R22 PILOT REPORTS NMAC WITH C152 AT 1000 FEET WHILE TRANSITING THE 7S3 TRAFFIC PATTERN.

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AFTER MANEUVERING FOR 5-10 MINS AT AROUND 1500-2000 FT I TOOK THE CTLS BACK; TURNED N DIRECT TO HILLSBORO AND DSNDED. I WAS ON THE TWIN OAKS CTAF THE WHOLE TIME AND HAD MADE 2 POS RPTS; BUT HADN'T SEEN OR HEARD ANYONE IN THE TWIN OAKS PATTERN. I MADE ONE LAST POS RPT 'HELI XXXX IS 2 NM W OF TWIN OAKS AT 1000 FT; NBOUND TO HILLSBORO' ON THE TWIN OAKS CTAF FREQ AND; AFTER HEARING NO REPLY; LOOKED DOWN TO SWITCH THE RADIO TO HIO TWR. I LOOKED BACK UP; AND SEVERAL CESSNAS AT OUR 10-11 O'CLOCK POS AT VARIOUS ALTS WITHIN ABOUT 2 MI OF US (N AND SBOUND) CAUGHT MY ATTN. I DETERMINED THAT NONE OF THEM WOULD BE A FACTOR FOR US; LOOKED AHEAD; THEN I SAW A CESSNA (PROBABLY A C152; WHITE WITH BLUE TAIL) HDG STRAIGHT FOR US AT ALMOST THE SAME ALT (MAYBE 20-50 FT LOWER THAN WE WERE); 12-1 O'CLOCK POS. I IMMEDIATELY MADE A R TURN AND SAW THE CESSNA DO THE SAME. WE PASSED EACH OTHER WITHIN MAYBE 150 FT; THE CESSNA STILL A BIT LOWER THAN WE WERE. I SWITCHED BACK TO TWIN OAKS CTAF AND ASKED IF 'THE CESSNA' WAS ON FREQ. THE CESSNA PLT CONFIRMED 'THAT WAS CLOSE!' WE WERE BOTH SURPRISED THAT WE HADN'T SEEN EACH OTHER. I SWITCHED BACK TO HIO TWR AS WE WERE APCHING THE D AIRSPACE AND LANDED WITHOUT FURTHER INCIDENT. WHAT COULD BE DONE DIFFERENTLY? THE OTHER PLT WAS APPARENTLY ON THE DOWNWIND IN A L TFC PATTERN FOR RWY 2 IN TWIN OAKS. I HAD UNDERESTIMATED THE SIZE OF A FIXED WING TFC PATTERN; AND WAS FLYING TOO CLOSE TO THE RWY WHILE DSNDING THROUGH THEIR TFC PATTERN ALT. THE AIRSPACE S OF HILLSBORO WAS VERY BUSY AT THAT TIME WITH NUMEROUS FIXED WING ACFT FLYING AT ALL ALTS BTWN HIO AND THE NEWBERG VOR; AND RWY 2 IN USE IN HIO. I WAS FLYING AT MORE THAN 1000 FT AGL. BECAUSE OF THE FIXED WING TFC; I SHOULD HAVE PROBABLY STAYED AT A MUCH LOWER ALT INSTEAD. ALSO; I LET MYSELF BECOME DISTR BY THE TFC ON MY FRONT/L SIDE AND DIDN'T SCAN STRAIGHT AHEAD FOR A MOMENT. SINCE THE OTHER ACFT WAS APCHING ALMOST HEAD ON; THERE WAS ALMOST NO RELATIVE MOTION; CAUSING ME TO SEE THE CESSNA ONLY AT THE LAST MOMENT. WITHOUT THE EVASIVE MANEUVERS; THIS COULD HAVE BEEN A MIDAIR COLLISION.