A Helmholtz coil is a magnetostatic device, operating at DC or low frequencies. The current in all windings is in phase and capacitive effects can be ignored.
At microwave frequency, the current in individual windings is phase shifted due to finite propagation speed and also affected by interwinding capacitance. You need to model the actual winding geometrie to get a realistic current distribution and field picture of the coil, which is quite different from a single loop.