MD550 PLT HAD LESS THAN LEGAL SEPARATION WITH A ROBINSON R-44 AT EMT; AFTER THE TWR WAS CLOSED.

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ON 04/THU/02 AT APPROX XA00; I HAD JUST CLRED THE LAX CLASS B AIRSPACE AND WAS ENRTE TO EL MONTE ARPT FOR FUEL AT THE END OF A 12 HR CREW DAY WHICH INCLUDED 7.1 HRS OF ACTUAL FLT TIME. I ANNOUNCED MY POS ON EL MONTE ARPT UNICOM (TWR WAS CLOSED) AND THAT I WOULD BE LNDG ON RWY 1. I ACTIVATED MY LNDG LIGHT AND ANNOUNCED MY POS SEVERAL TIMES AND I NEVER HEARD ANYONE ELSE ON UNICOM. AS I TURNED AN APPROX 1 MI FINAL AT 900 FT MSL; I NOTICED ACFT NAV LIGHTS AT MY ALT AT LESS THAN 1/4 MI. I TURNED MY ACFT TO THE R AND APPARENTLY THE OTHER PLT TURNED HIS ACFT TO THE R AND WE PASSED EACH OTHER AT THE SAME ALT WITHIN 500 FT HORIZLY OF EACH OTHER. I NOTICED THAT IT WAS A GOV ACFT AND I COULDN'T FIGURE OUT WHY I NEVER HEARD THEM ON UNICOM. I RECHKED MY FREQ AND NOTICED THAT I HAD INCORRECTLY SELECTED HAWTHORNE UNICOM (121.1) INSTEAD OF THE CORRECT FREQ OF 121.2 FOR EL MONTE ARPT. I THEN CALLED UP ON THE CORRECT FREQ AND SPOKE WITH THE PLT OF THE OTHER ACFT AND EXPLAINED THE MISTAKE I HAD MADE AND WHY I HAD NOT CALLED UP ON UNICOM. THE LESSON TO BE LEARNED FROM THIS INCIDENT IS TO CHK YOUR CHARTS FOR THE APPROPRIATE FREQ INSTEAD OF RELYING ON MEMORY. IN ADDITION; FATIGUE WAS PROBABLY A FACTOR SINCE I WAS AT THE END OF MY CREW DAY AND MOST OF MY FLT TIME INVOLVED A GOV MISSION WHICH STARTED IN INDIO AND ENDED IN THE LOS ANGELES BASIN.