DEPARTING SMA ACFT HAD NMAC WITH A PATROLLING ACFT OVER THE TOP OF THE DOWNWIND LEG OF THE TFC PATTERN.

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OPF TWR CLRED ME AND MY CFI COPLT (WHO WAS COMPLETING MY FLYING CLUB CHKOUT IN THE ACFT) TO DEPART RWY 9L AT OPF IN A L DOWNWIND DEP TO THE W. NO OTHER INSTRUCTIONS WERE GIVEN. I CLBED OUT AND TURNED DOWNWIND IN A CLB TO 1800 FT. (THERE IS A 2000 FT VERT RESTRICTION AT OPF BECAUSE OF THE PROX TO MIAMI TCA.) AS I PASSED THROUGH 1500-1600 FT; I NOTICED A HIGH WING ACFT PASSING UNDER AND SLIGHTLY TO MY R TRAVELING IN A DIRECTION 180 DEGS TO MINE (EASTWARD). I HAD NO CHANCE TO TAKE EVASIVE ACTION BECAUSE THE ACFT APPEARED SO QUICKLY. OPF TWR IMMEDIATELY RECEIVED A CALL FROM THE HIGH WING ACFT STATING THAT AN ACFT (US) HAD PASSED OVER IT ABOVE 2000 FT. OPF CALLED US AND ASKED FOR OUR ALT AND I REPLIED 1800 FT (WHERE I HAD LEVELED OUT BY THAT TIME). OPF TWR STATED OUR MODE C ALSO READ 1800 FT. WE CONTINUED TO THE W BUT AFTER THE FLT LEARNED THE OTHER ACFT WAS A POLICE ACFT ON TFC PATROL ORBITING IN THE PATTERN. OPF TWR HAD NOT INFORMED US OF THIS ACFT NOR HAD THEY GIVEN US ANY ALT RESTRICTION FOR THE DOWNWIND DEP. THE POLICE ACFT WAS FLOWN BY AN FAA EMPLOYEE (AUX POLICE OFFICER) AND HE TALKED WITH OPF TWR REGARDING THE INCIDENT AFTERWARD. I THOUGHT THIS WOULD SOLVE THE PROB BUT I FLEW AGAIN ON SUN AND WAS ONCE AGAIN GIVEN A DOWNWIND DEP WITH NO ALT RESTRICTIONS OR WARNINGS OF THE TFC ACFT; HOWEVER; HE WAS THERE AGAIN. THIS TIME I LEVELED AT 1000 FT AND PASSED ABOUT 500 FT BELOW THE ACFT. COMMENT: I QUESTION THE ADVISABILITY OF FLYING AN ACFT IN A BUSY ARPT TFC PATTERN (ESPECIALLY AGAINST TFC) SOLELY FOR THE PURPOSE OF GIVING TFC TICKETS FROM THE AIR. POLICE CARS ARE AVAILABLE FOR USE IN HIGH DENSITY ACFT TFC AREAS AND FLORIDA HAS 1000'S OF MI OF HWY TO BE PATROLLED AWAY FROM BUSY ARPTS.