The Radio Kootwijk is located in the Veluwe, just north of the Hoge Veluwe park. It's an amazing building, looming in a scrubby sand-swept landscape.
We went for one overnight at the nearest Natuurkampterrein, in Kootwijk. The trekkersveld was rather full, but that is to be expected when the weather is nice.
(And for after the cycle trip, according to wikipedia, one named film has been filmed at the building- Mindhunters. Tip: If you can sit through the first appallingly clichéd 20 minutes, you can see the building. After the scene inside with the "smart" guy solving a Rubiks cube, I don't know what happens because I couldn't watch any more.)