NMAC ON BASE LEG BTWN A SMA SEL HIGH WING ON BASE LEG AND A MOONEY SMA TURNING OFF FINAL STRAIGHT TOWARD THE RPTR. THE RPTR DSNDED AND TURNED TO AVOID THE MOONEY ACFT.

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WE (ACFT #1) WERE DOING AN APCH WITH THE PLT WORKING ON HIS INST RATING FLYING THE ILS RWY 23 APCH AT MMU. WE WERE INSTRUCTED TO CALL WHEN BREAKING OFF THE APCH TO CIRCLE TO LAND ON RWY 30. WE DID SO; BUT RECEIVED NO ACKNOWLEDGEMENT FROM THE TWR AS OTHER ACFT ON THE FREQ WERE KEEPING THE CTLR VERY BUSY. WE KNEW THAT THERE WERE TWO OTHER ACFT INBOUND; BUT ASSUMED THAT ONE WAS A LONG WAY OFF. ON SEEING ONE ACFT ON FINAL PASS OUR WING; WE TURNED BASE. AT ABOUT THE TIME OF COMPLETION OF THE TURN; THE ACFT ON FINAL; A HIGH WING; ASKED THE TWR IF THERE WAS A LOW WING ACFT BELOW HIM. THE TWR SAID YES; AND THE NEXT THING WE KNEW; THE LOW WING; A MOONEY; HAD TURNED R AND WAS HEADING RIGHT FOR US. WE WENT LOW AND TO THE L AND AVOIDED THE ACFT. THE FIRST ACFT WE SAW LANDED; AND WE FOLLOWED HAVING GONE PAST THE CTRLINE OF RWY 30; BUT HAVING NO TROUBLE AGAIN LINING UP WITH THE RWY. THIS INCIDENT SEEMS TO HAVE OCCURRED BECAUSE OF TOO MUCH TFC HEADING INBOUND AT THE SAME TIME. THE CTLR SAID THAT OUR CALL WAS MISSED; SO THERE WAS NO AWARENESS OF OUR TURNING BASE WHEN THE MOONEY WAS WAVED OFF. ALSO; WE FAILED TO NOTE THE SECOND ACFT ON FINAL; BECAUSE WE WERE NOT EXPECTING A CONFLICT ON FINAL. I DO NOT BELIEVE THAT THIS TWR HAS RADAR. IT IS A BUSY AREA; AND THE CTLRS NEED THE ADDITIONAL EYES THAT RADAR PROVIDES IN HELPING TO EXACTLY POINT OUT THE POS OF THE ACFT. HERE; IF A CONFLICT WAS DEVELOPING; ONE OF THE ACFT SHOULD HAVE BEEN TOLD TO CHANGE COURSE OR DO A 360 DEG UNTIL PROPER SEQUENCING COULD BE ACCOMPLISHED. NOTE: IT WOULD HAVE BEEN BETTER FOR THE TWR TO HAVE HAD THE HIGH WING ACFT GAR AND TO HAVE LET THE LOW WING; AT LOWER ALT; LAND. (NO CONFLICT WITH US.)