As mentioned the circular LNB is in the wrong frequency range of 12.2 to 12.7GHz and you probably want 11.7 to 12.2GHz for FTA. Inside the DTV feed throat is a metal stepped septum, which delays one component of the signal to put it in phase with the other so the simple dipole in the LNB can pick it up as a linear pol signal. I doubt if you could get the septum out very clean unless you had a drill bit the OD of the feed throat ID to clean up the resulting mess. There are other types of circular pol feeds that use a plastic "dielectric card" in the feed throat for signal delay, which is usually easy to remove without any damage to the feed.
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