System-equivalent flux density ($\mathrm{SEFD}=2k_B{T}_{\mathrm{sys}}/A_e$)

The system-equivalent flux density is the unpolarized spectral flux density that would produce an antenna temperature equal to $T_{\mathrm{sys}}$. With effective collecting area $A_e$, $\mathrm{SEFD}=2k_B{T}_{\mathrm{sys}}/A_e$. The factor two follows from one polarization of waves measurement of an unpolarized source. State effective area and polarization of waves conventions when comparing sensitivities.^[ref]