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An investigation was made to obtain some fundamental information concerning the relation of the impedance of a vertical antenna over a finite ground plane as a function of the size and shape of the ground plane when dimensions of the ground plane are relatively small in terms of wavelength. It was found that the input impedance is a damped oscillating function of wavelength and ground-plane dimensions, the impedance of a circular ground plane varying from ±5 to ±20 per cent. Similar variations were observed on a square ground plane, which were approximately 50 per cent of those of the circular ground plane except when the dimensions of the ground plane were small. In general, it was found that the impedance is quite critical with respect to the size and shape of the ground plane, and relatively independent of the thickness of the antenna. Measurements were made at microwaves by a modified Chipman method capable of measuring small differences in antenna impedance. Recent emphasis on improved microwave measurement techniques led to the investigation of the merits of the Chipman method as compared to those of the more conventional slotted-line standingwave method. Although the two methods are shown to be closely related from a theoretical viewpoint, practical mechanical and electrical limitations are encountered which dictate preference of method entirely dependent on the application.