NMAC OVER AN UNCTLED ARPT BTWN AN SMA SEL AND SMA TWIN.

Narrative

I WAS COMING BACK FROM A XCOUNTRY WITH A STUDENT AND AN OBSERVER WHO WAS IN THE BACK SEAT. BEFORE ARRIVING AT HWO; I HAD MY STUDENT PRACTICE A DIVERSION TO X46. AS WE APCHED THE ARPT (X46) FROM THE N; WE LISTENED ON 122.9 (X46 CTAF). WE ALSO ANNOUNCED OUR POS AND INTENTION APPROX 10 NM N OF THE ARPT (X46). AS WE CONTINUED TO MONITOR THE FREQ; WE APCHED THE FIELD AT 1500 FT AND THERE WERE NO SIGNS OF OTHER TFC IN THE AREA (NO ONE WAS TALKING ON THE FREQ BUT US). WE FLEW OVER THE W SIDE OF THE UNCTLED FIELD AND STARTED A L TURN TO CIRCLE THE FIELD; LOOK FOR TFC (IF ANY) AND TO LOOK AT THE WINDSOCK. AS WE STARTED THE TURN; WE ALSO ANNOUNCED ON 122.9 THAT WE WERE CIRCLING THE FIELD AT 1500 FT. WHEN OUR AIRPLANE WAS GOING THROUGH A HDG OF APPROX 030 DEGS ON THAT FIRST L TURN; A LIGHT TWIN PASSED RIGHT IN FRONT OF US AT OUR SAME ALT FROM R TO L AND APPROX 200 FT HORIZ DISTANCE FROM US. MY REACTION WAS TO MAKE A HARD R BANK TO AVOID THE COLLISION. I WAS NOT ABLE TO GET A TAIL NUMBER OF THE ACFT DUE TO THE SPD IT HAD. THE ACFT LOOKED LIKE A PIPER SENECA. MY CONCLUSION WAS THAT THE LIGHT TWIN HAD JUST DEPARTED OPF AND WAS HEADED NW; AND DID NOT BOTHER TO ANNOUNCE HIS INTENTIONS ON 122.9 AS HE PASSED OVER X46 AT 1500 FT. THIS SIT COULD HAVE BEEN AVOIDED IF THE LIGHT TWIN WOULD ANNOUNCE HIS INTENTION ON THE FREQ OR CLB TO A HIGHER ALT.