The transition of coaxial line to microstrip line will generate unexpected radiated emissions, which degrades the electromagnetic compatibility of the system. Existing circuit models in the literature for this radiation phenomenon are either inaccurate or incomplete. In this work, we proposed a physically-meaningful three-element circuit model for the coaxial-to-microstrip transition, as well as formula to estimate the circuit parameters when the whole structure is electrically short (in this paper, under 1 GHz). Using this model and its analytic equations, the level of total radiated power of the transition can be estimated. To verify the accuracy of the model's prediction, full-wave simulations are performed using a commercial solver. The results calculated by the proposed model follows the same trend as the simulated results, especially at high frequency bands. Besides, it is found that the radiated power resulting from the transition is roughly proportional to omega to the power of six at low frequencies.