A LIGHT PLANE PILOT; TAKING OFF AT A NON-TOWER FIELD; EXPERIENCED AN NMAC WITH AN OPPOSITE DIRECTION AIRCRAFT WHEN HE FAILED TO USE THE PROPER CTAF.

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IFR FLT CAVU. DECIDED TO LAND ZZZ FOR FUEL. UNFAMILIAR WITH ZZZ FIELD. NO AWOS NOTED ON APCH PLATE; SO I DIALED UP ZZZ1 ATIS FOR WX. I USED COM #2 AND AFTER RECEIVING WX; STORED THE FREQ AND DIALED IN CTAF. LANDED WITHOUT INCIDENT; FUELED AND TAXIED TO TAKE OFF. TURNED ON COM #2 AND ANNOUNCED MY INTENTIONS TO DEPART RWY XX; L TURN TO DOWNWIND AND THEN NW TO DEST ON THE FREQ THAT CAME UP AUTOMATICALLY ON THE RADIO THINKING IT WAS CTAF. HEARING NO TFC; I DEPARTED AND FOLLOWED MY RTE. UPON REACHING 1000 FT AGL; I BEGAN TO HEAR ZZZ1 ATIS. REALIZING I WAS IN THE WRONG FREQ; I LOOKED UP CTAF ON THE APCH PLATE AND CHANGED FREQ. AT THIS MOMENT; I WAS APCHING THE NW END OF RWY. ANOTHER ACFT WAS ON FINAL AND WE HAD A NEAR MISS. UPON SPOTTING THE OTHER ACFT COMING DIRECTLY AT US; I TURNED SHARPLY W AS THE OTHER ACFT PASSED 100 FT OVER US. NEEDLESS TO SAY; THE OTHER PLT WAS JUSTIFIABLY VERY ANGRY OVER THE RADIO. THE IMPORTANT LESSON LEARNED IS TO ALWAYS DOUBLECHK FREQS AT AN UNFAMILIAR FIELD. I ALSO LEARNED AN IMPORTANT LESSON REGARDING MY RADIO. WHEN IT IS TURNED ON; A STORED FREQ WILL COME UP AND NOT THE LAST FREQ USED AS YOU MIGHT THINK.