Purpose
Apply a correction for distance change
Presumptions
All distances are in far field for the frequiencies being considered
Where
Applied in CISPR 16-1-4 for sVSWR calibration
Product
Is dB loss/gain for distance change
Loss for increasing distance
Gain for reducing distance
Python
from numpy import log10
distance_measurement = 340 # cm
distance_reference = 300 # cm
x = 20 * log10(distance_measurement / distance_reference)
# dB loss/gain for distance change
Example
\begin{gather*}
\begin{aligned}
\mathrm{distance}_{measurement} &= 340 \;
\\[10pt]
\mathrm{distance}_{reference} &= 300 \;
\\[10pt]
x &= 20 \cdot \log_{10} \left( \frac{ \mathrm{distance}_{measurement} }{ \mathrm{distance}_{reference} } \right) \\&= 20 \cdot \log_{10} \left( \frac{ 340 }{ 300 } \right) \\&= 1.087 \; \;\textrm{(dB loss/gain for distance change)}\\
\end{aligned}
\end{gather*}