On first flight in RV8 with Advanced 6600 panel, the com worked great for 30 minutes. I was able to transmit and receive with groundcrew just fine. About 30 minutes into the flight, I stopped transmitting voice, reception was excellent. Ground crew could hear me key the mike and the radio screen showed a Tx but no voice. On the ground I checked the intercom and all was good, no problem speaking and hearing. (two lightspeed Zulus) could not voice transmit over the radio from the back seat, same as front. The EFIS showed the COM only putting out .4 amp on TX which should be about 8.0, so it was not getting out. I checked the pre made antenna cable and it appeared good, but I replaced it with another to try, NO-JOY. It is a new RAMI antenna and mounted to unpainted skin on bottom of airframe. Advanced suspects a bad harness and is sending one. Any other help or direction to check out is greatly appreciated, I want to get back in the air quickly, but safely.