We report the effect of cathode structure on the field emission properties of individual carbon nanotubes. Experimental field emission data are obtained for two well-defined cathode structures: a multi-walled carbon nanotube (MWNT) attached to an etched Ni metal wire and a MWNT attached to a flat Ni-coated Si microstructure. We observed different macroscopic turn-on fields of 1.6 and 2.5 V µm⁻¹, respectively, for the aforementioned experimental structures. This effect is investigated by detailed finite element analysis. We demonstrate that the geometry of the cathode structures significantly affects the microscopic tip field, leading to different turn-on voltages and field distributions for such individual MWNT emitters. Simulations show that changing the support geometry from a hemispherically capped shank to a cylindrical shank produces an increase in the macroscopic threshold field of 0.91 V µm⁻¹. This effect is further investigated by varying the support radius from 0.5 to 30 µm for a cylindrically shaped support structure. The results show that such a variation in the radius of the support structure produces an increase in the macroscopic turn on field from 0.72 to 5.89 V µm⁻¹. We also report quantitative evidence for the nonlinear relationship between the field enhancement factor as a function of support structure radius for nanostructures of three different aspect ratios.