Expressions tabulated for the effective antenna noise figure Fa usually assume an electric field antenna, since most measurements of radio noise are made on the electric field of the noise. Furthermore, the International Radio Consultative Committee (CCIR) noise model predictions for Fa are made only for electrically short grounded vertical monopoles over a perfect ground. However, at frequencies lower than those traditionally used for communications, i.e., at extremely low frequencies (ELF; frequencies in the range 3 Hz to 3 kHz) and very low frequencies (VLF; frequencies in the range 3–30 kHz), it is common for magnetic field loop antennas to be used, and the tabulated expressions for Fa do not apply. This communication reports the derivation of an expression for Fa as measured by a small vertical magnetic loop antenna and its subsequent application to ELF/VLF radio noise measurements made at a variety of locations around the world. There is good agreement between the measured Fa values and estimates of maximum and minimum Fa values for the ELF/VLF range published by Spaulding and Hagn in 1978, but an improved fit to the measurements can be obtained by making moderate adjustments to the maximum and minimum values at both the low (10–100 Hz) and high (8–32 kHz) frequency limits.