Something does not add up

[Colin Taylor]

It is widely accepted that the Electra's ventral receiver aerial was damaged on take off at Lae. However, at 0758 Itasca local time Earhart reported she was circling and asked Itasca to transmit on 7500 kcs. She then said that she received the signal but cannot get a minimum. The DF loop control box could not be tuned above 1500kcs so how was she able to receive the 7500 signal if the ventral receiver aerial had been damaged at take-off at Lae? And if she could receive on 7500 why could she not receive signals on 3105 or 6210?
The attached pictures show the Electra before the crash at Hawaii (short V aerial attached to the dorsal mast at mid-cabin) and after the rebuild (dorsal mast at the front of the cabin). Before the crash there were two ventral receiver aerials supported by six masts (plus the trailing aerial mast) but after the crash and rebuild there was only one ventral aerial with three masts.
Previously when  the Hooven automatic direction finder was fitted, it needed both a rotating loop aerial AND a static aerial to resolve the bidirectional ambiguity of the loop aerial. Hence there were two ventral receiver aerials, one connected to the Western Electric receiver and the other to the DF receiver. Before the crash at Hawaii, when the Bendix RA1 receiver and the manual direction finder was fitted there were still two ventral aerials so presumably the RA1 was connected to one of them in which case the RA1 could be tuned to 7500 as a receiver or to 1500 as a direction finder.
 (It was not essential to connect the RA1 to the static aerial as the directional ambiguity could be resolved by the human operator but the RA1 would then be limited to the frequencies to which the loop aerial could be tuned).
After the crash and rebuild there was only one ventral aerial. Was that aerial connected only to the Western Electric receiver? In which case if the Bendix RA 1 receiver was not connected to the static aerial, she could not tune the Bendix receiver to 7500, only to 1500 through the loop antenna.
Or was the single aerial connected to both the WE and the Bendix receivers? Can two receivers be simultaneously connected to a single aerial? (I don't think so, both receivers would need to be tuned simoultaneously to the same frequency). Or is a switch needed to flip the aerial from one receiver to the other? Or is one receiver switched off when the other is in use?
Something does not add up. What do you guys think?

[Martin X. Moleski, SJ]

After the crash and rebuild there was only one ventral aerial.

Was that aerial connected only to the Western Electric receiver?

In which case if the Bendix RA 1 receiver was not connected to the static aerial, she could not tune the Bendix receiver to 7500, only to 1500 through the loop antenna.

Or was the single aerial connected to both the WE and the Bendix receivers?

Can two receivers be simultaneously connected to a single aerial? (I don't think so, both receivers would need to be tuned simultaneously to the same frequency).

Or is a switch needed to flip the aerial from one receiver to the other?

Or is one receiver switched off when the other is in use?

It has been years since this was an active topic here.

I have three summary articles dating from back then.

Radio equipment on NR16020

NR16020 antennas

Loop antenna

The transmissions were on the V-antenna. The transmitter was crystal-controlled, so AE could only transmit on three pre-tuned frequencies. She had "three-channel Western Electric equipment of the type then being used by the airlines to provide one channel at 500 kc and the other two at [3105 and 6210 kHz]."

The Western Electric Model 20B receiver could scan frequencies on four bands. A band was selected and the frequency adjusted by hand.

Excerpt from the article on radio equipment aboard the Electra:

Ric Gillespie: "I think there was a Bendix device aboard the aircraft that allowed the loop to be used with the Western Electric 20B receiver. I think it was integral to the Bendix MN-5 loop and was the same device described on page 42 of the August 1937 issue of Aero Digest magazine. Under the heading "Aero Radio Digest - The Newest Developments in the Field of Aircraft Radio" the first article is entitled "Bendix D-Fs". I quote: 'Bendix D-Fs are designed to operate in conjunction with Bendix Type RA-1 receiver, but will also give accurate and dependable bearings when used with any standard radio receiver covering the desired frequency range.'"^[8]
Ric Gillespie, Finding Amelia, p. 64.



Just what range of frequencies the Electra's homing device could cover is an important question but not a difficult one to answer. A hoop-shaped "loop" antenna mounted above the Electra's cockpit received the signals for direction finding. Numerous photos taken from the time of its installation just prior to Earhart's first world flight attempt in March until the final takeoff from Lae, New Guinea, in July leave no doubt that the loop antenna on Earhart's Electra was one of a new line of Bendix direction finders pictured and described in the August 1937 issue of Aero Digest magazine: "Bendix D-Fs are designed to operate in conjunction with Bendix Type RA-1 receiver, but will also give accurate and dependable bearings when used with any standard radio receiver covering the desired frequency range." The article also notes that these receivers can be used "as navigational direction finding instruments within frequency range of 200–1500 kilocycles."^[9] Those parameters generally agree with the limits described by Manning and Miller prior to the first world flight attempt ("Plane has direction finder covering 200 to 1430 kcs").^[10] They also agree with Putnam's message of June 25, 1937, saying that the plane's direction finder "covers range of about 200 to 1400 kilocycles."^[11] Where Earhart got the idea that her direction finder could cover "from 200 to 1500 and 2400 to 4800 kilocycles" is not clear, but the signals she requested on 7500 kilocycles were far beyond even those limits.^[12]

I think you have confused the limited range of frequencies that the Bendix could use for DF with a limited band of frequencies that could be received. The Bendix just fed what was received to AE's headset as she rotated the circular directional antenna on top of the fuselage. That antenna was probably tuned for the local frequencies in its design. High Frequency Direction Finding (huff-duff) was only in its infancy in 1937. The Bendix was from an earlier era.
Hooven was extremely unhappy that AE removed his system and regressed to the Bendix loop: "Before Miss Earhart took off on her Round-the-World flight she removed from her plane a modern radio compass that had been installed and replaced it with an older, lighter-weight model of much less capability. I am the engineer who had invented and developed the radio compass that was removed, and I discussed its features with Miss Earhart before the installation was made. I have reason to believe that it was the failure of her radio direction-finder to do what the more modern model could have done that caused her to be lost. The story is told herein, and it is plain to see why I have been so very much interested in the subject."

One More Good Flight, Ric Gillespie, 2024 (p. 139):

"Earhart ... turned on the Bendix direction finder and tuned her Western Electric receiver from 3105 kcs to 7500 kcs, heard the dit-dah, dit-dah, dit-dah through the static. Reaching above her head, she rotated the loop, trying to discern any change in the volume. If she could find the spot where the As were least audible, a needle on the instrument in front of her would give her the bearing to where the signals were coming from, but, try as she might, there was no change. Disappointing, frustrating, but hardly surprising: she had never been able to get the damn thing to work."

I think this answers your questions.

• Was that aerial connected only to the Western Electric receiver?
  Yes.
• Or was the single aerial connected to both the WE and the Bendix receivers?
  There was only one receiver on board.
• Can two receivers be simultaneously connected to a single aerial? (I don't think so, both receivers would need to be tuned simultaneously to the same frequency).
  I don't know. Not applicable to this situation.
• Or is a switch needed to flip the aerial from one receiver to the other?
  A switch was used to connect the single receiver to one aerial or the other.
• Or is one receiver switched off when the other is in use?
  There was only one receiver.

It is tragic that the thought never crossed their minds that changing antennas suddenly allowed them to hear transmissions from the ground. The fact that the Itasca was transmitting As as they requested meant that the Itasca could hear them. They seem not to have tried just tuning the receiver back to the voice frequency that they had told the Itasca to use in order to test whether they could use the loop antenna to communicate. Or they could have played twenty questions, with the Itasca sending yes or no answers via Morse code.

I don't blame them. I often make mistakes in thinking things through. They were exhausted, they were deafened by the prop tips breaking the sound barrier a few inches away, they must have been at least somewhat anxious as one thing after another failed them. Hindsight is twenty-twenty vision, and we don't know what hindsight taught them in the last days of their lives.

So sad!

[Colin Taylor]

The question remains, how did Earhart receive the 7500kcs signal if the ventral receiver aerial was damaged?  The fact that she reported hearing the 7500kcs morse code signal debunks the broken aerial theory. But why was she unable to tune 3105 or 6210? If there was only a single receiver with a static aerial and a loop aerial, that does not explain how she could tune one HF frequency and not another.
Elgen Long, in his book, is quite clear that Radioman Joseph Gurr briefed Earhart on the Bendix RA 1 receiver and direction finder before her departure from California on the second attempt. In Long's book there is a photo attributed to PAA mechanic F Ralph Sias in Miami about May 26  showing the cockpit with the RA1 above the window. Therefore there were two receivers in the Electra but only one ventral static aerial.

[Martin X. Moleski, SJ]

The loop antenna was a receiving antenna.

The loop was more directional than a straight-wire antenna.

Here are directions about how to build your own "resonant magnetic loop" to do DF.

Handheld Finding Loop Antenna for RFI Location

The fact that the only time they received any signals was when they kicked in the loop antenna bolsters the theory that something was wrong with the ventral (belly) antenna.

One receiver. Two antennas.

[Colin Taylor]

The loop antenna could not be tuned above 1500kcs hence no directional signal. The fact that the receiver could tune 7500 tells that the static aerial was functional not the opposite!
Ric claims that before the second start the Bendix RA 1 receiver was removed and the loop coupler connected to the Western Electric receiver. This because of a conversation in Miami between Earhart and Reporter Carl B Allen who apparently queried the absence of the 'marine frequency radio.' (p81 One More Good Flight)
Ric assumes Allen was referring to the Bendix RA1 receiver. But was Allen actually referring to the absent Morse Keys and possibly the absent trailing aerial which enabled communication on the 'marine frequency' 500kcs, rather than referring to the Bendix radio receiver?
Earhart replied in terms of communication rather than in terms of direction finding, preferring voice rather than Morse code. Also she said, '...we decided to leave it in California'. But the Bendix receiver was pictured in the aircraft in Miami, so it was not the Bendix receiver which was removed.
So, unless the Western Electric receiver was removed in California, the Electra had two receivers and one ventral receiving aerial.

[Martin X. Moleski, SJ]

The loop antenna could not be tuned above 1500kcs hence no directional signal.

There is no such thing as "tuning" an antenna in flight. When an antenna is designed, steps are taken to make it most resonant with the frequencies that will be used most often, so those designs can be tuned before installation.

The receiver could be tuned to that frequency.

The loop was not intended for HFDF. It is true that AE could not hear a null on it. I think is also true that she was able to hear the sequence of A's through it. Those two ideas are not incompatible with each other.

the Electra had two receivers and one ventral receiving aerial.

The Electra also had a loop antenna on top of the airplane. You can see it in a multitude of pictures if you look carefully for it.
Whether there were two radios or one, the loop antenna was an antenna that AE could select for doing direction finding.
When she selected that antenna, she could hear transmissions from Howland Island, even though she could not get a null on the signal by rotating the antenna on top of the fuselage.
It would have defeated the function of the loop antenna if the system had also connected the dorsal antenna to the DF system. It is true that other designs used two antennas, one fixed and one mobile, but this was not one of them. In order for rotating the loop antenna to detect null, the signal had to be delivered solely through the loop antenna, not through a comparison of signals received from two different antennas, one on top of the plane and the other on the bottom, in order to resolve the ambiguity that is inherent in finding a null. When one finds a null, the location of the transmitter may be in front of the antenna or behind it. Without other information, a single observation of the position of the antenna when the signal was weakest does not tell you which way to go.
Fourteen years ago, Gary LaPook gave an excellent summary that shows how confused AE was.

"And why 7500, why not some other random number like 8364 or 6350 or 7937, etc.? And when did she come up with this number? The first time it is mentioned is in the June 27th radiogram from Earhart to Itasca. in which she requested Itasca to send the letter "A" and call letters "ON HALF HOUR 7.5 MEGACYCLES." The next day Itasca radioed to Earhart that "ITASCA TRANSMITTERS CALIBRATED 7500 6210 3105 500 425 KCS CW" and that "ITASCA DIRECTION FINDER FREQUENCY RANGE 500 TO 270 KCS." This message acknowledged AE's request for 7500 Kcs, (7.5 Mcs.) It is tempting to think that AE was just making a mistake on radio terminology and that she actually  wanted Itasca to transmit on a 750 meter wavelength (which is the same as 400  Kcs) which was a common direction finder wavelength. The problem with this idea is that in the same June 27th radiogram she requested "ONTARIO STANDBY ON 400 KILOCYCLES" so it is very unlikely she would request the correct 400 Kcs from Ontario and then use different terminology to request the same 400 Kcs (750 meter wavelength) signal from Itasca. And again on July 1st AE sent another radiogram requesting Ontario to transmit on 400 Kcs so that  appears to be the terminology that she knew how to use. ( I am using the obsolete terminology of "Kcs" and Mcs" rather than the modern usage of "Khz " and "Mhz" for consistency and to avoid confusion.)"


In One More Good Flight, Ric describes the changes AE made after the Luke Field crash and the departure of Manning from the crew. I have added emphasis in bold.

"The rationale for installing the Bendix RA-1 receiver on top of a fuel tank in the cabin had been for Harry Manning to be able to communicate in code with ships at sea and take bearings on them to establish his position, but with Manning gone, that was no longer an option. The Bendix radio direction finder coupled to the Bendix receiver could alternatively be coupled to the Western Electric receiver under the copilot's seat, so Earhart decided to save weight by getting rid of the Bendix receiver and its sense antenna on the belly. By eliminating one crew member, the trailing wire, and the Bendix receiver, Earhart had made the Electra a bit lighter at the expense of seriously degrading the world flight's chances of finding Howland Island" (76).

Before the repair of the aircraft, one receiver was above a fuel tank in the back of the plane and the other was under the copilot's seat. No photographs of the cockpit instruments show a receiver. What you are seeing is the display that reports the direction in which the loop antenna is oriented by the operator cranking it in circles until a null is found. That cluster of controls is in the Miami pictures because the Bendix loop antenna was still in use. Nothing about the cockpit picture shows the presence of a Bendix receiver on board in Miami.