A radio jammer under control of the Greycollars.
Radar Towers are destroyable structures located within bases. While they are active, the radar tower will jam nearby enemy radios, preventing radio support within the area. Assaulting an enemy location under the effect of a radar tower is a much more difficult task, as you will be unable to supply airstrikes or reinforcements near it.
Radar Towers are typically deep within enemy territory, meaning a coordinated assault or a successful infiltration is key to disabling them. Much like other destroyable structures, C4 is highly recommended.
A disabled radio means you are currently near an enemy radar tower.