I've been looking into the famous missing antenna.
Steve, yea, I am aware of that contention and there is another one of those "anecdotal" statements made a few years after the flight by an airport worker, that he had found a length of antenna wire at the end of the runway.
R.E. Fullenwider told TIGAR that he "spent some time in Lae during World War Two courtesy of Uncle Sam." As he remembers it, the old-timers there often said they hadn't been surprised when Earhart was lost because "she left part of her trailing wire antenna laying on the runway"
IMHO it is kind of a moot point with all her other radio problems.
Transmitting Antennas
The high-frequency antenna installed aboard the Electra was the dorsal "Vee" type. The original antenna was a total of 46 feet, doubled back onto itself. This length did not include the lead-in wire. This antenna was by itself 15% longer than optimum; but since the radio equipment had been installed by Bell Labs we must assume that it was tuned properly at that time.
While a bit too long for the HF frequencies the 46 foot length was totally unsuitable for the low-frequency 500 KHz operation, as the total length would be miniscule compared to the wavelength at this frequency. At 500 KHz, the wavelength is 600 meters (1968 feet). Thus Lockheed installed the 250 ft long (1/8 wave) trailing antenna in the "from the factory" Electra
Joseph Gurr, a radio technician privately engaged by Lockheed to check out the communications equipment while the aircraft was under repair at the Burbank factory, proposed that if the fixed H-F Vee antenna could be lengthened, it would serve on both 500 KHz and the high-frequency channels. He moved the antenna mast on the fuselage forward increasing the Vee's total length to 54 feet. There was a further slight increase in the length of lead-in, because the location of the feed-through insulator was changed to a point lower on the fuselage side. He also removed the Western Electric loading coil used with the trailing wire – a relatively small value of inductance – and substituted a very large, home-made coil. The extra antenna wire did not bring the antenna any closer to a proper length for low-frequency operation; but it may have been just enough, judging by an antenna-current indication alone, to make things appear to be working on 500 KHz.
This was a serious mistake. For the most part, the transmitter's output power was being dissipated in the coil, rather than radiated by the antenna. Very little R-F was actually going out despite whatever antenna current reading may have been observed.
Any signal radiated by this system on 500 KHz would be extremely weak. To overcome the losses in the loading coil and achieve any meaningful signal output would have required tremendous transmitter power in order to afford being able to waste a lot of R-F – far greater power than the 50 watts produced by the 13 series transmitter. None of this mattered on the final flight as Earhart removed all Morse equipment to save weight. What is important here is whether Gurr's modifications included the removal of the antenna transfer switch as well. One function of this switch was to remove the loading coil from the antenna circuit on H-F. If the huge homemade coil was left in the circuit all the time, operation on 3105 and 6210 could have been even more affected. The coil could have acted like a radio-frequency choke, blocking part of the transmitted power as well as part of the received signal.
The lengthened antenna was now almost 33% longer than the optimum length specified for the 13 series transmitter, in the H-F range. As for 500 KHz, this extra eight feet made practically no difference. At H-F, however, the extra length radically altered the tuning.
The length of antenna plus extra lead-in was now very close to an odd fraction of a resonant length on both frequencies, 3/16 wavelength at 3105 and 3/8 wavelength at 6210 (about 57 feet in each case). The increase in length meant a radical change in the antenna's characteristic impedance at both frequencies. Complete retuning of the transmitter was necessary.
In actual practice, these odd fractions of a resonant wavelength are often difficult to properly match. The reason for such difficulties lies in the R-F voltage and current distributions over the length of the antenna wire, and resultant standing waves along the antenna which cause power to be reflected back into the transmitter rather than transmitted out as RF
The doubling-back of the antenna length in the Vee configuration could have played havoc with the antenna's characteristic impedance, as the two halves of the Vee may have reacted upon each other. Also, a Vee configuration meant that more of the antenna was in close proximity to the metal skin of the aircraft, which could have further affected its operation and tuning.
The transmitter could easily have become so mistuned that power was actually being radiated on the wrong frequencies. If the transmitter were mistuned, the modulation quality would suffer. The signal might become broad and the speech distorted. Indeed, observations were made at Lae, New Guinea, before Earhart's final takeoff which indicated such distortion on 6210 KHz. It would be interesting to know how much deviation from standard tuning procedure may have been taken by their engineers, but available records do not reveal this.
Receiver
Overall, the Model 20 series receiver was, at best, an average radio, built with cost in mind. In terms of the technology of the times, it was hardly the best choice for a flight of this magnitude. It suffered from several design deficiencies.
The most significant was that it seems to have been designed with a view to maximize performance on the lower-frequency bands, 188-420 and either 485-1200 or 550-1500 KHz. This was logical, because in the mid-1930s, many U.S. domestic control towers at civilian airfields were still transmitting on low frequencies below 400 KHz, while listening for aircraft calls on the H-F channels. Two-way H-F was widely employed by the military, but only gradually coming into use by civilian ground stations. The H-F bands in the Model 20 seem to have been an afterthought and these were the two bands that Earhart relied on and used
Antenna Inputs
The antenna input circuit of the Model 20 receivers was designed to accept connections from two separate antennas: one for low-frequency Bands 1 and 2, the other for high-frequency bands 3 and 4.
Normally, the high-frequency antenna would be shared with the transmitter, and connected to the receiver through the antenna changeover relay inside the transmitter. The low-frequency antenna could be a separate structure connected to the low-frequency antenna input. Since the Gurr modifications meant the same antenna would now be used for transmitting on both the L-F and H-F bands, however, it becomes unclear how the two inputs on the receiver might have been used. Perhaps they were simply jumpered together. If so, sensitivity degradation may have resulted from connecting the input circuits for the low – and high-frequency bands in parallel, due to one partially detuning the other; but the actual extent is not known.
There is debate as to the exact nature of the receiving antennas aboard the Electra. It is possible that the dorsal Vee antenna was not used for receiving at all, but transmitting only; and that the receiver was not connected through the antenna changeover relay, but directly to one or more antennas installed on the belly of the aircraft. If that is/was the case THEN we have the theory that the belly antenna was destroyed, when the mast or the wire made contact with the ground during the final takeoff from Lae, New Guinea. This scenario may explain why Earhart seems to have been unable to hear any transmissions from the U.S. Coast Guard Cutter Itasca at Howland Island, as she approached the end of this leg of her flight.
So while the existence of the belly antenna is an established fact, it may or may not have been employed for communications reception at all due to the Gurr modifications.
Bendix DF
The Bendix D/F package simply used a direction-finding adapter or loop coupler to match the loop antenna into the low-frequency antenna input of the Western Electric receiver.
It seems logical to me that some kind of second receiver should have been incorporated; for if the one and only radio receiver was tied up in direction-finding duties, it would not be available for receiving communications. Thus she could not direction find and communicate at the same time
To further complicate things, in such a system, using only the Western Electric receiver with a Bendix coupler unit, any change of reception frequency and/or antenna functions from communications to D/F would involve complex switchology: changing bands, considerable cranking of the coffee-grinder receiver control head, as well as tuning the Bendix coupler. As a result, given the pressures and consequent fatigue of the long flight, operational quirks of the equipment and Earhart's well known lack of radio acumen, Earhart's attempts at communications and D/F failed.
The Hooven Radio Compass which had been in the Electra did have its own receiver and required no operator input or monitoring once tuned to the desired broadcast frequency. AE only have had to follow an arrow, on a gauge, which simply pointed to the broadcasting station.
Why did Earhart not have any frequencies used by organizations such as Pan American Airways? This is especially puzzling, since by 1937 Pan Am had an extensive radio system, and direction-finding facilities across the Pacific. Fred Noonan had pioneered the Pacific routes for Pan Am and was well familiar with their system. It's also puzzling that the two midway "Guard" ships, Ontario and Swan had NO way to communicate with Earhart. All their equipment was LF and she was HF. AE and Noonan were supposed to visually see Ontario as the flew over it in the dark YET neither ship was fitted with any type of special lighting.