C172 PILOT REPORTS SEEING HELICOPTER DURING SHORT FLIGHT BENEATH SLC CLASS B; AND BEING TOLD AFTER LANDING THE HELICOPTER PILOT WILL FILE A NEAR MISS REPORT.

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I HAD LANDED AT 42U AND DECIDED TO TAKE OFF AND SCOUT OUT WX TO SEE IF I COULD DO MY AERIAL PHOTO MISSION. I TOOK OFF AND REMAINED CLR OF THE SLC CLASS B AIRSPACE. CONTINUING TO REMAIN BELOW IT; I FLEW TO THE ESE OF SLC AND NOTICED A HELI PASS BELOW AND IN FRONT OF ME; 400 FT BELOW ME; ABOUT 400 FT IN FRONT OF ME; BUT THOUGHT NOTHING OF IT BECAUSE IT WAS 100% NO FACTOR. SHORTLY AFTER I NOTICED I'D NEGLECTED TO TURN MY XPONDER TO 'ALT' AND SWITCHED IT FROM STANDBY TO ALT AND SOON AFTER DECIDED TO RETURN TO 42U. I LANDED AND WAS ASKED TO CALL SLC TRACON; WHICH I DID. I WAS TOLD THE HELI PLT WAS PLANNING ON FILING A NEAR MISS RPT; WHICH IS SURPRISING SINCE THERE WAS NEVER ANY DANGER FOR EITHER OF US. THIS WAS A NON ISSUE OTHER THAN MY FAILING TO HAVE MY XPONDER ON 'ALT.' THERE WAS NEVER AN 'OH *&^%!' MOMENT AND HAVING TO TURN AWAY QUICKLY -- AGAIN -- I JUST THOUGHT 'THERE'S A HELI' AND KEPT GOING SINCE THE TFC WAS NO FACTOR AT ALL. FROM NOW ON; I WILL LEAVE MY XPONDER ON 'ALT' AND DOUBLE-/TRIPLE-CHK ITS POS.