Mark

ELT's output two frequencies. 121.500MHz and 406.025MHz.

The 121.50 signal is just a tone. Rescue operators use a device to look at signal strength and direction to find the location of the ELT transmitter.

406.025MHz is a digital burst of data that reports encoded aircraft/owner information, a unique identifier number, and Latitude/Longitude position information.
A good explanation can be found here .

We usually try not to send data or tone out in to the open air during testing. Either a direct co-ax cable can be connected between the ELT antenna output or a shield can be placed around the antenna with a co-ax leading to the tester. Direct hookup is the preferred method.

This is not to say we can't just send signals out without shielding. The allotted time frame for testing in the USA is during the first five minutes after the top of the hour. We usually call the tower to inform them that a test will be conducted.