The radiation behavior of the fractional-order, resonant mode within a circular sector cavity radiator is revealed at first, and then, it is employed to design a novel, planar quasi-isotropic magnetic dipole antenna. A set of closed-form formulas is derived and employed to determine the key parameters of the proposed antenna. The resultant circular sector magnetic dipole antenna operates at its dominant TM(2/3), 1 mode. It is numerically verified and experimentally validated at the 2.45-GHz band. It is seen that the antenna exhibits a good non-uniformity of less than 5.7 dB within the three principal planes, and an average radiation efficiency up to 82% within its impedance bandwidth from 2.4 to 2.5 GHz (for reflection coefficient smaller than - 10 dB). Good agreement between the theoretical, simulated, and measured results has evidently verified the proposed antenna design approach.