This scan is exactly what I get when DVB-T is disconnected from noise source and a small length of coax is left connected to the dongle. Seems your dongle is not getting feed from noise source. Are you using right type of MCX connectors? If you use RP-MCX Male instead of regular MCX Male, there will be no connection of central wire. The RP-MCX Male (thread inside) does not have a center pin. The regular MCX Female (thread outside) on the noise generator does not have a pin either. If RP-MCX Male is fitted over regular MCX Female, the center wire will not connect due to missing pin.This is my scan. My Step is different to yours. Not sure where this gets set.
Your latest filter design looks interesting