Couple more data points....
In conversations with an engineer at one of the popular ELT manufacturers we have learned the following.....
The transmit power of the 406 Mhz ELT's is 50X higher than it was for the traditional 121.5 ELT's, and their transmit signal has been repeatedly detected by COSPAS when no antenna was connected to the unit (usually on an uninstalled unit inside a building, etc., that was inadvertently activated).
There was some crash simulation testing done in conjunction with NASA and in two of the 3 test cases, the aircraft ended up upside down. In some of the tests the ant. was torn off the aircraft or the cable became disconnected, but in all test cases the ELT activated and was detected by COSPAS.
It was mentioned that as expected, the highest power transmission is with an intact externally mounted antenna.
If you look at enough crash wreckage, it quickly becomes apparent that there is no one ant. location that will work for all possible crash scenarios, when you consider the potential for it being ripped off, buried under wreckage, etc.
So it comes down to a personal decision in my mind. The ant. on my RV-6A is under the emp. fairing, because I have a high level of confidence that in any survivable crash scenario it will remain connected to the elt., and in good condition for transmitting. I accept the trade-off of potentially some level of shielding caused by the emp.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")