According to a calculation by a senior EADS radar expert, the Captor-E, which will use 1,426 T/R modules and is scheduled to be integrated onto the Eurofighter Typhoon in 2015, is capable of recognizing the F-35 at around 59 kilometers away.
He acknowledged that the chance is high for the F-35 to detect and fire missiles first against fourth-generation jets, such as the Eurofighter or Boeing’s F-15, but claimed that the latter are capable of dodging missiles and successfully counterattacking at such a long range.
His calculation shows that the F-35’s APG-81, which allegedly has 1,400 T/R modules, will be able to recognize the Eurofighter or semi-stealth fighter at 120 kilometers or farther based on the assumption both radars have the same capability.
If he can't get the correct T/R module count for the F-35, I'm not sure I'd trust his calculations; he may be a senior radar expert, but if you put garbage in, you'll get garbage out (particularly in regards to what values he's used for the F-35's RCS).