A Novel Gain Reference Dipole Antenna for Accurate Calibration of Over-the-Air Measurement Systems

Gain reference antenna is an important part of over-the-air (OTA) measurement system using antenna gain substitution method. Without an accurate gain reference antenna, the measurement results of such system are envisaged to lead to a biased evaluation of the antenna gain. Furthermore, such antenna might be susceptible to undesirable currents on the coaxial cable during the measurement, whereby the actual measured radiation pattern can be distorted causing significant differences from its expected radiation pattern. These undesirable currents are known as common-mode (CM) current. In this work, a novel gain reference dipole antenna (GRDA) with minimal CM current effects has been proposed. It achieves less than 0.2-dB radiation pattern ripple from the ideal omnidirectional radiation performance within the 4.2–4.9-GHz frequency band. The GRDA achieves vertical polarization and omnidirectional radiation by employing an array of four folded wideband high-efficiency electromagnetic structure (WHEMS) antennas with dimensions of
$22.1\times 22.1\times46.2$
mm. This proposed design overturns the classic design concept of the traditional dipole antenna. To suppress the CM current, a microstrip type-III balun with two ground points placed symmetrically around the middle of the radiating element has been designed. This design allows for the integration of a type-III balun into the radiator, so as to achieve the effect of the balun while maintaining the desired GRDA pattern. The measurement results show that the GRDA proposed in this work achieves a symmetrical pattern, suppresses CM current, and attains a quantifiable gain of around 1.5 dBi within a 15% bandwidth, making it an ideal choice for gain calibration of OTA measurement systems.