You used to be able to easily visit the Duga Radar, legally, via one of the many tour companies that offered Chernobyl tours. Back in 2018 I visited myself, I believe with this company, as part of a conference: https://chernobyl-tour.com/tours_to_the_chernobyl-2_duga-1_e... At least then Chernobyl had been turned into quite the tourist destination with enough tours running that there even was a giftshop at the main checkpoint.
Unfortunately due to the proximity to the Russian/Belarus borders the whole area is closed right now due to Russia's invasion (Russia even occupied Chernobyl briefly). But it'll probably reopen sooner or later.
Unfortunately due to the proximity to the Russian/Belarus borders the whole area is closed right now due to Russia's invasion (Russia even occupied Chernobyl briefly). But it'll probably reopen sooner or later.