AN IFR PA28 LOST COM 10 MIN AFTER TKOF. AT NIGHT BUT VFR; THE PILOT RETURNED TO HIS DEP ARPT BUT DSNDED VFR AND NORDO THROUGH RESTRICTED AIRSPACE.

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I WAS WITH A STUDENT TO PICK UP THE STUDENT'S RECENTLY PURCHASED ACFT. AFTER A SHORT CHK-OUT FLT (EVERYTHING OK) WE ELECTED TO START THE JOURNEY. WHILE ON AN IFR FLT PLAN FROM ZZZ TO ZZZ1 (1ST LEG OF THIS JOURNEY) I SUFFERED A COMPLETE COMS FAILURE ABOUT 10 MINS AFTER DEP. SYMPTOMS WERE THAT I COULD NEITHER RECEIVE NOR XMIT. I TRIED TO RAISE APCH; CTR AND ALSO THE FSS WITHOUT SUCCESS. I WAS IN VFR CONDITIONS AND THEREFORE ELECTED TO CONTINUE THE FLT UNDER VFR AND LAND ASAP. I SQUAWKED 7600. UNFAMILIAR WITH THE AREA (IT WAS NOW NIGHT) AND HAVING ONLY VOR NAV EQUIP AVAILABLE; I RETURNED TOWARDS THE NEAREST UNTWRED FIELD ZZZ2. REALIZING THAT ZZZ2 WAS ONLY 7 MI FROM ZZZ; I ELECTED TO RETURN TO ZZZ TO HAVE THE VENDOR REPAIR THE COMS EQUIP. UPON LNDG AT ZZZ I PHONED APCH WHO INFORMED ME THAT I HAD DSNDED INTO RESTR AIRSPACE DURING MY DIVERSION (THIS AIRSPACE WAS NOT HOT AT THE TIME). I SUSPECT THAT THE COMS FAILURE OCCURRED WITHIN THIS RESTR AIRSPACE (WHICH I HAD BEEN CLRED THROUGH). THERE ARE SEVERAL LESSONS WHICH COME FROM THIS: 1) ALWAYS CARRY A HANDHELD TRANSCEIVER AS A BACK UP (I DID NOT HAVE ONE WITH ME). 2) THOROUGH CHKOUTS OF THE AVIONICS ON A NEW ACFT ARE AS IMPORTANT AS CHKING OUT THE AIRFRAME OR PWR PLANT. 3) INITIAL FLTS IN A 'NEW' ACFT ARE PROBABLY BEST DONE VFR IN DAYTIME.