relationship between lambda and resonant frequency when simulating monopole on hfss

hi all, i was trying to simulate a simple monopole on hfss. however, when i was doing the simulation, i noticed that the length of the monopole doesn't exactly follow the lambda/4 eqn; the resonant frequency seems to be 2 or 3 times higher than the antenna was designed for. i think it has to do w/ the radius of the monopole, as well as the size of the ground plane, since they are all part of the antenna. can any1 explain exactly the relationship between length of the monopole, its radius and the size of the ground plane related to the resonant frequency of the antenna. thank you