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We present design equations for a two-element closely-spaced interferometerfor measuring the noise temperature of a uniform sky. Such an interferometer isuseful for observing highly diffuse radio sources such as the Milky Way andCosmological signals. We develop a simple equivalent circuit based onradiophysics and antenna theory to describe the interactions between key designparameters such as antenna self and mutual impedance and noise parameters ofthe receiver; the latter is considered internal noise. This approachstraightforwardly facilitates design studies as the response of the uniformsignal and the systematic error due to internal noise coupling can be analyzedusing the same equivalent circuit. The equivalent circuit shows that mutualcoherence due to internal noise coupling is non-negligible and an inherentproperty of a closely-spaced interferometer. A realistic example designinvolving two closely-spaced horizontal dipoles over a lossy ground forCosmological signal detection from 50 to 100 MHz is discussed as anillustration.

Design Equations for a Closely-Spaced Two-Element Interferometer Including Internal Noise Coupling