exposure-3094140
I just can't understand that if let's say you got Asus in front of you 316mW (distance 10cm) then you should get 25dBm reading - ok let's be generous and give +-10dBm (a lot) error margin- still you should get 15dBm so 31mW so then not minus scale.
Furthermore:
http://www.rfcafe.com/references/electrical/decibel-tutorial.htm
https://en.wikipedia.org/wiki/DBm
Same: 0dBm = 1mW, what if you had 10W antenna - app would still show less than 0dBm