The center hole of a bifilar coil determines the strength of the magnetic field. The smaller the hole, the stronger the magnetic field.
But at the same time, the dielectric field voltage rise, doesn seem to be affected by the size of the hole.
I have not conlcusivly found the longitudinal wave form, but, I suspect it is the same frequency as the transverse resonant frequency.
Since the enrgy seems to be the same in the dielectric field voltage, but the magnetic field is weaker, I suspect the energy in the longitudinal field is stronger.
In other words, a small center hole, is a strong magnetic and dielectric field. (weak longitudinal?)
A large center hole, is a stong longitudinal field and dielectric field. and a weak magnetic field.
If I would visualise the fields, and what happens when opening the hole (enlarging the diameter of the hole), I see a vortex, focussed in the smallest center hole. When the diameter gets bigger, this vortex transformes, becoming a wider and wider vortex, the field is erecting, becoming a beam, beaming straight of the coil, like a flat speaker, creating pressure waves, longitudinal. but also still rotating.
With a speaker you get akoustic feedback from the back and the front speaker, thats why its enclosed in a cabinet, so the pressure zones dont shortcut around the speaker.
This might also be needed in case of the bifilar coil. would this mean, having wide enough widings area? What would stop these longitudinal ether presure waves (tsunami's).