Ham Radio - RF Units Conversion 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 dBµv = 20 log [Signal (µV)/1µV] Decibels relative to one microvolt across same resistance Decibel Conversion: Microamps dBµA = 20 log [Signal (µA)/1µA] 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 dBµv dBµv = dBv + 120 Conversion from dBv to dBµv. Voltage to Power Conversion: dBµv to dBm dBm = dBµv - 107 Where the constant 107 is as follows: * RF systems are matched to 50Ω P = V2 / R 10Log10[P] = 20Log10[V] - 10Log10[50Ω] V = (PR)0.5 = 0.223 V = 223000 µV For a resistance of 50Ω and a power of 1 mW: 20Log10[223000µV] = 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 = dBµV/M - 115.8 Where the constant 115.8 is as follows: P=|E|2/Zo Where Zo is the free space characteristic impedance (Ω), equal to 120π. Change this equation to decibels, converting dBW/M2 to dBmW/M2 for power density and dBV/M to dBµV/M for the electric field. This yields 115.8 Electric Field Voltage V/M = 10{[(dBµV/M) -120]/20} Electric Field Voltage in volts per meter Electric Field Current dBµA/M = dBµv/M - 51.5 Where the constant 51.5 is a conversion of the characteristic impedance of free space (120π) into decibels: 20Log10[120π] = 51.5 A/M = 10{[(dBµA/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 Ω) = 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) dBµv/M = (Vo) dBµv + (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 = dBµA/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: µo = 4π x 10-7 H/M Convert from T to pT and from A/M to µA/M, and take the Log: 240 - 120 + 20Log10[4π x 10 -7] = 2.0 Units dB = decibels (Log10) m = milli = 10E-3 µ = micro = 10E-6 p = pico = 10E-12 dBi = decibels relative to an isotropic antenna dBw = decibels relative to one watt dBm = decibels relative to one milliwatt dBv = decibels relative to one volt dBµv = decibels relative to one microvolt dBµA = decibels relative to one microamp dBpT = decibels relative to one picoTesla π = pi = 3.14159265 V = Volts A = Amps I = Current R = Ohms (50) W = Watts P = Power H = Henrys T = Teslas AF = Antenna Factor M = Meters BACK Copyright © 1997-2005 W5AM, eHamRadio.com. All rights reserved.