A PRIVATE SMA PLT MADE SEVERAL ERRORS AND HAD AN NMAC. HE WAS VFR IN IMC AND MAY HAVE PENETRATED THE SFO TCA.

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DEPARTED MONTEREY FOR FLT N ALONG COAST 3500 FT UNTIL I PASSED ANO NUEVO RESERVE N OF SANTA CRUZ -- NO FOG -- VISIBILITY 10 MI. WENT TO 500 FT MSL OVER OCEAN CLOSE TO COAST. AS I NEARED HALF MOON BAY; CALLED UNICOM TO ANNOUNCE POS. ANOTHER ACFT ANNOUNCED IMC AROUND HALF MOON BAY; WHICH I DIDN'T SEE. STARTED TO CLB TO 2500 FT BUT ENTERED IMC NEAR HALF MOON BAY ARPT. ANNOUNCED POS (BY LORAN) AGAIN -- NO PROBLEM IN IMC SINCE INST RATED; BUT COULD HAVE HAD COLLISION SINCE VISIBILITY EQUALED 0. CLBED TO NEAR 3000 FT MSL TO BE 1000 FT ABOVE FOG; BUT THIS IS TOP OF SFO TCA -- MAY HAVE INTRUDED INTO SFO TCA. PROCEEDED ALONG COAST TRYING TO STAY BELOW 3000 FT MSL. CLR ABOVE FOG. FLT INLAND TO WOODLAND AFTER POINT REYES WAS AT 5800 FT; HDG 130 DEGS. AROUND SONOMA ACFT FLIES UNDER ME APPROX 300 FT DIRECTLY BELOW. -- NEAR MISS. PROBLEM -- I WASN'T SCANNING ENOUGH. PROBLEMS -- REASONS: 1. HAD CALLED FOR WX DIRECT WOODLAND FROM MRY BUT AT LAST MIN FLEW UP COAST -- NOT AWARE OF FOG -- DIDN'T GET UPDATE WX. 2. USUALLY USE FLT FOLLOWING; BUT AT 500 FT MSL BELOW COAST RANGE -- NOT POSSIBLE. 3. NEAR MISS -- NOT SCANNING ENOUGH -- USUALLY USE FLT FOLLOWING BUT DID NOT WISH TO SPEAK TO ATC FOR FEAR OF DISCOVERY OF EARLIER POSSIBLE TCA INCURSION.