In-band amplitude measurement was conducted on a 63.4 km test path with an oversea section (normal reflection point was on the ground), using a frequency band from 4440 to 4500 MHz. Results on peak-to-peak amplitude dispersion and linear amplitude dispersion were obtained for both nondiversity and idealized space diversity receptions. Potential improvements by space diversity in terms of reductions of amplitude dispersion were remarkable in the large amplitude dispersion region. An analysis of the in-band amplitude pattern was made in order to identify which multiray is most responsible for amplitude dispersion during multipath fading. The main cause of amplitude dispersion was found to be waves irregularly reflected from the sea surface and waves reflected from the ground, with a long delay but small amplitude. A frequency correlation function for received power, which is necessary for estimating linear amplitude dispersion, was derived theoretically, for the three-path model. The theoretical frequency correlation agrees well with that measured on this path.