A PA34-200 STUDENT RPTED LOSS OF COM WHEN 6 NM NW OF LHM.

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MY INSTRUCTOR AND I WERE ON AN IFR FLT PLAN FROM KEMT TO KLHM. I WAS IN IFR TRAINING AND MULTI-ENGINE TRAINING. AFTER COMPLETING MOST OF THE JOURNEY; AT LEAST UNTIL THE HDOF TO SIERRA APCH; ALL RADIOS; XPONDERS; AND ALTIMETERS WERE FUNCTIONING PROPERLY. WHILE BEING VECTORED BY SIERRA APCH; WE WERE HEADING ON A BEARING THAT; IN A FEW MORE MINS; WOULD HAVE LED US INTO THE WESTERN SIDE OF THE SIERRA NEVADAS. MY INSTRUCTOR ADVISED ME TO CONTACT SIERRA APCH TO ENSURE CONTINUANCE ON LAST ASSIGNED HDG. I DID; AND THERE WAS NO REPLY. I HEARD OTHER ACFT TALKING; SO I TRIED AGAIN. NO ANSWER. MY INSTRUCTOR TRIED OUR OTHER RADIO. NO ANSWER. HE TRIED HIS HANDHELD. NO RESPONSE. ALL XMISSIONS WERE HEARD VIA ONBOARD RADIOS. STILL NO ANSWER. WE INPUT '7600' AS A SQUAWK CODE AND LOOKED AT OUR VFR CHARTS FOR TERRAIN CONSIDERATIONS. TERRAIN WAS APCHING. WE TURNED AROUND AND CLBED TO CLR IMC. AT THIS TIME; SQUAWK CODE MUST HAVE BEEN RECOGNIZED; AS WE WERE BROADCASTING IN THE BLIND OUR INTENTIONS. ANOTHER ACFT IN THE AIR WAS RELAYING MESSAGES TO US FROM SIERRA APCH. WE WERE INSTRUCTED IMMEDIATELY TO PULL UP. WE DID. THEN WE WERE VECTORED IN VIA RELAY PLANE; TO LAND; USING THE ILS APCHS AT LHM. ANOTHER TIME WE WERE RELAYED TO CLB. THEY STATED WE WERE AT 800 FT. HOWEVER; BOTH OUR ALTIMETERS WERE SET TO THE PROPER ALTIMETER SETTING AND WERE SHOWING 2200 FT DSNDING TO 1800 FT TO THE NEXT STEP DOWN. ALL APCH/LNDG CONDITIONS WERE PRESET AND WE LANDED. IMMEDIATELY; SOMEONE CAME OUT OF THE HANGAR TO GIVE US A NUMBER TO CALL SIERRA APCH. MY INSTRUCTOR CALLED AND SPOKE TO A CTLR. ALL WAS WELL. WE THEN DEPARTED VFR BACK TO KEMT. OUR ALTIMETER; WITH THE PROPER SETTING; WAS WITHIN 75 FT OF FIELD ELEVATION. WHILE ARRIVING IN THE CLASS DELTA AIRSPACE OF KEMT; WE WERE IDENTED VIA OUR MODE C BY ATC. ALL COMS WERE FINE. THE NEXT BUSINESS DAY AFTER LNDG; THE RENTAL ACFT COMS WERE CHKED. NO PROBS WERE FOUND. THIS LOSS OF COM WAS A COMPLETE MYSTERY.