I highly recommend not relying on SDR ADS-B reception alone and also checking online data systems for ADS-B that also use multilateration to track aircraft using their transponder when they aren’t ADS-B capable. Otherwise you might be operating under the assumption there are no contacts in your working airspace when there is aircraft present but not announcing their location.
You need both data sources—a number of planes are “delisted” from the commercial trackers and won’t show up, but they still broadcast ADS-B. You can find partial lists of such registrations through a keen Google search.
https://flightaware.com/adsb/mlat/