C210 PLT ON AN IFR FLT PLAN ENCOUNTERED AIRFRAME ICING AND DEGRADED COM AT 16000 FT. PLT DSNDED TO AN ALT BTWN LAYERS; REESTABLISHED COM AND CONTINUED FLT WITHOUT FURTHER INCIDENT.

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I WAS LEVEL AT 16000 FT IN CLOUD WITH NO ICE ACCUMULATION; OUTSIDE AIR TEMP WAS -30 DEGS C. CTR HAD JUST TOLD ME THAT TOPS OF THE LAYER BELOW HAD BEEN RPTED AT 12500 FT. WITH ATC'S PERMISSION I LEFT CTR FREQ TO GET UPDATED WX FOR STATIONS ALONG MY RTE. I HAD MY HEAD DOWN COPYING WX AND HAND-FLYING THE AIRPLANE FOR APPROX 2 MINS WHILE TALKING WITH FLT WATCH. WHEN I LOOKED UP TO CHANGE FREQ BACK TO CTR; I NOTICED THAT THE WINDSCREEN HAD BECOME OPAQUE WITH ICE AND THAT ABOUT 1/4 INCH HAD ACCUMULATED ON THE LEADING EDGE OF THE GEAR MIRROR -- A TELLTALE OF THE WING LEADING EDGE. UPON RETURNING TO CTR FREQ; I COULD HEAR ONLY STATIC AND AN OCCASIONAL SPOKEN WORD; WHEN SYLLABLES BECAME AUDIBLE; I COULD NOT TELL WHETHER CTR OR AN ACFT WAS XMITTING. THE SAME ISSUE AFFECTED MY SECOND COM RADIO. AFTER LISTENING FOR ABOUT 10 SECONDS; I DECIDED THAT EFFECTIVE 2-WAY COM WITH ATC WOULD NOT BE POSSIBLE GIVEN CURRENT CONDITIONS; WHICH I ASSUMED WAS DUE TO ICE ON THE COM ANTENNAS. AFTER BRIEFLY CONSIDERING THE ALTERNATIVES; I CHOSE TO DSND TO SEEK AN ALT BTWN THE LAYERS. I BROADCAST IN THE BLIND TO ATC THAT I HAD RETURNED TO THE FREQ AND HAD LEFT 16000 FT FOR LOWER. (I MIGHT HAVE SAID; 'DUE TO ICE;' BUT MY RECOLLECTION IS VAGUE ON THAT POINT.) AT THE SAME TIME; I LOOKED AT THE ENRTE CHART TO FIND LCL TERRAIN ELEVATION; NEARBY AIRWAYS AND MEA'S. (I WAS OFF-AIRWAY; DIRECT AT THE TIME.) DURING THE DSCNT; I COULD MAKE OUT ATC ASSIGNING A FREQ CHANGE TO ME; HOWEVER ONCE ON THE NEW FREQ; I HEARD NOTHING BUT STATIC. I RETURNED TO THE ORIGINAL FREQ AND; AFTER A SHORT TIME; WAS ABLE TO REESTABLISH COM. I LEVELED OFF BTWN 2 CLOUD LAYERS; AND THE ICE THAT HAD ACCUMULATED SLOWLY DISSIPATED. (THE OUTSIDE AIR TEMP WAS NOW -9 DEGS C.) THE REMAINDER OF THE FLT WAS NORMAL. I CONTINUED WITHOUT INCIDENT TO MY DEST. I'VE REFLECTED ON THIS EVENT FROM A COUPLE OF PERSPECTIVES. FIRST; I DID NOT EXPECT TO ACCUMULATE ICE AT SUCH A COLD AMBIENT TEMP. I HAD READ AND BEEN TAUGHT THAT; IN STABLE AIR; AIRFRAME ICING STOPPED AT TEMPS BTWN -15 DEGS TO -20 DEGS C; WHERE THE AIR WAS TOO COLD TO HOLD SUPER-COOLED WATER. HOW TO PREVENT A RECURRENCE? FIRST; DISCARD THE IDEA THAT ICE WON'T FORM BELOW -20 DEGS C. ALSO; ONCE DETERMINING THAT UNANTICIPATED FLT IN CLOUD WILL BE REQUIRED ABOVE THE FREEZING LEVEL; BE QUICKER TO TALK WITH FLT WATCH ABOUT WX AND PIREPS. FINALLY; WORK WITH MY AVIONICS SHOP TO ENSURE THAT I HAVE AT LEAST ONE SOLID WORKING RADIO IN CASE OF ANTENNA ICING.