Monday, June 25, 2007

Amelia Earhart on KHAQQ

Amelia Earhart was declared dead on January 5th 1939. Her story as a pilot is well known and well covered elsewhere and I'll avoid repeating it here. What we care about are only the arcane details of radio am I right?

Her plane vanished near Howland Island in the Pacific, that too is well-known. Radio contact was lost and then so was she and her plane the Electra. Her last voice transmission received on Howland indicated Earhart and Noonan were en route to land at Howland Island to refuel. they sent position and fuel data by voice and Morse code. After many failed attempts to hail Howland, they ceased to transmit. transcripts indicate that they could not orient themselves because they were not receiving signals. Why?
The U.S. Coast Guard ship the Itasca had been standing off Howland Island for some days to act as a radio contact for Amelia. Radio communications in the area were very poor because the Itasca was overwhelmed with commercial radio traffic that the flights publicity had generated. Perhaps the plane was also inundated? (Transcript here) At 20:14 GMT She broadcast on 3105 and 6210 KHz "KHAQQ calling Itasca. We must be on you but cannot see you...gas is running low..."

It is also interesting to note that in a feat of short sightedness killed their own radio. They had on board a modified superheterodyne receiver of the highest sensitivity an early radio compass. But ditched it to spare the plane 30 lbs of ballast. This left her with only her all-wave receiver, connected to a directional loop antenna. Because of the bidirectional nature of a loop, signals are loudest when the plane is headed in two directions say east and west. They will disappear when the plane is headed perpendicular to that, north or south in that example. This is how bearings are taken, but note that as a basic means of direction finding it fails. (loop visible in the pics)

Also important to note that she not wanting to share credit with a man, installed the radio in her compartment and not that of her male navigator. Yes, the navigator had no access to the navigational equipment...
Directional bearings taken on some of the signals by Pan American Airways stations around the Pacific seemed to indicate that the transmissions originated from the area of the Phoenix Group of islands, which lie about 350 miles southeast of Howland but distress calls from the downed Electra, were received by operators across the Pacific indication other locations. Some must be hoaxes of course. And also the radio would have to have been powered by the battery in the plane, which is charged by the engine.. i.e. there was still gas left.

What's often ignored is that Earhart knew little of direction finding by radio and more often used a sextant. The frequencies she was using were not well suited to direction finding. She had actually ditched her low-frequency reception and transmission equipment at her last depot. That hardware might have enabled Itasca to locate her.
The supposed distress signals from the plane were heard intermittently for up to five days following the last transmission received by the Itasca.

None of them none of them allowed search parties to find the location of the downed plane. Incredibly, a couple of short wave radio listeners on the US mainland may have heard distress calls on upper harmonic frequencies. On shortwave frequencies this has been known to occur. Notably in Rock Springs, WY, 16 year old Dana Randolph, heard "This is Amelia Earhart. Ship is on a reef south of the equator.'' And near Tampa, FL 15-year-old Betty Klenck heard Earhart's followed by pleas for help as well. but Betty listened for hours, taking notes in a school composition notebook so we have that record today. That uniquely has an air of legitimacy despite the freakish reception scenario.

This book was written by Paul Rafford, Jr, a Pan Am radio engineer and is more technical in its historical account due to his background. Well worth reading.