C182 PILOT REPORTS RADIO MALFUNCTION THAT PREVENTS HEARING CALLS AND CAUSES NMAC WITH LEAR 45 AT CTAF ARPT.

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BEFORE TKOF; I PERFORMED A RADIO CHK ON THE CTAF. I RECEIVED NO RESPONSE; BUT BEING IT WAS EARLY IN THE MORNING ; I THOUGHT UNICOM IS NOT AVAILABLE AND NO ONE ELSE WAS FLYING THIS EARLY. IT WAS MY FIRST FLT AT THE ARPT; I WAS COMPLETING THE FLT PORTION OF AN INTERVIEW WITH A SKYDIVING OP ON THE FIELD. THE PAX AND I COULD HEAR ONE ANOTHER OVER THE INTERCOM LOUD AND CLR. THE MAJORITY OF THE FLT WAS UNEVENTFUL; UNTIL LNDG. I STARTED MY APCH HIGH; A MISTAKE I'M GLAD I MADE. I CONDUCTED RADIO CALLS THROUGHOUT THE APCH TO LNDG. UPON TURNING FINAL (200 FT HIGHER THAN NORMAL); I SAW A LEARJET 45 PASS BENEATH US. I IMMEDIATELY PERFORMED A GAR AND SET UP ANOTHER APCH AND LNDG THAT WAS UNEVENTFUL. I BELIEVE THE CAUSE WAS LACK OF COM. WHILE CONFIGURING THE RADIO SO MY PAX AND I COULD COMMUNICATE; WE UNKNOWINGLY PREVENTED OURSELVES FROM USING THE CTAF FREQS. AFTER THE INCIDENT; WE CLEANED THE CONTACTS AND RECONFIGURED THE RADIOS WHICH WORKED FOR THE REST OF THE DAY. TO PREVENT THIS FROM HAPPENING AGAIN; I WILL MAKE RADIO CHKS BEFORE THE FIRST FLT OF THE DAY TO A HANDHELD TRANSCEIVER AND A SECOND PERSON TO CONFIRM THE RADIO CHK.