The formula for W to dBm conversion is X (dBm)=10 log Y (mW)
so when we convert the smaller power as 0.3 mW
that will be written as :-
X dBm = 10 log 0.3
= 10 log 3^(-1)
= – 10 log 3
= – 10*0.477
= – 4.77 dBm
gives 0.3 mW = -4.77 dBm
so, it is clear here why -ve sign pops out to the value when we calculate the Rx level, as Rx level is always smaller, but Tx is always +ve because it is near the transmitter giving much power.
thanx