Antenna Calculations
The following is a list of various antenna calculations should you find it necessary to convert from one parameter to another:
Conversion of signal levels from watts to volts in a 50 ohm system:
P = V2/R
where:
P = power in Watts
V = voltage level in Volts
R = resistance in ohms
Note that 1mW = 10–3 W and 1 µV = 10–6 V.
For power in dBm, and voltage in dB (µV):
VdB(µV) = P(dBm) + 107 dB
Power density to field strength:
An alternate measure of field strength is power density:
Pd = E2 / 120π
where:
E = field strength in V/m
Pd = Power density in W/m2
Power density at a point:
Pd = PtGt / (4πr2)
This equation is only valid in the far field, where electric and magnetic fields are related by the characteristic impedance of free space:
where:
Pd = power density in W/m
Pt = power transmitted in Watts
Gt = gain of transmitting antenna
r = distance from the antenna in meters