SMA HAS AN NMAC WITH A BEECH DUCHESS WHILE DSNDING TOWARD P10 ARPT. NO TIME FOR EVASIVE ACTION.

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ON DSCNT TO THE CHANDLER ARPT I CONTACTED PHX APCH CTL ON 123.7 AT 6500 FT. I WAS GIVEN A SQUAWK AND TOLD I WAS OUT OF RADAR CONTACT. AT XB00Z WITH A GPS READING OF 286/20 FROM THE CHANDLER ARPT; I SUDDENLY OBSERVED A DUCHESS APCHING FROM MY L SIDE BELOW ME. THE DUCHESS PASSED DIRECTLY UNDER MY ACFT. I OBSERVED IT GOING AWAY TO THE R AND THEN TURNING TO THE L ABOUT 2-3 MI FROM MY POS. THE DUCHESS WAS 500 FT OR LESS FROM MY ACFT WHEN IT WENT UNDER ME. I NOTIFIED APCH CTL. THEY NOW HAD ME IN RADAR CONTACT. THE CTLR SAID THE DUCHESS'S CALL SIGN WAS DUCHESS XXX. I CALLED THE TRACON SUPVR AND TOLD HIM OF THE INCIDENT. I DO NOT FEEL THE CTLR OR THE DUCHESS PLT OR MYSELF DID ANYTHING WRONG. THERE DOES APPEAR TO BE POOR RADAR COVERAGE IN THE AREA. THIS NEAR MISS OCCURRED ON THE 115 DEG RADIAL AT 38.5 NM FROM THE PHX VOR. THE 'BIG SKY' THEORY BEGINS TO FAIL WHEN A LOT OF AIRPLANES CONVERGE ON 18 ARPTS WITHIN A 40 MI RADIUS. THE CLOSER IN YOU GET; THE TIGHTER THE 'BIG SKY' BECOMES. P.S. I NEVER SAW THE DUCHESS UNTIL IT WAS TOO LATE TO TAKE EVASIVE ACTION.