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A common problem with current planar ultra-wideband (UWB) monopole antennas is that their performance is heavily affected by the size of the ground plane. This is due to the fact that dimensional changes influence the characteristic modes excited on the ground plane, which directly affects the impedance and the radiation behaviour of the antenna. A technique of cutting slots on the ground plane is proposed to solve this problem. Owing to the presence of slots, the slot modes will be introduced. The optimal dimensions of the slots can produce strong coupling between the slot modes and the ground plane modes. This coupling makes the characteristic modes of planar UWB monopole insensitive to the changes in the size of the ground plane, hence effectively minimising the dependence of the antenna on the dimensions of the ground plane. Furthermore, a parametric study on the major geometric parameters of the slots is given to achieve an optimised design. Using this technique, two different UWB monopole antennas are investigated to observe the effectiveness. Both simulated and measured results demonstrate that this method can minimise the effects of dimensions of the ground plane on the impedance matching with very little effect on the other antenna parameters, such as radiation pattern, gain and time-domain performance.