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The equivalent external noise temperature of timevarying antennas is studied using the concept of cross-frequency effective aperture, which quantifies the intermodulation conversion of external noise across the frequency spectrum into a receiver's operational bandwidth. The theoretical tools for this approach are laid out following the classical method for describing external noise temperature of linear time-invariant antennas, with generalizations made along the way to capture the effects of time-varying components or materials. The results demonstrate the specific ways that a time-varying system's noise characteristics are dependent on its cross-frequency effective aperture and the broadband noise environment. The general theory is applied to several examples, including abstract models of hypothetical systems, antennas integrated with parametric amplification, and time-modulated arrays.

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