To convert a power reading in dBm into a field strength in dBµV/m, the antenna factor of the device used must be computed.
The antenna factor is the term used in EMC testing to convert a voltage or power level fed by an antenna to an EMI analyzer into the field strength units (usually dBµV/m) of the electromagnetic field producing that voltage or power.
In a 50 ohm system, the antenna factor (expressed in dB(m-1)) of an antenna of absolute gain G (expressed in dB) at a wavelength L (expressed in meters) is:
AF[dB(m-1)] = 19.8 - 20*log(L[m]) - 20*log(G[dB])
To convert a voltage reading V (expressed in dB(µV)) into an electric field E (expressed in dB(µV/m),) use the following formula:
E[dB(µV/m)] = V[dB(µV)] + AF[dB(m-1)]
To convert a power reading P across a 50 ohm impedance (expressed in dBm) into an electric field E (expressed in dBµV/M,) use the following formula:
E[dBµV/M] = P[dBm] + AF[dB(m-1)] + 107
AF(dB(m-1)) = 19.8 - 20*log(0.04) - 20*log(7) = 30.9 dB
If we measure -70 dBm on the receiver/spectrum analyzer, the E field is:
E = -70 + 30.9 + 107 = +67.9 dBµV/m