The first assumption relates to the measurement environment, namely that it is occurring in free space. Reflections from nearby objects, or the attenuation of the signal by anything other than distance, are not accounted for. A good Fully Anechoic Room (FAR) gives this kind of environment, an Open Area Test Site (OATS) less so because of complications like ground-plane reflections. With this in mind, the basic situation is shown in Figure 1.
Figure 1. Basic relationship between radiated power and field strength Where P= power (W), E= electricfield strength (V/m) and Pd= power density (W/m2)
dBμV/m into Effective Isotropic Radiated Power (EIRP)
P(dBm) = E(dBμV/m) – 95.2
In summary, bearing in mind the assumptions being made, the Effective Isotropic Radiated Power can be derived from the 3 m field strength test measurements supplied with the CNE and CGE reference sources by subtracting 95.2 from the numerical value given in dBμV/m. By following the same process for the case of 10 m test measurements, 84.8 should be subtracted instead.
More detail refer to : https://emc.wiki/thread-4295-1-1.html
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