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Basic Electrical Relationships
C: Basic Electrical Relationships
The decibel is used extensively in electromagnetic measurements. It is the log of the
ratio of two amplitudes. The amplitudes are in power, voltage, amps, electric field
units and magnetic field units.
decibel = dB = 10 log (P2/P1)
Data is sometimes expressed in volts or field strength units. In this case, replace
P with V2/R.
If the impedances are equal, the equation becomes:
dB = 20 log (V2/V1)
A unit of measure used in EMI measurements is dBμV or dBìA. The relationship of
dBμV and dBm is as follows:
dBμV = 107 + PdBm
This is true for an impedance of 50Ω.
Wave length (l) is determined using the following relationship:
λ = 3x108/f (Hz) or λ = 300/f (MHz)
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