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

The E-field 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. A 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 paper 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, 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 trade-off 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 multi-probe anechoic chamber is 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 multi-probe measurement system, and the subsequent precise evaluation of the performances of the antenna under test (AUT).