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Multi-frequency operation is obtained from the single-layer probe-fed square ring antenna by loading it with a gap or slit. Two additional modes are excited around the dominant TM11 mode of the square ring, equally spaced in the frequency spectrum, and have similar radiation patterns. To excite these additional modes, the loading has to be symmetric on one arm; however, the excitation has to be off-centred on another arm, or vice-versa. By choosing the location of the loading on vertical or horizontal arm, the polarisation of each mode can be controlled. These modes are investigated in greater detail with a parametric study. The effects of dielectric substrate parameters on these modes are also investigated. One of the loaded mode frequencies is much lower than the dominant mode frequency of the square ring that can be used to miniaturise the antenna to a great extent. Design equations are developed to relate loaded and unloaded mode frequencies. Simulation results are confirmed by measurements.