May be I can use analogy to explain.
Suppose you wanted to listen to 2 radio stations and measure how strong signal per average you can receive from them.
Station Argo is located very far, while station Aurli is located very near.
Therefore signal strength from Argo station is reduced ten times when it reaches you, wher as signal strength from Aurli stays the same.
Of course there is a minimal signal strength which your antenna can recieve, and register.
Suppose you register all signals sent from Aurli and Argo.
What you will get - From station Argo you will receive only those signals which were originally the strongest - those signals that exceed your antenna minimal signal by at least 10 times. From station Aurli you will receive almost all signals that exceeds your antenna minimal signal strength.
Now if you simply take the average of signals received you will not get a nice picture of what is signal strength coming from Aurli and Argo.
Signal coming from Argo will of course appear weaker that signal coming from Aurli. But much to the surprise signal coming from Argo will not be simply 10 times weaker - because your antenna has minimal signal strength it can recognise - you will lose a lot of small signals comming from argo.
So you can`t use your antena to compare average signal strength.
However you can measure and compare both stations strong signal average.
Those signals that are above some margin - In this case 10 times as strong as your antenna minimal signal strength recognition - because you wont register signals bellow this strenght coming from Argo station.
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