PVT PLT EXPERIENCES A LOSS OF COM FREQ IN A C172 IN THE TFC PATTERN AT SMF; CA.

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WAS PRACTICING TKOF'S AND LNDGS AT TWRED FIELD; AT NIGHT; AT SMF. SMF TWR HAD ME TURN OUT; AROUND; THEN CLRED FOR LNDG RWY 16L. THERE WAS SOME STATIC NOISE NOW ON COM RADIO. BROKEN XMISSION FROM TWR CAME THROUGH; SAYING MY XPONDER WAS NOW NON INTERROGATING. I CONTINUED ON DOWNWIND; THEN BASE TURN TO FINAL. (DURING THIS TIME; THERE MAY HAVE BEEN MORE XMISSIONS I DID NO HEAR). TURNING ON FINAL; THERE WAS MORE BROKEN XMISSIONS AND I COULD NOT CLRLY HEAR MYSELF XMITTING BACK. BROKE OFF APCH; WENT E AT 1000 FT; HEARD TWR THEN SAY 'C172; IF YOU CAN HEAR THIS XMISSION; LEAVE CLASS C TO THE E.' WHICH; IS WHAT I WAS DOING. AT THIS TIME; BECAME CLR THAT COM HAD SUDDENLY FAILED. I LEFT SMF C AIRSPACE; PWRED ALL ITEMS DOWN. WE WERE EXPERIENCING A CHARGING SYS PROB; WITH CONTINUOUS DISCHARGE ON AMMETER. TURNED BACK ON #1 COM AND COULD GET 2-WAY COM WITH SAC APCH. TURNED ON #2 COM; TOO MUCH DRAIN; NO COM AT ALL. LEFT #1 COM ON FOR HASTY RETREAT TO SUTTER COUNTY ARPT. IN FUTURE; WILL FLY WITH BATTERY-PWRED TRANSCEIVER AND GPS. I'M SUSPICIOUS THAT; IN THE FEW MINS THAT THIS OCCURRED; WE MAY NOT HAVE HEARD SOME OTHER XMISSIONS TO ME; FROM SMF TWR. THEY COULDN'T HEAR MINE.