Sorry for the late reply, I have not been getting email notifications (even though I should according to my preferences.)
As I don't know exactly the input to the amplifier, I can only guess at the power transmitted. However I will try to make a reasonable guess based on the following:
If I connect the SDR directly to the same donor antenna the amplifier (repeater) is using, I am unable to distinguish a signal in the frequency range (860 - 894 MHz) hence it is a very weak signal. Thus the reason for using an amplifier, so I can try and detect these signals. So it seems safe to assume then that the input to the amplifier when connected to this same antenna is less than -80 dBm, would it not? (Even if it is just white noise.)
If we go with that, lets calculate some link gains:
The gain of the amp has been measured to be 52 dB. The output is connected to an antenna with about 8 dB max.
-80 dBm + 52 dB + 8 dB = -20 dBm
So I will assume the output from the amplifier, including the output antenna gain is less than -20 dBm.
The input of the SDR is connected to a half-wave dipole 2 feet away.
Lets assume maximum antenna gain of the dipole to be 2.15 dB.
Free space path loss @ 870 MHz @ 2 feet = 27 dB.
-20 dBm + 2.15 dB - 27 dB ~= -45 dB
So I'm figuring at the very most -45 dBm at the input of the SDR.
Please tell me if this treatment seems reasonable to you (taking into account that I can only guess at the strength of the very weak signal I am amplifying.)