AN SMA PLT HAD AN NMAC WITH ANOTHER SMA NEAR RST. THE RPTR WAS HAVING DIFFICULTY WITH BOTH HIS VOR AND PILOTAGE NAV. ERROR ADMITTED. EVASIVE ACTION (ACFT Y).

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WE STOPPED FOR FUEL AT LACROSSE; WI; AND GOT A WX BRIEFING FOR A FLT FROM LACROSSE TO HARTINGTON; NE; VIA VORS AT MANKATO; REDWOOD FALLS; BROOKINGS (SD); SIOUX FALLS; YANKTON. RTE WAS TO GO AROUND A FRONT WITH LOW CEILINGS AND VISIBILITY AND TSTMS. AFTER LEAVING LACROSSE; I INADVERTENTLY FLEW S OF INTENDED RTE; WHICH WAS THE 270 DEG RADIAL FROM LSE. CORRECTED HEADING TO RETRACK 270 DEG RADIAL AND WAS ON RADIAL WHEN A VERY LARGE ARPT CAME INTO VIEW ABOUT 40 MI AFTER LEAVING LACROSSE. VISIBILITY WAS ABOUT 5 MI AND CEILING WAS ABOUT 3500 FT AGL. I WAS AT 2700 FT MSL; I WAS VERY SURPRISED. THE INTENDED RTE WAS 5 MI N OF ARPT AND 1 MI OUTSIDE OF CLASS D AIRSPACE. ABOUT 30 SECONDS AFTER SEEING THE ARPT AND WONDERING WHAT ITS IDENT WAS; I SAW A CESSNA AT MY 8 O'CLOCK POS (APPROX 200 FT LOWER AND APPROX 400 FT AWAY) MAKING AN EVASIVE MOVE TO THEIR R. I DID NOT REACT BECAUSE I WAS PAST THEIR DIRECTION OF FLT. I WAS USING 2 VOR RECEIVERS AND 2 NAV-COM RADIOS. BOTH WERE CHKED ON MAR/XX/95; BY AN FBO AND RADIO #2 WAS ADJUSTED 5 DEGS; RADIO #1 WAS OKAY. I WAS USING RADIO #1 AS THE PRIMARY GUIDE ON AUG/XX/96; RADIO #2 SEEMED TO BE A LITTLE OFF. AT THE TIME OF THE INCIDENT; I WAS 5 MI S OF INTENDED COURSE AND ABOUT 42 NM FROM THE LACROSSE VOR. I WAS ABOUT 65 NM FROM THE MANKATO VOR AND SO WAS RELYING ON THE LACROSSE VOR. I HAD NOT SET THE MANKATO VOR ON THE RADIO. ONE CONTRIBUTING FACTOR TO THE INCIDENT WAS THE LOW ACCURACY OF THE NAV-COM RADIO WHICH SHOWED I WAS ON THE 270 DEG RADIAL. I WAS USING A CURRENT SECTIONAL MAP FOR PILOTAGE; BUT THE REDUCED VISIBILITY REDUCED ITS USEFULNESS. I KNEW I SHOULD BE NEAR THE LARGE CITY OF ROCHESTER; MN; AND A MAJOR HWY; BUT COULD NOT SEE THEM; SO MY CONFIDENCE IN THE VOR AND NAV-COM RADIO WAS NOT HIGH. I WAS TRACKING IN A SLIGHTLY N HEADING TO TRY TO LOCATE THE MAJOR HWY BECAUSE I DID NOT CROSS IT PREVIOUSLY. I HAD BEEN FLYING ABOUT 25 MINS WHEN THE INCIDENT OCCURRED. EARLIER IN THE DAY I HAD DEPARTED OSHKOSH; WI; AND FLEW TO LACROSSE BY PILOTAGE AND VOR RADIAL WITHOUT PROBS. THE REMAINDER OF THE FLT FROM ROCHESTER; MN; TO VORS AT MANKATO; REDWOOD FALLS; SIOUX FALLS; AND YANKTON WERE ON COURSE.