Near-field characterizations of a miniaturized low-VHF omni-directional antenna using an electro-optical system

Near-field measurement technique has been widely used as an alternative to the standard far-field pattern measurement method for characterization of antennas with large electrical and/or physical dimensions. The advantages of the near-field method for such antennas include relatively small area for the measurement that does not need to fulfill the far-field range requirement and achieving a 3D pattern in one complete scan. Also, issues related to antenna positioning that can be antenna specific can be avoided. Similar advantages exist when even for electrically small antennas at HF and VHF bands far-field measurements in anechoic chambers are not possible. In order to precisely characterize the antennas with low gain or omni-directional pattern, however, spherical near-field scanning technique with complicated computational procedure which in turn leads to increasing processing time is required. Although planar near- to far-field transformation has the virtue of mathematical simplicity, the scanned area is confined to a relatively small area around the antenna boresight. Thus, the finite size of scanning surface results in the truncation error.