This paper presents a dielectric loaded slot antenna capable of maintaining omnidirectional patterns under varying ground plane dimensions. The antenna consists of four circumferential arranged slots, loading by a strategically adjusted dielectric structure. The loading dielectric structure changes the slots’ electric-field distributions to form an equivalent electric dipole with omnidirectional radiation pattern. Notably, the rearranged electric-field and the equivalent dipole doesn’t affect by the dimension increasing of the ground plane, ensuring consistent performance across diverse ground sizes. A prototype is fabricated and measured for validation, showing omnidirectional patterns with a maximum gain at θ=90° plane and a cross polarization of lower than -10 dB.