The researches of this thesis are divided into two parts. One is to investigate the applicability of ANSI C63.5 in high frequencies of 1 GHz to 6 GHz. In general, the Standard Site Method (SSM) of C63.5 is usually used to calculate the antenna factors for the calibration of broadband antennas. Through the theoretical calculations, the numerical simulations and measurements, we found that the discrepancies between the calibrated results of horizontal and vertical polarization related to the un-dipole-like pattern in higher frequencies. Furthermore, we analyzed the relations of the antenna factors, the heights of receive antennas and the radiation patterns. We also found that the predicted calibration results can be close to the practical ones by including the effects of radiation patterns. The other research is to study the issue about the ground plane size of a test site. The ground plane size of a qualified test site must be larger than the minimum size defined by C63.7. For this reason, we provide a method which combines the tilted antennas measurement with a trapezoid shape ground. By using this method in the smaller ground size, we can obtain the measured results close to those obtained in the larger ground size. This method can effectively reduce the required ground size and then decrease the installation cost of a test site. Meanwhile, this method can be applied for both of the horizontal and vertical polarizations.