ACFT EQUIPMENT PROBLEM RENDERED ALL ACFT RADIOS INOPERATIVE. EMERGENCY DECLARED.

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THE F/O WAS FLYING THE ACFT ON TKOF FROM RWY 30 AT IAD (DULLES). AFTER WE WERE AIRBORNE BOTH THE F/O AND I HEARD SOME CRACKLING IN THE RADIO BUT WEREN'T CONCERNED ABOUT IT AT THE TIME. WE WERE MAINTAINING OUR LAST ASSIGNED CLRNC WHICH WAS MAINTAIN RWY HEADING AND 4000'. THE TWR NEVER WAS HEARD TO TELL US TO SWITCH TO DEP FREQ SO I INITIATED A CALL TO THEM WITH NO RESULTS. I SWITCHED TO THE DEP FREQ AND TRIED THEM WITH STILL NO CONTACT. I THEN TRIED THE DEP FREQ ON OUR OTHER (#2) VHF COMM RADIO WITH STILL NO RESULTS. I THEN ASKED THE F/O TO USE HER MICROPHONE AND HER JACKBOX TO ATTEMPT CONTACT WHICH STILL PROVED UNSUCCESSFUL. AT THIS TIME WE SET THE TRANSPONDER TO 7700 THEN 7500 AND I ATTEMPTED CONTACT WITH ANYBODY ON 121.5 STILL WITH NO RESULTS. I WAS NOW FACED WITH REVIEWING MY OPTIONS AND SINCE MY DEST (MEMPHIS) WEATHER AND ENROUTE WEATHER WERE NOT VFR; I HAD DECIDED TO EXERCISE MY EMER AUTHORITY AND RETURN TO DULLES (SQUAWKING 7700) AND MAKE A NO RADIO APCH HOPING THAT THEY WOULD MOVE TFC OUT OF MY WAY. AT THIS TIME THE FLT ATTENDANT RANG THE COCKPIT WITH THE CALL SYSTEM. WHEN I PUSHED THE JACKBOX INTERPHONE BUTTON IN TO TELL THEM TO STANDBY; I FOUND THAT OUR COMMUNICATIONS RADIOS STARTED WORKING NORMALLY FROM THE F/O'S STATION AND THE OBSERVERS (JUMPSEAT) STATION AS LONG AS MY INTERPHONE XMIT BUTTON REMAINED PUSHED IN. SINCE WE NOW HAD COMMUNICATIONS WITH BOTH RADIO'S WE TERMINATED THE EMER WITH ATC AND CONTINUED UNEVENTFULLY TO OUR DEST. IN SUBSEQUENT TALKS WITH ATC AND FAA PERSONNEL I FOUND THAT ALL MAJOR CTL TWR AND APCH CTLS (THOSE HAVING ARTS III EQUIPMENT) HAVE AN ALARM THAT SOUNDS WHEN TRANSPONDER CODE 7700 IS RECEIVED AND THAT RETURNING IN IFR CONDITIONS WITH NO RADIO TO THE DEST ARPT WOULD NOT BE COMPLETELY DISRUPTIVE TO THE ATC SYSTEM AND IN SOME CASES MIGHT BE THE MOST APPROPRIATE COURSE OF ACTION. I BELIEVE THIS INFORMATION SHOULD BE ADDED TO FAR 91.127 AS A POSSIBLE ALTERNATIVE TO EXTENDED OPERATION NO RADIO IFR THRU MANY ATC SECTORS AND CENTERS AND BEING FACED WITH MAKING A NO RADIO APCH AT MANY OF THE AIRLINE HUB ARPTS.