A set of closed form expressions is proposed to compute the negative permittivity, the resonant frequency, the quality factor, the bandwidth, the input impedance and the gain of a coaxial probe fed microstrip metamaterial antenna. It is based on the cavity model, the transmission line model, the dynamic permittivity and the parallel resonant R-L-C circuit. The numerical results are compared with experimental results available in open literature and our new experimental results. We have also employed electromagnetic simulator to validate the model.