Measurement using the S-parameter method for radiation characteristics and mutual coupling of multiport antennas on a small ground

If a coaxial cable is connected to a small antenna, unbalanced current will be induced on the outside conductor of the coaxial cable. As a result, it may not be possible to measure the original antenna characteristics. In order to reduce the unbalanced current, a balun is often used. However, the operation frequency range of a balun is narrow in principle because the effect of current reduction depends on a resonance of a quarter-wavelength. Meanwhile, a measurement method using fiber optics was proposed. However, it was reported that impedance is altered by the influences of the optical/electric (o/e) converter mounted on the ground plane when the size of the antenna or the ground plane is much smaller than a wavelength. The S-parameter method is known to be one way to reduce the influence of an unbalanced current. Although the S-parameter method is an effective and useful method, it has only been applied for impedance measurements.