Two horns a few metres apart, pointed at the same patch of sky, make an interferometer. As a source drifts across their beams the path length to each horn changes, the two signals move in and out of step, and the combined power rises and falls in a fringe pattern. The spacing of the fringes is set by how far apart the horns are, which is what makes the arrangement worth building.
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Correlation and the Two-Element Interferometer - Where the fringes come from. Convolution, autocorrelation and cross-correlation, then why a single dish cannot reach the resolution, what the adding and multiplying arrangements each measure, and what a finite bandwidth does to the pattern. Comes with a notebook you can run.
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Additive Interferometry Using Two DSPIRA Radio Horn Telescopes - LightWork Memo 31, by John Makous. The theory of additive interferometry worked up from the path length difference, the two-horn set-up that tested it at Green Bank, and the fringe pattern it recorded from a transit of the sun on a 5.0 m baseline - measured fringe spacing 0.043 rad against 0.042 rad predicted. Multiplying the signals instead of adding them is noted at the end as work the group had done but had not yet written up.
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Setting Up a 2 Horn Interferometer - The practical companion: equipment list, wiring, aligning the two horns on an east-west baseline, and running either the adding or the multiplying spectrometer.