GA SMT ENTERED RWY FOR TKOF CAUSING GA SMA ON SHORT FINAL TO GO AROUND.

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I HAD FINISHED RUN UP AND PRE-TKOF CHKLIST. ON RUN UP PAD; FOR RWY 28; MY ACFT WAS FACING ON A 45 DEG HDG DURING RUN UP. ADVISED TRK UNICOM THAT TAXIING INTO POS ON RWY 28. (A LCL MECH WAS ACTING AS COPLT.) NEITHER COPLT NOR I HEARD ANY OTHER ACFT CALL IN THE TFC PATTERN PRIOR TO OUR CALL. NEITHER HE NOR I SAW ANY OTHER ACFT ON A LEFT BASE LEG; FINAL OR ANYWHERE IN THE VICINITY. I TAXIED INTO POS ON RWY 28; STOPPED; BROUGHT THE ENGS UP TO PWR; STABILIZED AND RELEASED BRAKES STARTED TKOF ROLL. ABOUT 50' INTO TKOF ROLL A MALE VOICE WAS HEARD ON THE RADIO ADVISING ME THAT I HAD PULLED OUT ONTO THE RWY WHILE HE WAS ON FINAL. ABOUT THAT TIME A SMALL; WHITE SINGLE ENG LOW WING ACFT PASSED ABOVE US AND TO OUR RIGHT SIDE ON A 280 DEG HDG. I IMMEDIATELY APOLOGIZED TO HIM; ADVISING THAT I HAD NOT HAD AN ACFT IN SIGHT BEFORE TAXIING ONTO THE RWY. MY CALL TO TRK UNICOM WAS MADE APPROX 1 MIN BEFORE ACTUALLY TAKING POS ON THE RWY; OR EVEN GETTING ONTO THE RWY. AT NO TIME DID WE HEAR A CALL THAT ANOTHER ACFT WAS ON FINAL APCH. FRANKLY I AM COMPLETELY AT LOSS AS TO WHY WE DID NOT SEE THE OTHER ACFT; AND ALSO CANNOT UNDERSTAND WHY HE MADE NO CALL; THAT WE COULD HEAR; ON BASE LEG OR FINAL APCH--ESPECIALLY ADVISING US THAT HE WAS ON FINAL APCH WHEN I MADE MY CALL THAT WE WERE TAXIING INTO POS ON RWY 28. I DID HEAR A VERY WEAK; GARBLED XMISSION WHEN HE WAS OVERHEAD AT THE END OF THE ARPT; BUT COULD NOT UNDERSTAND IT. MY RADIOS WERE WORKING FINE IN BOTH XMISSION AND RECEIVE; AS I HAD MADE A CHK WITH TRK UNICOM PRIOR TO TAXIING. WE POSSIBLY BOTH LOOKED INTO THE SUN FOR AN INSTANT ON OUR SCAN; PRIOR TO TAKING POS ON THE RWY; BUT DO NOT BELIEVE THIS WOULD NOT ENABLE US TO SEE AN ACFT ON FINAL. I HAVE BEEN OPERATING FROM THIS ARPT FOR OVER 4 YRS W/O INCIDENT; BUT IT JUST SHOWS EXTREME VIGILANT VISUAL SCANNING IS REQUIRED BY ALL PARTIES IN THESE SITUATIONS.