  Recent nuclear magnetic resonance experiments measuring the Knight shift in
$Sr_2RuO_4$ have challenged the widely accepted picture of chiral pairing in
this superconductor. Here we study the implications of helical pairing on the
superconducting state while comparing our results with the available
experimental data on the upper critical field and Knight shift. We solve the
Bogoliubov-de-Gennes equation employing a realistic three-dimensional
tight-binding model that captures the experimental Fermi surface very well. In
agreement with experiments we find a Pauli limiting to the upper critical field
and, at low temperatures and high fields, a second superconducting transition.
These transitions which form a superconducting subphase in the H-T phase
diagram are first-order in nature and merge into a single second-order
transition at a bicritical point $(T^\ast,H^\ast$), for which we find (0.8~K,
2.4~T) with experiment reporting (0.8~K, $\sim$ 1.2~T) [\textit{Phys. Rev. B}
\textbf{93}, 184513 (2016)]. Furthermore, we find a substantial drop in the
Knight shift in agreement with recent experiments.
