Thanks for the charts Hpasp, are they original declassified Northrop documents?

They show that the difference of RCS was expected to be about 2.5 times when comparing a x-band fire control radar and a VHF search radar such as the P-18.

For the S-400 it would mean that if used with the Nebo-M its detection range is against a ~ 2.5 times larger RCS target than the nominal detection range against those x-band golf ball and marble RCS statements of the US DoD. The 0.0014m² sphere becomes a 0.0042m² sphere. I dont have the radar equation at hand but a 500km class system such as the Nebo-M should have a useful range against a 0.0042m² (virtual RCS in VHF band) target. It should be added that this value should be even higher for VHF band radars as RAM and RAS is optimized and effetive for x-band and similar band radars much less effective in VHF band.

The question for a successful engagement is at which range the S-300/400 tracking radar will pick up the identified target. Here the brute force of a large aperture PESA able to create a concentrated pencil beam should be the key to put enough energy on the target. It should be possible that the VHF detection range would't be much higher than the tracking range again low-RSC targets.

*Edited for some corrections