INADVERTENT USE OF WRONG CTAF AT O17 RESULTS IN SIMULTANEOUS OPPOSITE DIRECTION TKOFS.

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APCHING NEVADA CITY; WE NOTICED NO RADIO CALLS ON THE CTAF AND DIDN'T EXPECT ANY; AS THERE WAS NO VISIBLE TFC IN THE AREA. WE LANDED ON RWY 7; AS WINDS WERE FAVORABLE. AFTER A SHORT BREAK; WE RETURNED TO THE PLANE; WATCHED 3 FIRE SVC ACFT TAKE OFF ON RWY 25 AND A DIAMOND MOTORIZED GLIDER LANDED USING THE OPPOSITE DIRECTION (RWY 7). THEN AFTER STARTUP AND AVIONICS ON; WE HEARD NO FURTHER RADIO CALLS; AND DIDN'T EXPECT ANY AS WE COULD SEE NO PLANES IN THE PATTERN OR OTHERS MOVING ON THE GND. WE DECIDED TO TAKE OFF IN THE DIRECTION FAVORED BY THE WIND; RWY 7. NEVADA CITY HAS A SLOPING AND CONVEX RWY; AND FROM THE HOLD SHORT LINE; DIDN'T HAVE A VIEW OF THE OPPOSITE END. WE MADE OUR CALL; AND BELIEVED WE WERE STILL THE ONLY ACFT ACTIVE ON THE CTAF. WE ABORTED THE TKOF ROLL AFTER NOTICING THE DIAMOND IN THE SAME STAGE OF TKOF COMING OVER THE RISE IN THE RWY. WE TURNED OFF THE CTRLINE TO THE R AS FAR AS POSSIBLE; AND THE DIAMOND WAS ABLE TO CLB WELL OVER OUR POS. ONCE IN THE AIR; THE DIAMOND OFFSET HIS TKOF TO THE R. WE EXITED THE RWY; TRIED TO CONTACT THE DIAMOND ON CTAF; BUT HEARD ONLY INTERMITTENT STATIC. WE MISTAKENLY CONCLUDED THAT THE DIAMOND WAS EITHER WITHOUT RADIO OR IN ERROR. AFTER A SUCCESSFUL TKOF ON RWY 7; WE REALIZED THAT IT WAS OUR RADIO THAT WAS IN ERROR; ONE DIGIT DROPPED (CORRECT IS 122.725; WE HAD 122.75). UNFORTUNATELY; DURING THE TIME WHEN OUR RADIO WAS ON (LNDG AND TAXI); THERE WAS NO REASON TO EXPECT CTAF RADIO CALLS; BECAUSE WE DIDN'T SEE ANY ACFT AND; THEREFORE; DIDN'T CATCH OUR MISTAKE. FURTHER; THE BEND IN THE SURFACE OF THE ARPT KEPT US FROM SEEING TAXI ACTIVITY AT THE OPPOSITE END.