PLT OF A SMA SEL LOST COM AND SQUAWKED 7700 CONTINUOUSLY BY MISTAKE.

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ENRTE TO LANSING; MI; FROM ALMA; MI; I ENCOUNTERED RADIO PROBS WITH MY SHIP 2-WAY COM UNIT. I HAD A BACK-UP HAND HELD TRANSCEIVER WHICH I HAD TESTED ON PAST FLTS AND KNEW WAS WORKING. I FELT I WAS COMING CLOSE TO THE 20 MI RADIUS-LANSING AIRSPACE AND WENT TO 2500 FT IN AN EFFORT TO MAKE RADIO CONTACT. I COULD HEAR TFC ON MY PORTABLE UNIT BUT APPARENTLY WAS UNABLE TO RAISE LANSING. AT THIS POINT I FELT I WAS WITHIN THE 20 MI RADIUS OF THE LANSING AIRSPACE AND THUS DECIDED TO SQUAWK 7600. I SQUAWKED 7700 FOR 1 MIN -- AND NEGLECTED TO SWITCH TO 7600. I COULD HEAR RADIO TFC ON MY TRANSCEIVER; BUT APPARENTLY CONTINUED TO BE UNABLE TO RAISE LANSING APCH. AS I APCHED THE 5 MI RADIUS OF LANSING; I DECIDED TO DROP TO 1500 FT MSL AND DIVERT TO DAVIS ARPT; E LANSING. AT ABOUT THAT TIME; I WAS ABLE TO COM WITH LANSING AND I WAS CLRED TO LAND ON 10 R. I WENT BACK UP TO PATTERN ALT AND MADE A NORMAL APCH TO LNDG. AT THAT TIME I ADVISED LANSING THAT I HAD EXPERIENCED A RADIO FAILURE AND WAS COMMUNICATING VIA TRANSCEIVER. ON THE GND; I DISCOVERED THAT I HAD BEEN SQUAWKING 7700.