Frequency domain interferometry (FDI) is a mesosphere-stratosphere-troposphere radar technique used for probing thin-layered structures of the atmosphere. The position and thickness of a scattering layer embedded within the illuminated volume can be deduced from the complex coherence of received signals at two closely spaced frequencies. This technique has permitted us to identify layered structures (called FDI layers) with thicknesses estimated to be around 50--200 m in the troposphere-stratosphere. However, its application needs very restrictive hypotheses. For example, the layers are assumed to have a large extent in the horizontal plane with respect to the horizontal extent defined by the -6-dB angular width of the two-way antenna pattern. This hypothesis seems not always to be verified in light of observations with other radar techniques. In this paper, theoretical calculations are performed in order to estimate the consequences of the limited extent of the scattering structure in the horizontal plane on the FDI parameters, i.e., the vertical extent and position in height of the single layer. The first analysis concerns the effects of the limited extent by itself, and the second one deals with the consequences of the advection of the scattering structure by the wind. It is shown that in some limiting cases, substantial biases in the FDI parameters can occur. Finally, this work stresses the necessity to confront the FDI data with other technique measurement results or to complete it with space domain interferometry data. A future paper will be devoted to the study of the effects of the tilt of a scattering layer on the thickness and position estimations. ¿ 2000 American Geophysical Union