Try an antenna from another scanner that you, or your friend, owns. See if that helps. Make sure that the center pin from the old antenna has not broken off in the BNC connector on the back of the scanner.
Has anyone keyed up a portable radio & transmitted close to this scanner or it's antenna?
The SDS200, as you know, is a database scanner. For
all database scanners from Uniden, Whistler, whoever, one of the first things to check is the memory card. Skim through dozens of old threads regarding various problems with any of the models. A very significant percentage of those reported issues are solved either with a replacement of the memory card, or by formatting it, then reloading everything.
Use this formatting app, recommended by Uniden & a number of others:
SD Card Formatter - SD Association
After formatting the card, use the 'Clear User Data' under the drop=down Scanner menu in Sentinel. This sets the card up with the folders needed for the database & Favorites lists. (If you are using Clear User Data either when setting up a new card, or formatting an existing card, you may need to select "Show all drives" so that Sentinel will recognize the card since it has been cleared of all scanner specific files.
After doing that, then reload all the Favorites lists & database updates through Sentinel's Write to Scanner link.
No guarantee that this may cure any issues, but more than a few have been corrected via these steps. Besides, other than a few minutes of your time, this procedure costs nothing but a few minutes of your time.