SIGHTSEEING HELI OPS AT GRAND CANYON ARPT HAVE PROB WITH NO COM AND 1 HELI DEPARTING AS 1 IS LNDG.

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I WAS OPERATING OUT OF GRAND CANYON WEST ARPT IN A BELL 206 LONGRANGER. THIS ACFT IS EQUIPPED WITH DUAL VHF RADIOS. THIS FEATURE ALLOWS THE PLT OF THE ACFT TO MONITOR BOTH GRAND CANYON WEST FREQ 122.9 AND THE CANYON SECTOR FREQ 121.95. I WAS TRYING TO DEPART THE ARPT HELIPAD WHICH IS LOCATED AT THE N END OF THE FIELD. A SECOND OPERATOR ALSO WAS OPERATING 2 HELIS FLYING TOURS TO THE BOTTOM OF THE CANYON FROM THE SAME LOCATION. ONE OF THE HELIS HAD ALREADY DEPARTED AND WAS ASSUMED AT THE BOTTOM OF THE CANYON; THE SECOND HELI WAS LOCATED ON THE HELIPAD E OF MY POS. THIS ACFT WAS LOADED WITH PAX AND WAS PREPARING TO DEPART TO THE BOTTOM OF THE CANYON. I TRIED SEVERAL TIMES TO CONTACT HIM ON BOTH 122.9 AND 121.95 WITH NO RESPONSE ON EITHER RADIO. THIS WAS WITNESSED BY MY DIRECTOR OF OPS; WHO WAS SEATED IN THE COPLT'S SEAT OF MY ACFT MONITORING THE RADIOS AS WELL. AFTER SEVERAL ATTEMPTS TO CONTACT HELI TO ADVISE THEM OF MY DEP AND I DID NOT RECEIVE ANY RESPONSE I ASSUMED THAT HE WAS ON A SEPARATE FREQ TALKING WITH THE HELI WHICH HAD ALREADY DEPARTED. I DECIDED TO WAIT FOR HIS DEP AND TO FOLLOW; RATHER THAN DEPART WITHOUT COM WITH HIM. IMMEDIATELY AFTER HIS DEP I ANNOUNCED ON 122.9 IN THE BLIND THAT I WAS DEPARTING BEHIND HELI TO THE BOTTOM OF THE CANYON. WHAT WAS NOT KNOWN AT THE TIME WAS THAT SECOND HELI WAS RETURNING FROM THE BOTTOM OF THE CANYON. HE WAS ON SHORT FINAL BEHIND ME. AT NO TIME TO THIS DAY DID I SEE THE SECOND ACFT; HOWEVER; AFTER MY DEP I DID HEAR HIM TALKING TO HIS SISTER SHIP STATING THAT I HAD DEPARTED IN FRONT OF HIM. THIS XMISSION WAS HEARD OVER THE SECTOR FREQ 121.95 ON MY #1 VHF RADIO. IT WAS LATER DETERMINED ONCE I RETURNED TO THE HELIPAD AT GRAND CANYON WEST THAT THE #2 VHF RADIO WAS INTERMITTENT. I DISCOVERED THIS WHILE FLIPPING FREQS BTWN RADIOS; AND FINALLY TALKING BTWN SHIPS ON THE GND BEFORE DEPARTING AGAIN. AFTER RETURNING TO THE SOUTH RIM OF THE GRAND CANYON; I RPTED THE INCIDENT TO MY CHIEF PLT AND FILED A FLT IRREGULARITY AND HAZARD RPT WITH HIM. THE RPT IS ON FILE AT COMPANY.