It appears that thethe US Government is forbidding the use of an advanced radar on the Korean FA-50 trainer derivative (subscription required).

Let’s make this clear, this is not a technology transfer issue. The radar, the Vixen 500E (PDF)

But in declining to permit advanced FA-50 capability, the U.S. side is only exercising rights that were set out when the then-General Dynamics combat aircraft division agreed in the early 1990s to support South Korean development of a fast jet. Under the agreement, the aircraft’s combat capability had to remain below that of the F-16, to avoid competition.

Korea Aerospace will struggle to sell the fighter overseas without the preferred radar, the Vixen 500E active electronically scanned sensor built by Finmeccanica’s Selex unit. But Korean industry officials assume that that was the intention of the U.S. authorities: To ensure the FA-50 would be unattractive on export markets and offer no competition to the Lockheed Martin F-16.

I’m really not sure why anyone with equibalent choices uses US systems, such as choosing the F-15K over the Rafale, which performed better in tests.