The apparent advantages of a Lloyd's mirror interferometer are enumerated. A detailed description is given of the installation which is a variable spacing interferometer whose spacing changes in a smooth and continuous manner from zero to six kilometers during about one half hour. The ionospheric effects which appear as fluctuations are discussed. The ionospheric horizon was observed to be peculiarly high in the east. This may be due to a bulge in the earth's atmosphere near the equator or to the proposed ring current circulating around the equator 5.5 earth radii distant. The radio astronomy of celestial sources will appear in Part II.