EMI measurement over 1GHz must be operated in a free-space, open area test site. The validation method of free-space condition is called "Site Voltage Standing-wave Ratio" method, which is standardized by CISPR 16-1-4. The standard defines a new procedure for validation of EMC test sites by the ratio of maximum and minimum of transmitted voltages measured at the only 6 locations within a range of 400mm length. The small number of measurement points can cause larger uncertainty. Although the standard requires the measurement at a frequency interval of 50MHz or less, the measurement is usually performed at the largest frequency interval of 50MHz. In this paper, we propose a new approach to reduce the measurement uncertainty with a narrower frequency interval. The approach was validated by measurement as well as simulation. Also the approach was found to reduce the uncertainty caused by the location error in the measurement setup.