There are several threads for converting circular Ku feedhorns to linear.
Mostly to bolt onto DishNet FSS 10750 LNBs, to get better reception than the horn DishNet used on their 105 bird.
Most recent was... by.. Magic Static (?), where he took a horn off an older DirecTV Phase III LNBF and mounted it to a DishNet LNB.
You could easily do this to a PrimeStar LNB (same mechanical interface), IF you had a round PrimeStar dish.
The feedhorn had little steps cast inside it to turn circular to linear - you just leave 'em there for this application.
edit:
Oh, on second thought...
if you have a round Primestar, you've got a long neck to shove a dielectric slab into (experiment with size).
Of course, if you have an elliptical Primestar, you're up the creek.
