When I served in the Russian Aerospace Forces as a senior radar operator, my lead officer, who was involved in the Serbian operation, told me how the Serbs shot down "invisible" US planes. The reason the F-117s were ineffective was the Americans' reliance on the centimeter wave range and modern detection systems. But the Serbs used the old Soviet P-18 radars, which had a one-meter wavelength range, giving the aircraft full visibility. That's one of the reasons why the F-117 turned out to be such crap.
By the way, stealth technology for aircraft is a Russian invention. The Americans simply "borrowed" it. I don't know why, but this fact makes someone American fanatic patriots' asses burn.
https://en.wikipedia.org/wiki/Pyotr_Ufimtsev 