I. Introduction
Since 2011, the Measurement and Testing Division of Technical center, the Viet Nam Authority of Radio Frequency Management has employed a field probe (with frequency range from 100 to 6 GHz) and standard antennas (with the frequency range from 80 MHz to 18 GHz) to measure the gain factor of antennas operating within the frequency range up to 18 GHz, primarily for radio frequency inspection purpose [1]. This calibration and measurement process followed two methods, including the reference field method and the reference antenna method.