A SAAB 340B RPTED TCASII TARGETS DROPPING OUT IN BUSY TERMINAL AREAS.

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IN SUM: ENRTE TO PBI FROM MIA; MIA APCH HAD INFORMED US OF A TARGET AT 12 O'CLOCK AT 3000 FT. MIA THEN TURNED US OVER TO PBI. PBI RECONFIRMED THE TARGET. WAS ISSUED A DSCNT TO 3500 FT AND THE TARGET WAS ON TCASII. SHORTLY AFTER THE CLRNC WAS ISSUED THE TARGET DISAPPEARED FROM THE TCASII. A TCASII RA SOUNDED. IT ISSUED A CLB COMMAND AND I BEGAN COMPLYING. I CAUGHT A GLIMPSE OF THE INTRUDER BEFORE THE NOSE BLOCKED MY VIEW. IT APPEARED TO BE A BIPLANE INVERTED OR PERFORMING A ROLL AT THE TOP OF A LOOP OR OTHER VERT MANEUVER. PBI WAS ADVISED WE WERE COMPLYING WITH A RA ADVISORY. WE RETURNED TO OUR ALT ONCE WE WERE CLR OF CONFLICT. APCH MADE NO MENTION AS TO WHETHER THEY HAD CONTACT WITH THE AEROBATIC TFC. WE WERE GIVEN A FURTHER DSCNT SHORTLY THEREAFTER. THE ABOVE MANEUVER DID NOT REQUIRE ABRUPT INPUTS. FLT ATTENDANTS RPTED NO PROBS IN THE CABIN. THE ANTENNA POS RELATIVE TO EACH ACFT IS STILL A PROB WITH KEEPING TCASII TARGETS ON THE SCREEN CONSTANTLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: WHEN OBSERVING A TARGET FOR A PERIOD OF TIME WE THEN HAD THE TARGET DROP OFF. IT MAY BE THE POS OF THE ANTENNAS INSTALLED ON THE AIRPLANE. WHEN ASKED IF THE AIRPLANE WAS EQUIPPED WITH A DIGITAL OR ANALOG AIR DATA COMPUTER THE RPTR SAID HE COULD NOT BE CERTAIN OF THE TYPE. THE REASON FOR THE QUESTION BEING THE ANALOG OUTPUT TO THE MODE 'S' XPONDER IS GETTING HUNG UP LOOKING AT THE ANALOG ALT INPUT EVERY TIME IT RECEIVES AN INTERROGATION. IN A HIGH DENSITY AREA THIS DELAY CAUSES DROP-OUTS.