The first active IFF transponder (transmitter/responder) was the IFF Mark I which was used experimentally in 1939. This used a regenerative receiver, which fed a small amount of the amplified output back into the input, strongly amplifying even small signals as long as they were of a single frequency (like Morse code, but unlike voice transmissions). They were tuned to the signal from the CH radar (20–30 MHz), amplifying it so strongly that it was broadcast back out the aircraft's antenna. Since the signal was received at the same time as the original reflection of the CH signal, the result was a lengthened "blip" on the CH display which was easily identifiable. In testing, it was found that the unit would often overpower the radar or produce too little signal to be seen, and at the same time, new radars were being introduced using new frequencies.