B737 Next Generation Undifferentiated
Air Traffic Control Fully Certified
Situational Awareness; Distraction; Time Pressure
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Human Factors; Procedure; Aircraft; Airspace Structure
Aircraft X was being vectored for the ILS Runway 13R approach at SAT airport. Aircraft X was 10 NW of SAT on a modified downwind given a pilot's discretion to 3;000 feet. At this point I saw a target depart 5C1 airport climbing low-level southbound initially away from Aircraft X. The unknown target aircraft then made a turn to the northeast heading about 050 degrees climbing out of 2;800 feet converging with Aircraft X who was approximately descending out of 4;000 feet heading 260 degrees. As soon as I noticed something had to be done to avoid the conflict someone else called and delayed my transmission. I then based Aircraft X a little early heading 220 to parallel the target to pass off Aircraft X right. Maybe 3 seconds later Aircraft X said he was turning to a 200 heading to further avoid the target so I told him to turn even further left heading 180 to ensure separation. The last control instruction resolved the conflict.This is a systemic problem with aircraft departing 5C1. This airport is located directly on the approach course to SAT to our most used runway. This airport is also on the base turn which makes it even worse. SAT controllers have gone to 5C1 to educate the pilots about climbing off 5C1 into the downwind/base turn without talking to Approach Control but nothing has changed. This situation happens every day. SAT has had countless RA'S (Resolution Alerts) due to this. This airport is often used by gliders that are most of the time not talking to us nor have a working transponder. It's a matter of time before a crash happens.To fix this problem the Class C at SAT needs to extend further out to cover 5C1.
SAT TRACON Controller reported an airborne conflict between an IFR and VFR aircraft along with a possible related airspace problem.