Contents
Introduction
The calculator converts from radiated power (EIRP) at the output of the transmitter in dBm to the electric field strength* dBmV/m (deciBels millivolt per meter).
*Electric Field Strength and Electric Field Intensity refer to the same thing
Typically this measurement is made at 3 meters for the purposes of FCC testing. However you can specify any distance in the calculator.
Calculator
???? dBmV/m to dBm
Example Calculations
For an EIRP of 0 dBm, the field strength at 3 meters is calculated to be 35.2 dBmV/m. As the distance increases, the field strength decreases.
Formula
EdBmV/m = PdBm + 35.2
Derivation
Consider an isotropic source that radiates equally in all directions.
The power density Pd = P/(4*Ï€*r2)
The power density decreases with increasing distance r from the source. The FCC stipulates radiated emissions at a distance of 3 meters away from the source and hence we will use r=3 in the equation above.
Pd is also given by the equation
Pd = E2/Zo
Where E is the field strength and Zo is the characteristic impedance of free space, equal to 377 Ω.
Therefore,
P/(4*Ï€*r2)= E2/Zo
which simplifies to
P = 0.3*E2
Converting Power from Watt to dBm and field from uV/m to dBuV/m gives
PdBm = EdBμV/m – 95.2
or
EdBμV/m = PdBm + 95.2
EdBμV/m = EdBmV/m + 60
therefore
EdBmV/m = PdBm + 35.2
Background
dBmV/m is a unit of measurement used in the field of electromagnetic wave propagation, particularly in radio and television broadcasting, to quantify the strength or intensity of an electromagnetic field at a specific distance away from the emitter or transmitter. It is often used to describe the signal strength or power level of a radio or television signal in a given area.
EIRP stands for “Effective Isotropic Radiated Power.” It is a measure of the total power radiated by an antenna in a specific direction, expressed as an equivalent power that would be radiated by an ideal isotropic antenna (a theoretical antenna that radiates energy uniformly in all directions). EIRP is typically used in the context of radio frequency (RF) and wireless communication systems to describe the strength of a transmitted signal.