  Light from any physical source diffracts over space, as spherical wavefronts
grow and energy density is spread out. Diffractive effects pose fundamental
limits to light-based technologies, including communications, spectroscopy, and
metrology. Polarisation becomes paraxial in the far field limit and, by
ignoring longitudinal field components, the rich physics of non-paraxial fields
which exist in near-fields or a beam's tight focus are lost. The longitudinal
field cannot, however, be ignored when transverse field components vanish (in a
transverse field zero) and carry a small non-paraxial region to infinity. We
show that a transverse field zero is always accompanied by non-diffracting
polarisation structures, whose geometries are independent of the distance to
the source, including an enclosing intensity ratio tube, and parallel,
non-diverging polarisation singularities. We illustrate these features in
multipole radiation and in double slit interference, two examples which have
time-fixed transverse field zeros. Non-diffracting structures with changing
position are coupled to time-varying zeros, which are present in all far field
radiation.
