SMA PLT SKIRTING TCA HAS XPONDER FAIL WITHIN 30 MI VEIL.

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I LIVE APPROX 70 MI S OF STL. 2 EVENINGS AGO I TOOK A FRIEND; VFR; FOR A SIGHTSEEING EXCURSION TO VIEW SPRING FLOOD WATERS DROWNING OUT SEVERAL COMMUNITIES N OF STL LOUIS. I FLEW OVER THE STL TCA IN EXCESS OF 10000 FT TO GET TO MY DEST. I DSNDED; N OF WHAT I VISUALLY JUDGED TO BE THE NORTHERNMOST STL TCA BOUNDARY AND STARTED MY SIGHTSEEING. MY LORAN ADVISED SEVERAL TIMES ON MY E AND THEN S RETURN TO HOME; THAT I WAS ABOUT TO ENTER THE STL TCA. I PLANNED TO FLY UNDER AND TO THE E OF THE STL TCA ON THE RETURN TRIP HOME. NIGHTFALL CAME ON THIS 2 HR FLT AND I FOUND MYSELF WITHOUT GOOD VISUAL LANDMARKS TO VERIFY THAT I WAS NOT ENTERING THE STL TCA. I FLEW WHAT I FELT WAS SUFFICIENT TO THE E BEFORE TURNING S. SEVERAL TIMES MY LORAN DISPLAYED 'ALERTS' THAT I WAS NEARING THE TCA. WHENEVER I SAW AN 'ALERT' DISPLAY; I DIVERTED MORE TO THE E BEFORE RESUMING MY TRAVEL TO THE S. I'M A LOW- TIME VFR PLT (APPROX 525 HRS) AND RECENTLY SOLD MY SMA X AND BOUGHT AN SMA Y. I HAVE 15 HRS IN THE SMA Y. I BELIEVE MY CONFUSION; OR ANXIETY; ABOUT WHETHER OR NOT I ENTERED THE TCA RESULTED FROM: 1) DARKNESS AND RESULTING POOR/DISAPPEARING GND VISUAL CLUES. 2) LOW TIME IN A NEW PLANE (TO ME). 3) DIFFERENT INSTS/BRANDS (IN DIFFERENT PLACES) TO GET ACCUSTOMED TO. I READ AND REREAD MY LORAN MANUAL UPON RETURN HOME AND BELIEVE THAT I REMAINED CLR OF THE TCA ALT-CTLED AREAS; BUT POSSIBLY ENTERED; AT SEVERAL POINTS; WITHIN THE 30 NM CIRCULAR RANGE OF THE TCA; FROM THE DATA DISPLAYED BY MY LORAN AND WHAT VISUAL CLUES I COULD ASCERTAIN FROM MY CURRENT ST. LOUIS TERMINAL AREA CHART THAT I HAD WITH ME NIGHT BEFORE LAST. ABOUT 2/3 OF THE WAY THROUGH THIS TRIP; MY XPONDER STOPPED WORKING. I HAD BEEN SQUAWKING THE STANDARD 1200 VFR CODE IN EXCESS OF AN HR BEFORE IT QUIT. THAT PROB WAS TRACED YESTERDAY TO A LOOSE CONNECTION; PROBABLY OCCURRING WHEN SOME AVIONIC WORK WAS DONE BEFORE I TOOK DELIVERY OF MY SMA Y. MORE TIME IN THIS PLANE AND ITS INSTS WILL ENABLE ME TO BETTER JUDGE WHERE I AM AND BETTER INTERPRET THE DATA COMING FROM ITS AVIONICS. IF I HAD TO DO IT OVER AGAIN; I WOULD HAVE LEFT AN HR EARLIER SO I WOULDN'T HAVE RUN OUT OF DAYLIGHT. A CLOUD COVER INDUCED DARKNESS EARLIER THAN WHAT I HAD HOPED IT WOULD BE.