Net Power

Net Power is defined as Forward Power minus the Reflected Power. This document describes the formula's that should be used to correctly calculate the Net Power.

The Net power always has to be calculated in linear power values. It is not possible to calculate the Net power directly from logarithmic values (dBW, dBm, dBpW, ...) without converting them to linear power values.

The following formulas should thus be used:

$P[W]=10^{\frac {P[dBW]}{10}}$ or $P[mW]=10^{\frac {P[dBm]}{10}}$

Formula -1: Logarithmic power value to linear power value

$P_{net}[W]=P_{fwd}[W]-P_{refl}[W]$

Formula -2: Net power calculation from forward and reflected power

$P[dBW]=10*^{10}log(P[W])$ or $P[dBm]=10*^{10}log(P[mW])$

Formula -3: Linear power value to logarithmic power value

$P[dBm]=P[dBW]+30$

Formula -4: dBW to dBm

Example

Let's assume that on a certain frequency (for example: 201,973 MHz), a forward power of 50 dBm is measured, with a reflected power of 30 dBm.

First the power values should be calculated from dBm values to linear values.

$50\ dBm=10^{\frac {50}{10}}\ mW=100.000\ mW=100\ W$

$30\ dBm=10^{\frac {30}{10}}\ mW=1000\ mW=1\ W$

Then the net power should be calculated by subtracting the linear values

$P_{net}=P_{fwd}[W]-P_{refl}[W]=100\ W-1\ W=99\ W$

And the linear value should be converted to a dBm value again:

$P[dBW]=10*^{10}log(99W)\approx 19,95635195\ dBW\approx 49,95635195\ dBm$

The Net Power of 50 dBm forward and 30 dBm reflected power is thus 49,96 dBm.