First off, that's not good code: there is no limit on Azimuth in there, despite the comment that it should range from 0 to 360.
Each time the timer ticks, you add the same item to the _radar collection - so your change of the Azimuth property immediately above that affects all of the items, not just one. To add different values, you need to create a new instance of whatever
is each time you get a new Tick event.
Your if condition just looks weird - unless
start with very specific values it's unlikely to work. Pretty much,
would have to start between 40 and 75, and
would have to be 1 or 5 ...
And there is noting there at all about serial data: start here and see what you can get to: SerialPort Class (System.IO.Ports) | Microsoft Docs