Characteristic Impedance of Coax on HFSS

Could anyone please simulate a coax based on the equation Zo = 138/√er * log(b/a) on HFSS and return the port impedance (use filling other than air). I did and it return the right value. But after I place a boundary box around it, the returned port Zo is no longer true. Any comments would be helpful, thanks

chose outer radius :0.16 cm
inner radius 0.07 cm
it gives 49.66 impedance