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Loop antennas are used to measure the magnetic field emitted from electric and electronic devices, power-transmission lines and so on. The antenna factor of a receiving loop antenna must be accurately calibrated as one of antenna's characteristics. The common loop antenna with the diameter of several ten cm is electrically small antenna when it is used for the frequency lower than 30 MHz. In such a low frequency range, the major measurement method is "Standard Field Method ". The current detector using thermo-couple is used to decide the exciting current of transmitting loop antenna. The generated magnetic field is utilized to measure the antenna factor of receiving loop antenna. The fragile structure is one of the drawbacks to keep stability and reproducibility. Besides, a common technique of three antenna measurement is promising for a reliable method for loop antenna measurement. We adopted the technique and examined the uncertainty of the loop antenna measurement using ah automatic network analyzer. In this paper, we discuss the uncertainty of "3-Antenna Method", for loop antenna calibration. The "Standard Field Method " is commonly used, and a current detector with a thermocouple is used. It is easily damaged by the exciting current itself because strong current (sometimes about 1 A) is needed to make the magnetic field enough for measurement. However, in "3-Antena Method", it is possible to use an automatic vector network analyzer. So it is expected that we can obtain a wide dynamic-range and stability by automatic measurement. The magnetic antenna factor of a loop antenna is defined as the ratio of the incident plane magnetic field strength to the output voltage across the load impedance (50 ?). However, it is difficult to apply such actual plane wave to a loop antenna and to realize far-field condition in the low frequency. Consequently there is non-uniformly distributed magnetic field inside the loop. Therefore the average magnetic field inside the receivi- ng loop antenna is ordinarily used instead of the ideal plane magnetic field. We adopted this average magnetic field for "3-Antenna Method" to calibrate loop antennas. The target of the paper is the evaluation of "3-Antenna Method" using the averaged magnetic field and the estimation of uncertainties for this calibration method using automatic vector network analyzer.