Loop Antennas for Accurate Calibration of OTA Measurement Systems: Review, Challenges, and Solutions

The E- and H-field standard antennas are commonly used for calibration of measurement systems in various test applications. The standard H-field antenna is a horizontally polarized omnidirectional probe antenna, with its radiated electromagnetic field being like that of an ideal virtual magnetic dipole. An H-field antenna is physically realized by a consistent current loop fed through a coaxial cable, thus is often referred to loop antenna. This review article offers a careful description of all aspects involved in the design of a loop calibration antenna; specifically, the concept of a calibration loop antenna is introduced, and the state of the art and the recent advances of loop antenna designs and performances are reviewed. The design of a calibration loop antenna needs to take into account comprehensive indicators, such as gain ripple, common-mode current, cross polarization, impedance matching, and radiation pattern. A design of this type of calibration antenna is a challenging task since all the above aspects need to be taken into account simultaneously to find the best tradeoff among them. In the context of the example calibration loop antennas, the techniques to improve the bandwidth, the performance of omnidirectional radiation, and the effects of suppressing common-mode current are discussed, which provides new ideas for the design of calibration loop antennas. Finally, the principles and steps of the horizontal polarization calibration in the multiprobe anechoic chamber are introduced and discussed; the errors caused by the common-mode current to the calibration results are evaluated through calibration examples to demonstrate the relevant impact of a well-designed loop antenna for the accurate calibration of a multiprobe measurement system and the subsequent precise evaluation of the performances of the antenna under test (AUT).