TIGHAR

Electra before the crash.jpgElectra after the rebuild.jpg
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?

Quote from: Colin Taylor on February 02, 2026, 09:26:26 AMAfter 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 (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020)

NR16020 antennas (https://tighar.org/wiki/index.php?title=NR16020_antennas)

Loop antenna (https://tighar.org/wiki/index.php?title=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:

QuoteRic Gillespie (https://tighar.org/wiki/index.php?title=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 (https://tighar.org/wiki/index.php?title=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] (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020#cite_note-8)}
Ric Gillespie (https://tighar.org/wiki/index.php?title=Ric_Gillespie), Finding Amelia (https://tighar.org/wiki/index.php?title=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] (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020#cite_note-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] (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020#cite_note-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] (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020#cite_note-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] (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020#cite_note-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 (https://tighar.org/Projects/Earhart/Archives/Documents/Hooven_Report/HoovenReport.html) 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!

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.

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 (https://www.arrl.org/files/file/Antenna%20Book%20Supplemental%20Files/24th%20Edition/Handheld%20DF%20Antenna%20-%20NA6O.pdf%20antenna)

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.

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.

Quote from: Colin Taylor on February 16, 2026, 08:04:31 AMThe 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.

Quotethe 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 (https://tighar.org/smf/index.php?topic=573.msg8475#msg8475) 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.

Here are pictures of the Electra cockpit in Miami 26th May, next a Bendix RA1 receiver, loop control box and the loop aerial. Also, the loop control box shown to Earhart prior to fitting.

Which of these is fitted above the window in the Electra cockpit? Note the characteristic shape of the tuning dial window on the receiver but bear in mind that Earhart's radio was a prototype or early production model. Also, all of these boxes have tuning controls and dials, but none have aerial direction indicators. The relative bearing of the loop aerial is read off the dial at the base of the loop aerial.

I went looking for the "Miami photo" and found a Research Bulleting from 16 years ago that shows that the photo cannot be dated to Miami and that other photos do not show the remote control.

The "Miami" Cockpit Photo

https://tighar.org/Projects/Earhart/Archives/Research/Bulletins/53_MiamiPhoto/53_MiamiPhoto.htm (https://tighar.org/Projects/Earhart/Archives/Research/Bulletins/53_MiamiPhoto/53_MiamiPhoto.htm)

"Lockheed documents and Bureau of Air Commerce inspection reports indicate that there was a single Western Electric 20B receiver aboard Earhart's Electra. Photos show that from early March 1937 onward, the aircraft was equipped with a Bendix MN-5 loop antenna. The Bendix loop could be used with the Western Electric receiver ..."

"The [Bendix/Hooven] Radio Compass was removed in early March 1937. In other words, this photo was taken sometime between November 1936 and early March 1937. NR16020 was never in Miami during that period. This photo was not taken in Miami. So where was this hangar? The type of construction appears to be consistent with the hangars at Burbank where the Electra was based."

"So the box was present when the Bendix/Hooven Radio Compass was aboard the aircraft. Was it still there after the Radio Compass was removed? Two very similar photos of AE in the cockpit suggest that the box went away at the same time the Radio Compass went away."

Thanks for the links. I agree with you. So, the 'Miami' photo apparently shows the Electra cockpit in a hangar at Burbank, California before the first start! The mystery box appears to be the Hooven ADF control box and I see the ADF left/right indicator in the instrument panel above the turn and slip.
Oh dear, I cannot believe what is written in Elgen Long's book? I thought the internet was bad enough!
So, I am inclined to agree that there was only one receiver, hence only one ventral aerial and the loop control box was connected to the WE receiver.

So, how could she tune 7500 but not hear anything on 3105 or 6210? I guess we will never know.

Quote from: Colin Taylor on March 11, 2026, 05:02:57 AMSo, how could she tune 7500 but not hear anything on 3105 or 6210? I guess we will never know.

The loop antenna worked on 7500. She did not experiment with staying on that antenna and tuning to one of the other two frequencies. 

My guess is that after failing to DF on 7500, she then switched back to the circuit for the belly antenna, on which she never heard anything. 

One receiver, two antenna circuits. 

One antenna worked once. The other one never did.

Her transmission antenna -- on top of the plane -- worked well enough for Howland to record messages from her that increased in signal strength from 2:40 AM until 8 AM or so.

https://tighar.org/wiki/index.php?title=Transmissions_heard_from_NR16020 (https://tighar.org/wiki/index.php?title=Transmissions_heard_from_NR16020)

The 7500kcs signal could not be received through the loop antenna because the loop control box could not be tuned above 1500kcs. Therefore it must be received through a static aerial.

However, according to Michael Everette it is possible that the upper two bands of the Western Electric receiver were connected to the dorsal aerial through switch gear in the transmitter.

https://tighar.org/Projects/Earhart/Archives/Research/ResearchPapers/ElectraRadios/ElectraRadios.htm#16

Alternatively the ventral receiver antenna was undamaged.

That means that the problem was to do with switch selection. To receive a morse signal on 7500kcs would require selection of the upper band, tuning 7500 AND selecting CW (or BFO).

Whatever the aerial configuration, a small adjustment of the tuning and deselection of CW would have enabled Earhart to receive a voice transmission on 6210kcs.

The undated picture shows the loop aerial, the loop control box (under the roof panel at left) the receiver remote control (to right of throttles by her left hand) and the five switch panel containing the CW switch (to the right of that)

Quote from: Colin Taylor on March 16, 2026, 05:00:12 AMThe 7500kcs signal could not be received through the loop antenna because the loop control box could not be tuned above 1500kcs. Therefore it must be received through a static aerial.

The control for the frequency of the loop was in the one and only RECEIVER.

Amelia configured the RECEIVER to receive on 7500 kcs and connected it to the loop.

The loop could not do any DIRECTION FINDING on that frequency.

Those are two different issues.

If the loop was not acting as an antenna, it could not be used for direction finding.

The unit she was using employed only the loop for direction-finding. The more sophisticated unit that Hooven had designed that was originally installed on the aircraft used two antennas. It was able to resolve the question of whether the transmission was coming from ahead of or from behind the plane.

Hooven wrote a paper about the loss of the aircraft and crew.

The loop control box has LOOP TUNING printed on it. It has a tuning dial and a band selector. I guess it needed to be connected to a receiver for signal amplification, audio output and BFO function. The receiver would need to be tuned to the same frequency but would be limited to the frequency range of the loop control box. The loop alone will give two nulls. Combining the signals from the loop and the static antenna does not prevent the user finding the null but it gives a single null giving an unambiguous direction.

Quote from: Colin Taylor on March 17, 2026, 11:53:05 AMThe loop control box has LOOP TUNING printed on it. It has a tuning dial and a band selector. I guess it needed to be connected to a receiver for signal amplification, audio output and BFO function. The receiver would need to be tuned to the same frequency but would be limited to the frequency range of the loop control box. The loop alone will give two nulls. Combining the signals from the loop and the static antenna does not prevent the user finding the null but it gives a single null giving an unambiguous direction.

I am not sure what picture you are looking at.

If you mean the control box that was removed from the aircraft after the crash at Luke Field, the labels on it are meaningless.

Let's examine some more excerpts of the research paper (https://tighar.org/Projects/Earhart/Archives/Documents/Hooven_Report/HoovenReport.html) written by Frederick J. Hooven, the designer of the more sophisticated direction finder that had been originally installed on the aircraft.

QuoteA much improved type of radio direction finder had recently been developed and Miss Earhart had had one of them installed on her plane, but had removed it to save its extra weight of about 30 lb., the weight of five gallons of fuel. The improved device was then called a radio compass, although it was later automated and was called the "adf". It incorporated a superheterodyne receiver of the highest sensitivity, and receivers then were just as sensitive as the best receivers are today. It used a regular antenna in addition to its loop, so that the operator always heard the signal that was being tracked. Most importantly the combination of loop and antenna made it possible to provide a visual left-right indicator that gave a single, unambiguous direction for the signal.

 The improved sensitivity made it possible to use a much smaller loop than the older device, and in Fig. 2 may be seen the smaller, streamlined housing of the newer loop as it appeared when installed on the Earhart plane in October, 1936. Miss Earhart did not realize that this streamlined loop reduced air resistance to an extent that would have saved several times the five gallons of fuel that she had added by removing the radio compass and replacing it with the older device.

1. The Hooven system had its own receiver.

2. The Hooven system had a "much smaller loop antenna" under a streamlined cover.

3. The Hooven system used two antennas: "a regular antenna in addition to its loop."

4. The Hooven system receiver may or may not have had a limit of 1500 kcs.

5. The control box for the Hooven system was removed when the Hooven system was removed. If your picture is of the control box for the Hooven system, it is utterly irrelevant to the Bendix loop in use on the fatal flight.

6. The Hooven system did not require the user to "find a null" in the transmission. The system found the null, eliminated the ambiguity, and provided "a visual left-right indicator" that showed the direction the plane need to turn to fly toward the transmitter.

7. AE had intended to detect the null by ear as she turned the loop over her head. That would have been difficult even if she had given a frequency that would work with the Bendix loop due to the stress on her hearing after so many hours sitting so close to the tips of the propellor, which were very noisy.


Original Hooven system: two receivers, two antennas, and a control box. The null was found by the system and displayed visually.

Obsolete Bendix system: one receiver, tuned by the band selector and the dial that were used for voice reception, and one loop antenna in use for direction finding. The null was to be found by the operator listening to the strength of the transmission.



Here again is a link to the research bulletin that discusses the removal of the control box before taking off for Miami to begin the second attempt to fly around the world:

I am referring to the cockpit photo in my March 16th post which shows the loop aerial above the cockpit and the loop control box 2024-12-17.png2026-03-20 (10).pngabove the starboard pilots window. You are saying that this loop control box was removed along with the Bendix RA 1 receiver before the second attempt. Why do you think that the loop had a control box for use with the Bendix receiver but not with the Western Electric receiver? I think the loop control box was part of the loop aerial and did not count as a separate receiver.

The Miami Photo article which you linked to shows a photo of Earhart and the Electra attributed to Carapito Venezuela June 2nd or 3rd 1937 and the caption says no 'Box' is apparent. The unattributed picture attached here shows Earhart and Noonan boarding the plane and a 'Box' is visible above the pilots window. Earhart is wearing the same clothes in both pictures so I think this shows the loop control box fitted for the second attempt. The third picture shows Earhart and Noonan boarding the plane at Lae. The corner of the loop control box is just visible through the side window. 

Quote from: Colin Taylor on March 16, 2026, 05:00:12 AMThe 7500kcs signal could not be received through the loop antenna because the loop control box could not be tuned above 1500kcs. Therefore it must be received through a static aerial.

Quote from: Colin Taylor on February 16, 2026, 08:04:31 AMThe 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!

Joe Gurr discussed this via his Q and A with Fred Goerner (https://tighar.org/wiki/index.php?title=Modifications_by_Joe_Gurr#Questions_from_Goerner,_Answers_from_Gurr).

    Goerner: According to the Itasca (Coast Guard vessel) radio log, Earhart asked for homing signals to be sent to her on 7500. Did her DF have that range? Could she have taken a bearing on that frequency?

    Gurr: Yes.... The DF loop on the airplane was designed for low frequencies .... It would be much less efficient on 7500, and then the signal would have to be quite strong to overcome the greatly detuned (from resonance) condition. (emphasis added)

This seems to be at the heart of the matter.  The loop antenna was designed for use as a DF on low frequencies (up to around 1500 kcs), but it would still function as an antenna, albeit not an ideal one, for receiving on higher frequencies.  When Earhart switched the antenna input on her WE receiver from the belly antenna (which may have been damaged on takeoff) to the loop antenna to listen for Itasca's A's on the hour, she was close to Howland (within 100 miles if you're a crash and sinker like me, 225 miles if you're a Tighar true believer), and the antenna would still function, as suggested by Gurr, albeit degradedly.  She then set the WE receiver to 7500 and listened through her headphones that were connected to the WE receiver.  The antenna was a mismatch to the receiver, so the A's came in as detuned and weak, and, accordingly, they were audible but could not be heard not well enough for AE to get a minimum.

Relatedly, you seem to take the existence of a "Loop Tuning" dial on the Bendix MN-5 coupler to mean that the coupler must be tuned to a particular radio frequency (with its max being 1500) and that it therefore couldn't receive a signal sent on 7500.  Based on much clearer photos below of a similar Bendix coupler, one can see that the "Loop Tuning" dial on a Bendix coupler is marked 1-100, which would suggest that it is not a means of tuning the antenna to a particular frequency.  Rather, when I read the instructions on the control box in the pic I attached, it suggests to me that you turn the Loop Tuning dial while trying to listen for a max signal, and that perhaps the turning of the dial makes some kind of internal tuning adjustments to the antenna while you zero in on the max signal (i.e. "tuning it").  Or perhaps it just sets something on the compass dials that help to read you a bearing? I didn't entirely understand the directions, but it seems pretty clear that they don't refer to setting a radio frequency with the Loop Tuning dial. Take a look at the directions on the coupler pics below and see what you think.

Just fyi, although these pics are of a slightly later model (1942), you can see that the Loop Tuning dial and its markings appear to be the same as the one you have posted. They are found here. (https://www.worthpoint.com/worthopedia/ww2-bendix-radio-directional-antenna-1934792843)

If LOOP TUNING does not mean what it says, then I am lost!

Is there some instrument associated with this set-up? What is the ARROW, the POINTER, the MASK and the UNMASKED PORTION OF THE DIAL?

Quote from: Colin Taylor on March 22, 2026, 06:17:31 AMIf LOOP TUNING does not mean what it says, then I am lost!

I may not have explained it very well, so I cheated and asked chatgpt to explain what tuning the loop meant. Here is its reply (lightly edited for length):

"That instruction is referring to tuning the loop as a resonant circuit. A WWII Bendix loop like that isn't just a piece of wire—it's actually a tuned LC circuit (inductance + capacitance), very similar to the front-end of a radio. A loop antenna inherently has: Inductance (from the wire loop itself) and a variable capacitor (usually built into the loop assembly or its control box). Together, these form a resonant circuit based on the principle of Resonance.

"What 'tune loop for max signal' means: Even though the receiver is tuned to the station frequency, the loop must also be tuned to resonate at that same frequency (emphasis added). When you adjust the loop's tuning control: You're changing the capacitance, and this shifts the loop's resonant frequency. At resonance means: The circulating current in the loop is maximized; the induced voltage is highest; and signal transfer into the receiver is strongest.

"In practice, the operator would: Tune the receiver to the known beacon frequency, adjust the loop's tuning knob, watch/listen for peak signal (often via headphones or meter), then rotate the loop to find the null (minimum signal) for bearing.

"Even if the receiver is perfectly tuned: If the loop is not resonant, it behaves like a weak, broadband pickup, and the coupling between antenna and receiver becomes inefficient. (emphasis added)

"Think of it like two tuned circuits: The loop (antenna circuit) and the receiver input circuit.  Both need to be aligned for best performance."

QuoteIs there some instrument associated with this set-up? What is the ARROW, the POINTER, the MASK and the UNMASKED PORTION OF THE DIAL?

I also asked chatpgt about this. Here's its reply (again lightly edited for length):

'Arrow will point to station': The loop assembly has a mechanical pointer (arrow) linked to its rotation. Once you've found the peak,the physical orientation of the loop is aligned with the incoming wave. The pointer is therefore indicating the direction of arrival. In other words, the antenna itself becomes a compass for the signal.

'Adjust mask so that zero mark is over pointer': The 'mask' is basically a rotatable compass card or dial overlay. You're doing a quick calibration. While the loop is aimed for maximum signal, you rotate the dial (mask), you line up the 0° mark with the pointer, now the system is 'zeroed' to the station.  After that adjustment: The loop's current direction = 0° reference. Any further rotation gives you relative bearings from that station."

The pics I posted may be size limited, but if you go to the website (https://www.worthpoint.com/worthopedia/ww2-bendix-radio-directional-antenna-1934792843) on which I found the pics and look at the one with the side view of the loop, you can see two printed arrows on the directional control wheel that are parallel to the loop and what looks like possibly another one (not printed, but rather perhaps a physical attachment or cutout?) approximately perpendicular to the loop.

Hope this helps.

Thanks for the message. I want to read the information on the side view - I am sure that will tell about the tuning bands.

Fwiw, it's not universally accepted that the belly antenna was pulled off or broken at Lae. My colleague Gary LaPook believes that AE established two-way communication with Lae during a position or weather report, meaning she had reception capability after takeoff from Lae (can't recall his exact reasoning, but something about the wording reported by Lae suggested that AE must have answered them at some point). I'm guessing if you search Gary's posts here and the terms Lae, position report, etc., you might be able to ifnd it. 

I believe Elgen Long also looked at the possibility that it was Itasca's transmitter on 3105 that was the problem, not Earhart's reception capability. He may have been prompted in this by the fact that in the days before AE's (non) arrival at Howland, there was cable traffic between Itasca and San Fran Div in which San Fran noted problems in Itasca's transmitter on 3105 and ordered them to get it fixed. Just going from memory on this, can't give you any details. But I assume you can find something on it in Tighar's vast archives, maybe discussions here on the forum, and probably in one of Ric's books or Elgen Long's book. 

Colin, I think this conversation came to an end a few posts ago (https://tighar.org/smf/index.php?msg=45441) when you wrote, "So, I am inclined to agree that there was only one receiver, hence only one ventral aerial and the loop control box was connected to the WE receiver."

I do not see what you see in the photos.

You have not shown the face of the control box in a reliably dated photo.

I am of the opinion that the loop antenna did not require a separate control box from the receiver.

The crank to turn it was above AE's head, directly under the loop.

The controls to tune the radio and to select antennas were elsewhere in the cabin.

If you agree that there was 1 receiver and that there were 2 different antennas, we are in agreement.


I find the sequence of pictures in Ric's article (https://tighar.org/Projects/Earhart/Archives/Research/Bulletins/53_MiamiPhoto/53_MiamiPhoto.htm) persuasive. Your photos do not persuade me that you are seeing the same box that Ric showed in the so-called "Miami Cockpit photo." He shows the position of the box in the cockpit and then shows a picture taken from the same angle later that shows that that box is missing.

The more modern Hooven system needed a control box because it did have its own receiver that needed to be tuned to the desire frequencies.

The obsolete Bendix system did not have its own receiver and was tuned through the normal radio controls, as explained in the Hooven Report. (https://tighar.org/Projects/Earhart/Archives/Documents/Hooven_Report/HoovenReport.html)

Hi Marti

I think the Bendix Loop Control Box was mounted above the pilots window and can be seen in the Caripito and Lae photos. It replaced the Hooven Remote Head in the Miami photo. As Diego pointed out, the Loop Control Box is not a receiver but is some kind of antenna adjusting device.

The point of this thread is to ask how she was able to receive a signal on 7500khz but not on 3105 or 6210.

I note that although she said in her final message that she would repeat on 6210khz, Itasca never heard anything on that frequency and never transmitted on that frequency.

The only time AE was listening on 7500, she was using the loop antenna for directional finding.  All other times, she was listening on 3105 or 6210.  The only time she reported hearing something was on the 7500 and loop antenna.

Quote from: Colin Taylor on March 27, 2026, 10:07:04 AMThe point of this thread is to ask how she was able to receive a signal on 7500khz but not on 3105 or 6210.

That has been well and thoroughly answered.

AE's one and only receiver could be turned across a full spectrum of usable frequencies.

You can click on this link to read more on this topic than I am quoting here: Radio Equipment on NR16020 (https://tighar.org/wiki/index.php?title=Radio_equipment_on_NR16020)

Western Electric Model 20B

"The receiver aboard NR16020 was a Western Electric Model 20B. This receiver was designed for communications purposes. It contained no circuitry to enable its use as a navigation receiver.

"The tuning range was divided into four bands. Originally these were:

Band 1, 188-420 KHz (beacon and marine);

Band 2, 550-1500 KHz (standard broadcast);

Band 3, 1500-4000 KHz;

Band 4, 4000-10000 KHz.

"As the requirement for 500 KHz operation existed in Earhart's case, the Band 2 tuning range was factory modified to 485-1200 KHz, covering the lower frequencies at the expense of the upper part of the broadcast band. A 1939 source lists a Model 20BA receiver, with Band 2 covering 485-1200 KHz. Earhart's equipment may have been the prototype for this off-the-shelf model.

"The Model 20B receiver was a remote-control model, with tuning dial, band switch, volume control and other controls located in a Model 27A remote control head linked to the receiver by means of tach-shafts. The remote head was mounted in a center console below the instrument panel in NR16020; the receiver itself was mounted beneath the right seat in the cockpit."

QuoteI note that although she said in her final message that she would repeat on 6210khz, Itasca never heard anything on that frequency and never transmitted on that frequency.

There is an amazing wealth of discussion in this Forum and on the website about the oddities of radio reception.

This is a link to a table of all of the transmissions logged during the last flight: Transmissions heard from NR16020 (https://tighar.org/wiki/index.php?title=Transmissions_heard_from_NR16020)

No transmissions were heard from AE on her daytime frequency for the first four hours of her flight.

Presumably she was transmitting at the times she had planned to do so.

Apparently, there was something odd about her daytime frequency that meant it was hard to receive it when she was close to the receiver.

It is not odd at all that the Itasca could hear her with increasing clarity and volume when she was on her nighttime frequency but heard nothing when she switched to her daytime frequency.

You can click on this link to learn more about the discussion: Radio propagation (https://tighar.org/wiki/index.php?title=Radio_propagation)

So here again is the set of facts that we have covered:

1. There was only one receiver on board during the fatal flight.

2. The loop antenna did not have its own receiver.

3. The one and only receiver on board could be connected to the loop or to the belly antenna.

4. The only transmission AE heard was on 7500 kcs while using the loop antenna to attempt to find a null.

5. When her system was configured to hear voice on her daytime or nighttime frequencies, she never heard any transmissions.


Inference: the DF loop antenna received the Morse code on 7500 kcs. There was something wrong with the belly antenna itself or the circuit that was supposed to connect it to the receiver.

Which is essentially what Randy Jacobson said in his post.

Dr. Jacobson is one of the most remarkable people whom I have met through TIGHAR. Not only does he possess excellent technical credentials but he has spent countless hours transcribing every message about the fatal flight from government archives and designed a Monte Carlo simulation to test various theories about weather conditions and fuel consumption.

When Randy speaks, I listen.

This is a link to a page about him and his work: Randall S. Jacobson, Ph.D. (https://tighar.org/wiki/index.php?title=Randall_S._Jacobson,_Ph.D.)

His work has been phenomenal.

No one has done more than he has to fill in the blanks about the history of the final flight.

Oh my....Marty: your $1000 check is now in the mail! 
Thank you for your kind words.

Quote from: Randy Jacobson on March 29, 2026, 08:29:54 AMOh my....Marty: your $1000 check is now in the mail! 
Thank you for your kind words.

The kind of respect and gratitude that I feel toward you can't be bought.

You did outstanding work. We are all in your debt!