AN INSTRUCTOR PLT IN A PA28-161 INADVERTENTLY PENETRATES THE CLASS C OF SHV WHILE GIVING AN STUDENT PLT XCOUNTRY INSTRUCTION ON A NIGHT OP.

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DURING A NIGHT XCOUNTRY INSTRUCTIONAL FLT OF A STUDENT PLT; WE WERE XFERRED FROM LONGVIEW DEP AND DIRECTED TO CONTACT SHV APCH IN 10 MI. DUE TO MY STUDENT HAVING SOME TROUBLE WITH HIS FLT LOG; MY ATTN WAS FOCUSED INSIDE AND I LOST TRACK OF OUR TIME AS WE APCHED SHV CLASS C AIRSPACE. WHEN WE GOT AROUND TO CALLING UP SHV APCH; WE WERE PROBABLY 1 MI OUTSIDE THEIR AIRSPACE. OUR RADIO FAILED TO PICK UP ANYTHING ON THEIR FREQ; HOWEVER; AND BY THE TIME I REALIZED OUR SIT WE WERE ENTERING THE AFOREMENTIONED AIRSPACE. WE EXITED THEIR AIRSPACE BY DSNDING BELOW THE OUTER SHELF. OUR RADIO WAS CONFIRMED TO BE BROKEN ON THE GND; BUT JUST ON 2 FREQS SHV APCH 119.9 AND GGG ATIS 119.65. OP ON GGG TWR FREQ OF 119.2 WAS FINE. THE OFFENDING RADIO WAS A BENDIX KING KX155 TSO. I BELIEVE MY RELATIVELY LOW LEVEL OF EXPERIENCE AS A FLT INSTRUCTOR WAS PART OF THE CAUSE. STAYING AHEAD OF THE PLANE AND THE STUDENT SEEMS TO BE THE KEY. ALSO; I DIDN'T KNOW THAT A DIGITAL RADIO COULD FAIL ON JUST 1 OR 2 FREQS AND LEARNING THAT FACT USED UP WHAT LITTLE TIME I HAD GIVEN US TO ESTABLISH COM BEFORE ENTERING CLASS C AIRSPACE.