Antenna equivalent radius
The equivalent radius of an antenna electrical conductor is defined as: where ℓ {\displaystyle \scriptstyle \ell } denotes the conductor's circumference, L {\displaystyle \scriptstyle {L}} is the length of the circumference, x {\displaystyle \scriptstyle {\boldsymbol {x}}} and y {\displaystyle \scriptstyle {\boldsymbol {y}}} are vectors locating points along the circumference, and d x {\displaystyle \scriptstyle {dx}} and d y {\displaystyle \scriptstyle {dy}} are differentials segments along it. The equivalent radius allows the use of analytical formulas or computational or experimental data derived for antennas constructed from small conductors with uniform, circular cross-sections to be applied in the analysis of antennas constructed from small conductors with uniform, non-circular cross-sections.
Source: Wikipedia — Antenna equivalent radius (CC BY-SA 4.0)