  We present design equations for a two-element closely-spaced interferometer
for measuring the noise temperature of a uniform sky. Such an interferometer is
useful for observing highly diffuse radio sources such as the Milky Way and
Cosmological signals. We develop a simple equivalent circuit based on
radiophysics and antenna theory to describe the interactions between key design
parameters such as antenna self and mutual impedance and noise parameters of
the receiver; the latter is considered internal noise. This approach
straightforwardly facilitates design studies as the response of the uniform
signal and the systematic error due to internal noise coupling can be analyzed
using the same equivalent circuit. The equivalent circuit shows that mutual
coherence due to internal noise coupling is non-negligible and an inherent
property of a closely-spaced interferometer. A realistic example design
involving two closely-spaced horizontal dipoles over a lossy ground for
Cosmological signal detection from 50 to 100 MHz is discussed as an
illustration.
