  We report development of a simple and affordable radio interferometer
suitable as an educational laboratory experiment. With the increasing
importance of interferometry in astronomy, the lack of educational
interferometers is an obstacle to training the future generation of
astronomers. This interferometer provides the hands-on experience needed to
fully understand the basic concepts of interferometry. The design of this
interferometer is based on the Michelson & Pease stellar optical
interferometer, but operates at a radio wavelength (~11 GHz; ~2.7cm); thus the
requirement for optical accuracy is much less stringent. We utilize a
commercial broadcast satellite dish and feedhorn. Two flat side mirrors slide
on a ladder, providing baseline coverage. This interferometer resolves and
measures the diameter of the Sun, a nice daytime experiment which can be
carried out even in marginal weather (i.e., partial cloud cover). Commercial
broadcast satellites provide convenient point sources for comparison to the
Sun's extended disk. We describe the mathematical background of the adding
interferometer, the design and development of the telescope and receiver
system, and measurements of the Sun. We present results from a students'
laboratory report.
