PLT OF A PIPER PA28-161 ON A SHORT BASE LEG AT AN UNCTLED ARPT TOOK EVASIVE ACTION BY GIVING WAY TO A LEARJET ON FINAL BY TURNING AWAY AND DSNDING.

Narrative

AT 1 1/2 MI I RPTED INTENT TO ENTER MID-FIELD L DOWNWIND RWY 31 ST AUGUSTINE. AS I NEARED THE FIELD I BEGAN TO TAKE CAREFUL NOTICE OF ACFT; THEIR RPTED POS AND THEIR ACTUAL POS AT ST AUGUSTINE. A LEARJET WAS ON FINAL FOR LNDG; A CHEROKEE WAS ON XWIND FOR TOUCH AND GO AND A CHEROKEE WAS DOWNWIND FOR LNDG. I POSITIONED MY ACFT TO ENTER THE L DOWNWIND FOR RWY 31 BTWN THE CHEROKEE ON XWIND AND THE CHEROKEE PRESENTLY AT MID-FIELD DOWNWIND. I ENTERED THE L DOWNWIND APPROX 1/2 MI BEHIND THE CHEROKEE WITH THE SECOND CHEROKEE STILL ON XWIND. THE SECOND CHEROKEE RPTED TURNING DOWNWIND RWY 31 ABOUT 30 SECONDS LATER. THE CHEROKEE IN FRONT OF ME RPTED TURNING L BASE RWY 31 (SGJ). I CONTINUED DOWNWIND. THE CHEROKEE IN FRONT (ON BASE) RPTED TURNING FINAL RWY 31 (SGJ). ALSO; OTHER RPTS WERE COMING FROM NEARBY AIRFIELDS THAT WERE ASSIGNED UNICOM 122.8 (EVB). SOME PLTS WERE CALLING RWY NUMBERS WITHOUT IDENTING THE ARPT. THE RADIO TFC WAS QUITE HVY. THE CHEROKEE ON FINAL RPTED AGAIN THAT IT WAS ON FINAL FOR ST AUGUSTINE. I HEARD AN ACFT RPT 2.5 MI FINAL BUT DID NOT HEAR THE ARPT OF INTENDED LNDG. THE CHEROKEE THAT WAS ON FINAL WAS NOW AT MY 10 O'CLOCK POS AND I RPTED TURNING BASE FOR RWY 31 ST AUGUSTINE AND STATED THAT I WAS #2 TO LAND. AS I PROCEEDED ON THE BASE LEG (ABOUT 2/3 DOWN BASE) AT 800 FT MSL; 20 DEG FLAPS; 80 KTS; I LOOKED R AND SAW THE LNDG LIGHTS OF THE LEARJET. IT WAS AT MY APPROX ALT AT MY 2 O'CLOCK POS ABOUT 1/2 MI AWAY. I BANKED HARD TO THE R AND PITCHED THE NOSE DOWN. I CAME AROUND (180 DEGS) IN A STEEP R BANK DSNDING TO 500 FT OVER THE RIVER. I RECEIVED A RADIO RESPONSE THAT SAID 'I'M GOING AROUND;' BUT NO ACFT OR ARPT IDENT. I CONTINUED ON THE REVERSE BASE COURSE FOR 4-5 MI AND RETURNED TO THE SGJ; EXECUTED A STANDARD L ENTRY INTO THE PATTERN FOR RWY 31 SGJ. THE APCH AND LNDG WERE UNEVENTFUL. FACTORS: 1) THE LEARJET DID NOT USE RECOMMENDED PATTERN ENTRY TECHNIQUE. 2) THE RADIO TFC WAS HVY. IT WAS DIFFICULT TO DETERMINE ACFT POS (IE; WHICH ARPT). 3) MY FAILURE TO SEEK CLARIFICATION CONCERNING WHAT ACFT HAD RPTED 2 1/2 MI FINAL AND AT WHICH ARPT. 'PLEASE EVERYONE; USE STANDARD ENTRY PROCS.'