HELICOPTER PILOT REPORTS NMAC AT 700 FEET AGL WITH DA40 FLYING STRAIGHT IN VISUAL TO ZZZ.

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WHILE FLYING NW AT APPROX 700 FT AGL THE DA40 CROSSED OUR PATH; L TO R; APPROX 30 FT AHEAD; AND 5 FT BELOW THE MAIN ROTOR OF THE HELI. I BELIEVE THE WINGS OF THE ACFT MAY HAVE EVEN PASSED BELOW THE MAIN ROTOR. THE DA40 WAS IN A BLIND SPOT ON THE HELI (BEHIND THE L-HAND DOOR FRAME) AND COULD NOT BE SEEN UNTIL IT PASSED. I DIDN'T HAVE TIME TO TAKE ANY EVASIVE ACTION. THE DA40 PASSED TOO QUICKLY. THERE WERE SEVERAL FACTORS WHICH CONTRIBUTED TO THIS NEAR MISS. 1) WE WERE XING THE FINAL APCH PATH FOR RWY 4 AT ZZZ HOWEVER WE WERE 4 MI AWAY AND BELOW TFC PATTERN ALT (TPA). ALTHOUGH WE WERE SCANNING FOR TFC; WE COULD NOT SEE THE DA40 AS IT WAS IN A BLIND SPOT. 2) THE DA40 WAS NOT TALKING ON THE CORRECT FREQ FOR ZZZ. AFTER CONFRONTING THE OTHER PLT; I DISCOVERED HE WAS MAKING RADIO CALLS ON THE ARPT'S OLD CTAF FREQ. (CURRENTLY IT IS UP-TO-DATE ON CHARTS AND THE ARPT FACILITIES DIRECTORY.) 3) THE DA40 WAS NOT ABOVE TPA PRIOR TO ENTRY AS THE AIM SUGGESTS. FURTHER; THE DA40 DID NOT FLY A TFC PATTERN; HE LANDED STRAIGHT IN. 4) UPON CONFRONTING THE DA40 PLT; HE CLRLY DEMONSTRATED 2 OF THE HAZARDOUS ATTITUDES LISTED IN MULTIPLE FAA PUBS: MACHO AND INVULNERABILITY. I BELIEVE THE ONLY WAY TO PREVENT THIS TYPE OF INCIDENT FROM HAPPENING AGAIN WOULD BE FOR THE DA40 PLT TO RECEIVE REMEDIAL TRAINING ON ARPT TFC PATTERN OPS; AND THE IMPORTANCE OF USING AN UP-TO-DATE CHART AND ARPT FACILITIES DIRECTORY FOR PREFLT PLANNING.