Lately stuck on WW2 history.

[Gawdzilla Sama]

Bear in mind:
A) The US had only just started using RADAR, having been given access to the technology by Britain the previous year. They probably didn't have much idea how to use it properly or much faith in its effectiveness.
B) There were actually a shocking number of warning signs that the government ignored, up to and including a war game in 1932 where Fleet Admiral Harry Yarnell managed to take Pearl Harbor in the same way that the Japanese would nine years later, going in the same direction, using the same persistent cloud bank, and even starting his attack on a Sunday the Seventh. If he used real bombs and bullets, things probably would have been worse than the actual attack. And even after Vice Admiral Ernest King managed to repeat the same tactic with the same results in 1938, things didn't change.

Actually, the radar technology the US received from Britain the previous year was the cavity magnetron that allowed radar to be made much more compact and with higher resolution.   That hasn’t translated to operational radar set yet at the time of Pearl Harbor.

The radar operating in Pearl Harbor is based on older, indigenous American technology that actually dates to before 1939.

The SCR-270B model radar used an oscilloscope "screen", like the one below. The returns were measured in amplitude, the bigger the target the higher the spike. No other information about the "blip" could be learned except for rate of approach.