BE33 PLT HAS NMAC WITH C172 ON BASE LEG AT UNCONTROLLED AIRPORT.

Narrative

WAS ON A POST MAINT CHK FLT; VFR WITH FLT FOLLOWING S OF ZZZ1. AFTER MAINT ITEMS WERE CHKED STARTED TO RETURN TO HOME ARPT; BUT ON SPUR OF THE MOMENT DECIDED TO DIVERT TO ZZZ FOR PRACTICE LNDGS. ADVISED ATC; CANCELED FLT FOLLOWING; MADE A DSNDING 270 DEG TURN AWAY FROM ARPT; SO AS TO BE AT PATTERN ALT WHILE APCHING ARPT. LEVELED OUT ABOUT 3.5 MI; MADE STANDARD 45 DEG ENTRY TO DOWNWIND PATTERN. ZZZ DOES NOT HAVE AN INST APCH; SO DID NOT HAVE A REF PATTERN ADVISORY FREQ IN THE APCH PLATES; NO TIME TO DIG OUT THE SECTIONAL; SO I SCANNED FOR CONFLICTING TFC CAREFULLY. AS I TURNED DOWNWIND; I SAW 1 ACFT JUST LIFTING OFF DEP END; NOTHING ELSE. CONTINUED TO LOOK FOR TFC; CONFIGURED THE ACFT FOR LNDG; WAS STABILIZED ON ALT AND AIRSPD; BUT MAY HAVE BEEN (IN RETROSPECT) A LITTLE WIDE IN THE PATTERN. AT THE 180 DEG; TURNED FOR L BASE AND BEGAN DSCNT. BEFORE I WAS WINGS LEVEL FOR BASE; SAW A C172 PASS VERY CLOSE (50 YARDS OR LESS) FROM MY 8 O'CLOCK POS; XING MY FLT PATH JUST BEHIND ME. I ABANDONED THE APCH AND DEPARTED THE PATTERN TO HOME BASE. CONSIDERATIONS: SPUR OF THE MOMENT DECISION LED TO IMPROPER PREPARATION; EVEN THOUGH NOT REQUIRED; I DID NOT HAVE THE GREAT ADVANTAGE OF BEING ABLE TO QUERY AND ANNOUNCE IN THE PATTERN. I TRIED TO COMPENSATE FOR THIS BY CAREFULLY SCANNING FOR ACFT; UNSUCCESSFULLY. MARKINGS MAY HAVE HELPED THE ACFT TO BLEND INTO BACKGND.