Personally, I think this bit was the key: The Serbs had spies outside the Italian airbase most of the bombers operated from. When the bombers took off, the info

Personally, I think this bit was the key: The Serbs had spies outside the Italian airbase most of the bombers operated from. When the bombers took off, the information on what aircraft they, and how many, quickly made it to Zoltan and the other battery commanders. My guess is, if you know how many airplanes are on their way and at what time, you can prepare. Knowing the ground speed, you can estimate when they're going to be above you and quickly light up your radar, take a pot shot, and then turn it off and scoot.