Results of a spaced-receiver experiment using several very low radio frequencies to probe the irregular structure of the lower ionosphere at night are presented. A multiple frequency sounding facility was used whose transmissions were received via a nearly vertically incident first-hop sky wave. The present paper describes some of the statistical characteristics of the phase and amplitude fluctuations in the diffraction pattern at the ground. The data are compared with a theoretical analysis of the diffracting properties of an irregular screen [Bowhill, 1961] to determine the feasibility of using a single receiver near a standard VLF transmitter to continuously monitor the scale size of the irregularities in the nighttime D region by measuring the phase and amplitude of the first-hop sky wave. Some implications are pointed out regarding the functional forms of the spatial spectrum and the correlogram of the irregularities at the height of reflection.