The experimental lines are from
(1) M. C. McCarthy, P. B. Changala, and P. B. Carroll, 2024, J. Mol. Spectrosc. 400, Art. No. 111886.
Only K_a = 0 and 1 transition were detected thus far. The extrapolation should be good up to possibly 150 GHz for these K_a; transition frequencies involving higher values of K_a should be viewed with increasing caution.
Spin-statistics matter for the intensities. The nuclear spin-weight ratio is 3 : 1 for ortho-H₂C₃Si : para-H₂C₃Si. The ortho states are described by K_a odd, the para states by K_a even.
The dipole moment is from an unpublished quantum chemical calculation mentioned in (1).