Decibel Conversion: Power
dB = 10 log [P2/P1]
Decibels relative to Power
Decibel Conversion: Voltage
dB = 20 log [V1/V2]
Decibels relative to Voltage across same resistance
Decibel Conversion: Current
dB = 20 log [I1/I2]
Decibels relative to Current through same resistance
Decibel Conversion: Milliwatts
dBm = 10 log [Signal (mW)/1mW]
Decibels relative to one milliwatt
Decibel Conversion: Microvolts
dBmv = 20 log [Signal (mV)/1mV]
Decibels relative to one microvolt across same resistance
Decibel Conversion: Microamps
dBmA = 20 log [Signal (mA)/1mA]
Decibels relative to one microamp through same resistance
Power Conversion: dBw to dBm
dBm = dBw + 30
Conversion from dBw to dBm.
Voltage Conversion: dBv to dBmv
dBmv = dBv + 120
Conversion from dBv to dBmv.
Voltage to Power Conversion: dBmv to dBm
dBm = dBmv - 107
Where the constant 107 is as follows:
* RF systems are matched to 50W
P = V2
/ R
10Log10[P] = 20Log10[V] - 10Log10[50W]
V = (PR)0.5 = 0.223 V = 223000
mV
For a resistance of 50W and
a power of 1 mW:
20Log10[223000mV] = 107 dB
Power Density
dBw/M2 = 10Log10[V/M - A/M]
Decibel-Watts per square meter.
dBm/M2 = dBw/M2 + 30
Where the constant 30 is the decibel equivalent of the
factor 1000 used to convert between W and mW:
10Log10[1000] = 30
Electric Field to Power Density
dBm/M2 = dBmV/M - 115.8
Where the constant 115.8 is as follows:
P=|E|2/Zo
Where Zo is the free space characteristic
impedance (W), equal to 120p.
Change this equation to decibels, converting dBW/M2
to dBmW/M2 for power
density and dBV/M to dBmV/M
for the electric field.
This yields 115.8
Electric Field Voltage
V/M = 10{[(dBmV/M) -120]/20}
Electric Field Voltage in volts per meter
Electric Field Current
dBmA/M = dBmv/M - 51.5
Where the constant 51.5 is a conversion of the characteristic impedance of free space (120p) into decibels:
20Log10[120p] = 51.5
A/M = 10{[(dBmA/M) -120]/20}
Electric Field Current in amps per meter
Antenna Factor
AFdB = EdB - VrdB
where:
AF = Antenna Factor in dB/M
E = Field strength at the antenna in dBµv/M
Vr = Output voltage from receiving antenna in dBµv
AF (for 50 W) = 20 log f (MHz) - G(dBi) - 29.78 dB.
where f is the measured frequency (MHz), G is the antenna gain (dBi) over isotropic.
(E) dBmv/M = (Vo) dBmv + (AF) dB/M
AF is the antenna factor of the measuring antenna (as per calibration or per antenna manufacturer).
E is the unknown or measured electric field strength.
Vo is the adjusted (calibrated for cable & connector losses) spectrum analyzer output.
Magnetic Flux Density
dBpT = dBmA/M + 2.0
Where the constant 2.0 is as follows:
The magnetic flux density B
is in Teslas (T)
The permeability of the medium is in Henrys per meter (H/M)
The permeability in free space is: mo
= 4p x 10-7
H/M
Convert from T to pT and from A/M to mA/M, and take the Log:
240 - 120 + 20Log10[4p x 10
-7] = 2.0
Conversion Units
dB = decibels (Log10)
m = milli = 10E-3
m = micro = 10E-6
p = pico = 10E-12
dBw = decibels relative to one watt
dBm = decibels relative to one milliwatt
dBv = decibels relative to one volt
dBmv = decibels relative to one microvolt
dBmA = decibels relative to one microamp
dBpT = decibels relative to one picoTesla
V = Volts
A = Amps
I = Current
R = Ohms (50)
W = Watts
P = Power
H = Henrys
T = Teslas
AF = Antenna Factor
M = Meter
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