TCA PENETRATION WITHOUT CLRNC.

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WHILE CLBING TO 4500' (AS PER MY FILED FLT PLAN). I CONTACTED SEA DEP ON FREQ 120.4 TO ASK FOR FLT FOLLOWING TO ILLWACO ARPT; AT JUST BELOW 3000' MSL. AT 3000'; CONTACT WAS ESTABLISHED. PLT (ME): 'REQUEST FLT FOLLOWING TO ILLWACO. I'M AN SMA AT 3000' CLBING TO 4.5; 10 MI E OF BOEING FIELD.' DEP: 'SMA STANDBY FOR SQUAWK CODE.' (A FEW SECONDS LATER): 'SMA RADAR CONTACT; DSND IMMEDIATELY TO 3000'; YOU ARE IN THE TCA; I CAN'T CLR YOU FOR THAT ALT W/O A MODE C TRANSPONDER.' PLT: 'CAN YOU PICK UP MY ALT NOW?' (SWITCHING ON THE MODE C). DEP: 'SMA OK; I HAVE YOU NOW; PROCEED TO 4.5.' CONTRIBUTING FACTORS TO TCA VIOLATION: SMA IS A RENTAL AIRPLANE (FBO; BOEING FIELD) EQUIPPED RECENTLY WITH THE REQUIRED ALT ENCODER TRANSPONDER. AS I ENTERED THE RWY (CLRED FOR TKOF) I SWITCHED THE TRANSPONDER FROM 'STANDBY' TO 'ON;' BUT NEGLECTED TO CONTINUE TURNING KNOB TO THE 'ALT' POS. ALTHOUGH THE ATC HAD ME IN CONTACT ON RADAR; THEY COULD ONLY ASSUME I HAD NO MODE C; SINCE IT WAS NOT ACTIVATED. CORRECTIVE ACTION: ASSURE TRANSPONDER IS IN THE 'ALT' POS; AND BE SURE I AM 'CLRED INTO TCA' BEFORE ENTERING.