Ron and Tom--

But don't the "whip" antennas supplied with the 406 MHz ELTS require a ground plane? The antenna that comes with the ACK E-04 is clearly dedigned for external mounting on a metal skin. See: http://www.aircraftspruce.com/pages/av/elts_zack/whipantennaE04.php   What I am looking for is either a quarter-wave "rubber duck" antenna that mounts directly to the ELT, or preferably a copper foil half-wave dipole that I can bond to the inner surface of my aircraft.

Being a Ham Radio operator in a former life, I know that a quarter wave whip antenna is cut for one specific frequency and performance is degraded at all other frequencies unless some kind of "trap" or impedance-matching device is installed. Theoretically, a quarter-wave antenna for 406 MHz should be somewhere around 7" long. For 121.5 MHz it should be around  23" long. Clearly the whip antennas that come with the ELT units are cut for some intermediate frequency which is a compromise that works "OK" at both 406 and 121.5.

And what about polarization? The 121.5 and 243MHz signals should be vertically polarized like your COMM antenna(s), but technically the 406 MHz should be circularly polarized which would either require a helical antenna or two crossed dipoles. Of course the polarization would depend upon whether your aircraft was upright or not :-(