How to determine the power of a signal source?

Question

Signal processing

There is a radio source. I want to measure the power of the source. I measure the signal strength at 4 different points.
The following measurements are obtained (distance from the source, signal strength):
1) 2.5m -42dbm
2) 4.5m -56dbm
3) 7m -69dbm
4) 10.5m -72dbm I
enter these data into excel, build a logarithmic trend.
It turns out I have the following function: y=-21.93*ln(x)-22.928.
In 0, it will not work, so I took numbers close to zero.
At 0.1m I got 27dbm, at 0.001m 128dbm. And the closer to zero, the greater the value of power, but it cannot really be like that. There should be about 35dbm in fact.
How correctly using this method it is possible to determine the power of the signal source? Or is this method obviously wrong? Then what would be right?

Answer 1

Well, the first thing I can say is that, usually, extrapolation of non-linear values ​​is a bad idea.
It seems that you need to know the gains of the antennas of the radio transmitter and radio receiver:
digteh.ru/UGFSvSPS/power
But something I didn’t manage to calculate like that. Is it an omnidirectional antenna? And what do you measure?