The radiation resistance of a short planar monopole antenna with a small ground plane is studied. The monopole of length h (λ/100 <; h <; λ/10) is located orthogonally to the edge of a rectangular ground plane of length L and width W . First, a parametric study is realized in order to determine the effect of the ground plane dimensions on the radiation resistance. Then, the two limit configurations, long and wide, are studied analytically with a model based on triangular current distributions, and comparisons to numerical results are shown. Finally, impedance matching by addition of one or two dipole arms is studied. We explain why this addition, besides reactance compensation, further increases the radiation resistance.