Calibration loop antenna for 5G OTA measurement systems

The advent of the fifth generation New Radio (5G NR) is pushing toward an extension of the traditional Sub-6G communication bands to 6 GHz and above. Such trend calls for an update of multi-probe measurement systems for an accurate Over-The-Air (OTA) characterization of such new devices. The new measurement requires the precise calibration of the measurement system within a frequency band beyond the traditional 6 GHz limit. Thus, a new calibration antenna working over 6 GHz is required, which should achieve an adequate comprehensive behavior in terms of suppression of common mode current, small gain ripple and good impedance matching. In order to meet this objective, an analytical expression is proposed in this paper to optimize each of the above aspects of the calibration loop antenna. Based on the proposed objective function, a calibration loop antenna operating at 5.9–7.12 GHz is designed. The proposed design is made of six Vivaldi antennas with a compact size of 35 × 35 × 1.524 mm. In order to effectively suppress the common mode current, a three-layer PCB is designed to reduce the mutual coupling between the antenna and coaxial cable. Slotted balun structures and arc slots are designed on each Vivaldi antenna radiator to lower the gain variation to 0.2 dB or less in the horizontal plane as well as further suppress the interference of the cable. Measurement results reveal that the proposed calibration loop antenna achieved a radiation performance similar to the standard antennas, but with a higher bandwidth of 19%, since it is characterized by |S11| <−10 dB between 5.9 and 7.12 GHz, a small gain ripple less than 0.2 dB as well as a high cross-polarization ratio larger than 27 dB.