You can get Ku feedhorns for prime focus dishes. For example Invacom makes a ku band prime focus feedhorn with a C120 flange. They also make a C120 flange LNB that will mate up perfectly to this feedhorn. If it were me, I'd use something along these lines, meaning a separate feedhorn and LNB, rather than an LNBF. It's likely the scaler ring on the LNBF is NOT adjustable and on a prime focus dish you want an adjustable scaler ring to get proper illumination. The best setup would probably be a feedhorn that has a polar servo and accepts wr-75 flange LNBs, like norsats, but then youd need a reciever with polar servo control or some other method of controlling the servo. This would be best as it would allow fine tuning of skew (on some satellites the skew is deliberately set different) and you could use better LNBs, like PLL lnbs. If you'll also note the prime focus LNBF linked to by satelliteAV lists the support F/D ratios as .5 to .7. Does your dish have an F/d ratio in this range? I'd bet not as most prime focus dishes I've seen aren't. While it says that LNBF is for a prime focus and the scaler ring in the picture looks like one for a prime focus those specs suggest differently. Anyway, for an idea what's available search on ebay using the words "Ku feedhorn". I'm not saying you should buy on ebay, but this will at least let you see the kind of Ku feedhorns that are out there.
