Final answer:
The external antenna associated with the 406 MHz ELT is a three-frequency blade or rigid whip style antenna, specifically engineered to send out distress signals upon activation.
Step-by-step explanation:
The question pertains to the two antennas associated with the 406 MHz Emergency Locator Transmitter (ELT) used in aircrafts for emergency situations. One of them is an external antenna attached to the aircraft, which is known as a three-frequency blade, or rigid whip style antenna. This particular antenna is designed to emit signals across multiple frequencies when the ELT is triggered, ensuring the distress signal can be detected by search and rescue satellites and ground stations. The second, portable antenna is intended for use when the ELT is removed from its mounted position, typically in a scenario where the aircraft is down and the survivors are mobile. This portable antenna is usually stowed within the ELT's mounting tray, ready to be deployed when necessary. It is crucial in maintaining the capacity to send out distress signals even when the ELT is not attached to the aircraft. Various experiments and studies, like the Take-Home Experiment: Antennas, indicate that the efficiency of an antenna depends on its size relative to the wavelength of the signals it is designed to transmit or receive. This is also seen in broadcast antennas, which are most efficient at one-fourth the wavelength of the broadcast frequency. In the context of television reception, the ideal length for cross wires on a TV antenna is one-half the wavelength of the signal to ensure optimum reception. These principles are vital in designing antennas for various purposes, including emergency locator transmitters.