I didn't carefully read your post.
In your LNB you have dielectric?
"''Linear'' signals in C-band are signals that have ''v'' or ''H'' as their polarization. Even Ku signals are ''linear'' signals. And as u would have noticed with ur ku signals, doesn't need the insertion of the dielectric inside the throat of the lnb.
Whereas the signals with ''R'' or ''L'' polarization are known as circular signals/polarization and requires the insertion of the dielectric."