With an increase in the probe diame

With an increase in the probe diameter, the impedance plot shifts downward on the Smith chart providing better
matching.

The BW of the antenna increases significantly to 1.3 GHz (38%) for d = 0.4 cm. The AR remains below 0.1 dB in the entire BW.

This is theoretical BW, which assumes that the inputs at four feeds remain equal with 0°, 90°, 180°, and 270° phase difference over the entire BW, which is not practically feasible.