C421 TAXIING TO THE RAMP AT MYF ENTERED RWY WITHOUT ATC APPROVAL.

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I MADE AN ILS APCH TO RWY 28R AT MYF. AFTER LNDG I WAS INSTRUCTED BY THE TWR TO CROSS RWY 28L AND THEN CONTACT GND CTL. I THOUGHT I HAD TAXIED OFF RWY 28R ONTO TXWY G BUT INSTEAD I HAD TAXIED ONTO RWY 23. I DID NOT NOTICE THE HOLD LINE MARKINGS FOR RWY 28L AND I MADE A L TURN ONTO RWY 28L. WHEN I TURNED ONTO WHAT I THOUGHT WAS TXWY H; I SWITCHED MY RADIO TO 118.2; TOLD GND CTLR I WAS CLR OF RWY 28R AND WANTED TO TAXI TO FBO. WHEN I DID NOT RECEIVE A REPLY; I CHKED THE FREQ AND DISCOVERED I HAD SET IN THE WRONG FREQ. I SET IN THE CORRECT FREQ AND MADE CONTACT WITH GND CTL; WHO INFORMED ME THAT I HAD TURNED ONTO RWY 28L. I EXITED RWY 28L ON TXWY D. MY UNFAMILIARITY WITH THE ARPT; MY INATTN TO MY LOCATION ON THE AIRFIELD; NEGLECTING TO ASK FOR TAXI INSTRUCTIONS FROM THE TWR; AND MIS-SETTING THE PROPER RADIO FREQ CAUSED ME TO ACCIDENTALLY TURN ONTO AN ACTIVE RWY. I HAD THE ARPT DIAGRAM WITH ME IN THE COCKPIT AND I SHOULD HAVE REFERRED TO IT BEFORE LNDG. OTHER FACTORS THAT MAY HAVE CONTRIBUTED TO THE INCIDENT WERE THE LOW SUN ANGLE THAT OBSCURED MY VISION; AND FATIGUE DUE TO THE LATE AFTERNOON ARR. I HAD RECENTLY TAKEN AND PASSED A RWY INCURSION TEST ONLINE; AND I FEEL I AM UP-TO-DATE ON AIRFIELD MARKINGS AND SIGNS; BUT I WAS IN A HURRY TO EXIT THE BUSY RWY AND FAILED TO FOLLOW THE PROPER TAXI PATH.