Unless you have a matched antenna for the channel you are transmitting on much of the power is being reflected back into the modulator. (bad SWR) This will cause the output package in the BT mod to fail in short order.
This is transmitter 101
My experience is the DTV converters dont last if you leave them on 24/7. Take note on how hot the microprocessor runs in the converter. Running it with the top off will help but a small fan would be better. Still, dont expect it to last very long, They were designed to pacify the public not to last.
Heres a video I made some time ago on how to make a basic antenna. Having the antenna analyzer or a spectrum analyzer to sweep the antenna is a must.
http://youtu.be/z4dhRt7tmd4