RWY INCURSION WHEN A LONE CFI IN HIS SMA ACFT MISSES A TURN OFF ON AN ACTIVE RWY DURING AN ATC FAC XMISSION PROB AT LAF; IN.

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ON 04/FRI/02; I WAS LNDG TO A FULL STOP ON RWY 05 AT LAF (PURDUE UNIVERSITY ARPT) AFTER A SERIES OF TOUCH AND GO'S. THE PATTERN AND FREQUENCY WAS EXTREMELY CONGESTED AND THE TWR TRANSMISSION WERE SOMETIMES GARBLED AND HARD TO HEAR. AFTER LNDG ON RWY 05; I HEARD ANOTHER GARBLED INSTRUCTION. I BELIEVE I HEARD MY INSTRUCTION WAS TO MAKE A TURN ONTO RWY 10-28 AND THEM TO EXIT ON CHARLIE 1; CONTACT GND ON 'POINT NINER.' I TURNED ONTO THE RWY AND THOUGHT I WAS TO TURN ONTO CHARLIE 1. AFTER I MISSED THE BRAVO INTERSECTION TWR HAD INFORMED ME THAT I HAD MISSED MY TURNOFF OFF THE RWY AND TO CONTACT GND ON 121.9. UPON EXITING OFF RWY 10-28 I CONTACT GND AND I WAS ASKED TO CALL THE TWR ONCE INSIDE THE FBO. DURING MY CONVERSATION WITH THE TWR I WAS INFORMED THAT TWR WAS HAVING MAINT TROUBLE WITH THEIR RADIO XMISSIONS AND WAS GOING TO 'PULL THE TAPES' TO HELP THEIR POINT OF NEEDING FUNDING FOR TWR RADIO REPAIRS. I WAS ALSO INFORMED THAT THE MIS-COM REGARDING WHICH EXIT POINT OFF THE RWY WAS GOING TO BE CONSIDERED AN 'INCURSION.' I AM A 1700 HR FLT INSTRUCTOR WHO HAS BEEN TEACHING OUT OF THE LAF ARPT DAILY FOR THE LAST TWO YRS. LAF IS ALSO CONSIDERED A 'TRAINING ARPT' BECAUSE OF THE HIGH VOLUME OF TRAINING FLTS THAT TAKE PLACE. BECAUSE OF THIS TRUISM; THERE ARE FREQUENT MIS-COMS WITH STUDENT PLTS AND STUDENT CTLRS. I BELIEVE THAT HAVING TWR RADIOS THAT SHOULD HAVE BEEN XMITTING PROPERLY AND LESS CONGESTION IN THE DELTA SURFACE AREA COULD HAVE PREVENTED THIS 'INCURSION.'