I have recently explained elsewhere how the 2 radars are operated (at least in FOTRSU mod but it works that way elsewhere too I think). Please read my response here at post #1363:
https://www.subsim.com/radioroom/sho...243064&page=91
In the TMO mod you can raise or lower the SD radar antenna at periscope depth, and there is NO option to turn the radar off AFAIK, it will always be working whatever the radar station looks like. In the FOTRSU mod, as explained in the link above, you can switch off both radars.
When setting max range with the knob you don't get a good idea of the range, I would switch it on middle range and 'sweep'.
Focus: the radar wave is directed in one precise bearing without the radar antenna turning around, this yields a fast update on located targets.
Sweep: The radar antenna turns around to locate all possible targets in a 360° arc. Air targets may also be detected, the blip doesn't differ from that of a ship.
In the FOTRSU mod, when leaving radar station the game always changes from sweep to focus, nothing you can do, but it doesn't matter for target detection. Whatever method the radar uses, all targets will appear on the NavMap as grey icons. Those icons which read "speed very fast" are planes.
Theoretically warships could triangulate your position if more of them detect your radar waves, but I have not seen this in the early war stage during game. Maybe in the late war years.
Cheers,
XS